**Quaternary Geochronology Using Accelerator Mass Spectrometry (AMS) – Current Status of the AMS System at the TONO Geoscience Center**

Akihiro Matsubara, Yoko Saito-Kokubu, Akimitsu Nishizawa, Masayasu Miyake, Tsuneari Ishimaru and Koji Umeda

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/58549

**1. Introduction**

## **1.1. Background**

The Tono Geoscience Center (TGC) of the Japan Atomic Energy Agency (JAEA) has been conducting research into the long term (several million years) stability of underground environments, in order to provide the scientific knowledge needed to ensure safety and reliability for the geological disposal of high-level radioactive waste [1–3]. The time scale for occurrence of the relevant geoscientific activities, as shown in Figure 1, i.e., earthquake/fault and volcanic activities, behavior of groundwater flow, uplift/subsidence and erosion of the ground surface, and so on, corresponds well to the duration of the Quaternary Period geology. Geochronology of the Quaternary Period has been strongly enhanced by measurement of terrestrial *in situ* cosmogenic radionuclides, such as 10Be, 14C, 26Al, and 36Cl, produced by secondary cosmic rays (e.g., neutron, muon) which are generated by interaction between the atmosphere of earth and primary cosmic rays that originate from the sun and galactic systems.

Applications of accelerator mass spectrometry (AMS) using those rare radionuclides for geological studies have been summarized by various authors [4–7]. It is a well-known fact that 14C has been widely utilized in several disciplines, including geology, environmental science, archaeology, and biomedicine. With regard to research into underground geological disposal of waste, radiocarbon dating of organic samples (e.g., bulk organic, humic acid, and humin fractions) taken from faults provide an historical archive of typical conventional applications

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

to the investigation of seismic activity [7,8]. The dates obtained, combined with other scientific and historical information, help to determine whether or not the fault is a so called "active fault", and to estimate cyclicity of seismic events and probability for serious large fault movements during the post-closure duration of geological disposal.

with alpha-particle/neutron emitted from radioactive elements such as Th and U within the sediment or rock dozens of meters or more underground, where there is no cosmogenic

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Our ongoing efforts, therefore, have been dedicated to development of a multi-nuclide AMS for measurement of the rare radionuclides 10Be, 14C, 26Al, and 36Cl. In this article, the current status of the AMS system at the JAEA-AMS-TONO and our activities leading to development

The next section shows the history and present-day status of our AMS system. The detail of the AMS system and its configuration are described in Section 3. The current status for 14C, 10Be and 26Al measurements is presented in Sections 4, 5, and 6, respectively. Section 7 provides the research and development related to improvement of the isobar discrimination for the

The history of the JAEA AMS system is depicted in Figure 2. The AMS system was installed at TGC in 1997, and routine measurement of 14C started in 1998 [9]. The preliminary development of the 10Be-AMS started around 2002 [10], intensive development was implemented from 2010 to 2012 [11,12], and progress of which will be described in Section 5. After that, the routine measurement of 10Be started at the beginning of fiscal year 2013. At present, we have initiated the development of the 26Al-AMS (described in Section 6) [13, 14]. Furthermore, as a part of preparatory activity for the development of the 36Cl-AMS, we have started to investigate the nature of the pulse trace that is disturbed by interfering particles in the heavy ion detector, in order to improve the discrimination performance for

radionuclide production [5].

**1.2. Purpose and contents of this article**

of a multi-nuclide AMS are presented.

**Figure 2.** Timeline of the JAEA-AMS-TONO development.

the detector system (presented in Section 7) [15].

**2. Operation status**

ionization chamber. Finally, Section 8 presents a summary.

**Figure 1.** Long-term geological activities relevant to the geological disposal of high-level radioactive waste.

Long-lived cosmogenic radionuclides, such as 10Be, 26Al, and 36Cl enable us to apply to the exposure dating methods on boulder and bedrock surfaces for exposure ages up to 107 years [4–7]. These methods can provide information relevant to geological disposal with respect to geomorphological evolution, i.e., erosion rates of rock surfaces, burial histories of rock surfaces and sediments, fault slip rates, and so on. One of the typical radionuclides for surface exposure dating is 10Be, or both 10Be and 26Al. The half-lives of 10Be and 26Al are long enough (1.6 Myr and 0.7 Myr, respectively) to span the entire Quaternary timescale. They can be produced simultaneously in a single sample of quartz where 26Al and 10Be are mainly produced through nuclear spallation from 28Si and 16O, respectively. Concentration of each depends on balance between *in situ* production and surface erosion and generally, there are two unknown variables. The simultaneous measurement of 26Al and 10Be concentration has the advantage of solving for the two variables: exposure age and erosion rate. On the other hand, 36Cl is produced through multi-channel reactions: spallation on Ca and K with neutron and muon, and the thermal neutron capture on 35Cl. This feature provides an advantage to the 36Cl exposure dating that is not restricted to specific rock types or minerals (such as carbonates or silicates) under a number of conditions. Furthermore, decomposition of contribution for 36Cl production into spallogenic, mugenic, and thermal neutron can increase the amount of information for one sample, with the potential for greater erosion-exposure history accuracy.

The measurement of 14C and 36Cl is also applicable to hydrogeologic investigations: studies of groundwater age, origin and mixing. Most of these nuclides are produced through interaction with alpha-particle/neutron emitted from radioactive elements such as Th and U within the sediment or rock dozens of meters or more underground, where there is no cosmogenic radionuclide production [5].

## **1.2. Purpose and contents of this article**

to the investigation of seismic activity [7,8]. The dates obtained, combined with other scientific and historical information, help to determine whether or not the fault is a so called "active fault", and to estimate cyclicity of seismic events and probability for serious large fault

**Figure 1.** Long-term geological activities relevant to the geological disposal of high-level radioactive waste.

Long-lived cosmogenic radionuclides, such as 10Be, 26Al, and 36Cl enable us to apply to the exposure dating methods on boulder and bedrock surfaces for exposure ages up to 107

[4–7]. These methods can provide information relevant to geological disposal with respect to geomorphological evolution, i.e., erosion rates of rock surfaces, burial histories of rock surfaces and sediments, fault slip rates, and so on. One of the typical radionuclides for surface exposure dating is 10Be, or both 10Be and 26Al. The half-lives of 10Be and 26Al are long enough (1.6 Myr and 0.7 Myr, respectively) to span the entire Quaternary timescale. They can be produced simultaneously in a single sample of quartz where 26Al and 10Be are mainly produced through nuclear spallation from 28Si and 16O, respectively. Concentration of each depends on balance between *in situ* production and surface erosion and generally, there are two unknown variables. The simultaneous measurement of 26Al and 10Be concentration has the advantage of solving for the two variables: exposure age and erosion rate. On the other hand, 36Cl is produced through multi-channel reactions: spallation on Ca and K with neutron and muon, and the thermal neutron capture on 35Cl. This feature provides an advantage to the 36Cl exposure dating that is not restricted to specific rock types or minerals (such as carbonates or silicates) under a number of conditions. Furthermore, decomposition of contribution for 36Cl production into spallogenic, mugenic, and thermal neutron can increase the amount of information for one sample, with the potential for greater erosion-exposure history accuracy.

The measurement of 14C and 36Cl is also applicable to hydrogeologic investigations: studies of groundwater age, origin and mixing. Most of these nuclides are produced through interaction

years

movements during the post-closure duration of geological disposal.

4 Geochronology - Methods and Case Studies

Our ongoing efforts, therefore, have been dedicated to development of a multi-nuclide AMS for measurement of the rare radionuclides 10Be, 14C, 26Al, and 36Cl. In this article, the current status of the AMS system at the JAEA-AMS-TONO and our activities leading to development of a multi-nuclide AMS are presented.

The next section shows the history and present-day status of our AMS system. The detail of the AMS system and its configuration are described in Section 3. The current status for 14C, 10Be and 26Al measurements is presented in Sections 4, 5, and 6, respectively. Section 7 provides the research and development related to improvement of the isobar discrimination for the ionization chamber. Finally, Section 8 presents a summary.

**Figure 2.** Timeline of the JAEA-AMS-TONO development.

## **2. Operation status**

The history of the JAEA AMS system is depicted in Figure 2. The AMS system was installed at TGC in 1997, and routine measurement of 14C started in 1998 [9]. The preliminary development of the 10Be-AMS started around 2002 [10], intensive development was implemented from 2010 to 2012 [11,12], and progress of which will be described in Section 5. After that, the routine measurement of 10Be started at the beginning of fiscal year 2013. At present, we have initiated the development of the 26Al-AMS (described in Section 6) [13, 14]. Furthermore, as a part of preparatory activity for the development of the 36Cl-AMS, we have started to investigate the nature of the pulse trace that is disturbed by interfering particles in the heavy ion detector, in order to improve the discrimination performance for the detector system (presented in Section 7) [15].

The evolutions of the measurement time and the number of sample cathode (target) are shown in Figure 3. Total, cumulative, measurement time (the blue line) has increased more or less continuously for 15 years, and reached 15,000 hours this fiscal year. Around 2005, routine measurements ceased for a while due to system maintenance by the lab-staff. As shown on the bar chart, the average number of samples measured annually is between 800 and 1000, and the total number of samples will exceed 12,500 within the next few months. After the devel‐ opment of the 10Be measurement has been intensive since the start, the proportion of 10Be samples to the total sample number has increased rapidly. In fiscal year 2012, the proportion of 14C, 10Be and 26Al sample cathodes are 76%, 20%, and 4%, respectively.

**Figure 4.** Proportion of 14C-AMS samples measured in the various investigation fields (left) and, on the right, those

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The AMS system is a versatile system based on the PelletronTM tandem accelerator (Model 15SDH-2, 5 MV terminal voltage) [17]. The same type (5 MV Pelletron) of the AMS system has been used in other facilities, for example, at the Micro Analysis Laboratory, Tandem accelerator (MALT) at the University of Tokyo, Japan [18], the AMS system at the National Institute for Environmental Studies (NIES) of Japan [19], at the Scottish Universities Environmental Research Center (SUERC) in the United Kingdom [20,21], and at the Uppsala 5 MV Pelletron

This AMS system is designed for the AMS analysis with most radio-isotopes including 10Be, 14C, 26Al, 36Cl, and 129I. Although technological advances in recent years have enabled practical use of compact AMS systems below 1 MV allowing the measurement of 10Be, 14C, and 26Al [24– 26], the relatively wide range of high terminal voltage greater than several megavolts has, even now, been generally recognized to be beneficial to efficient suppression of signal background,

Figure 5 is a schematic of the AMS system layout. The system can be divided into five major subsystems: the ion sources, the sequential injection system, the tandem Pelletron accelerator, the post-accelerator beamline with the high-energy mass spectrometer components, and the heavy ion detection system by means of the ionization chamber. There are eight vacuum turbomolecular pumping systems attached along the beamline, where, several beam steerers

tandem accelerator developed in the Uppsala University, Sweden [22,23].

resulting in further potential for expandability for a multi-nuclides measurement.

requested by JAEA users to by others in fiscal year 2012.

**3. AMS system**

**3.1. Overall features**

**3.2. System description [17]**

**Figure 3.** Evolution of the measurement time and sample number.

In Figure 4, the pie chart on the left shows the proportion of 14C-AMS samples measured for the various study fields in fiscal year 2012. Geoscience accounted for about 60%, while environmental studies accounted for most of the balance. The proportion labeled as "Analysis" stands for the cathode number used in our technical development. The pie chart on the right in Figure 4 illustrates the relative proportion of measured samples requested by users in JAEA to other users. Almost all of the samples were requested by JAEA users. The measurement of the other samples were performed under JAEA's common-use facility program for non-JAEA users [16]. This program started in 2006, in order to enlarge and expand the public use of JAEA's facilities. The study fields using the program were mostly in environmental science and archaeology.

Quaternary Geochronology Using Accelerator Mass Spectrometry (AMS) – Current Status of the AMS System… http://dx.doi.org/10.5772/58549 7

**Figure 4.** Proportion of 14C-AMS samples measured in the various investigation fields (left) and, on the right, those requested by JAEA users to by others in fiscal year 2012.

## **3. AMS system**

The evolutions of the measurement time and the number of sample cathode (target) are shown in Figure 3. Total, cumulative, measurement time (the blue line) has increased more or less continuously for 15 years, and reached 15,000 hours this fiscal year. Around 2005, routine measurements ceased for a while due to system maintenance by the lab-staff. As shown on the bar chart, the average number of samples measured annually is between 800 and 1000, and the total number of samples will exceed 12,500 within the next few months. After the devel‐ opment of the 10Be measurement has been intensive since the start, the proportion of 10Be samples to the total sample number has increased rapidly. In fiscal year 2012, the proportion

In Figure 4, the pie chart on the left shows the proportion of 14C-AMS samples measured for the various study fields in fiscal year 2012. Geoscience accounted for about 60%, while environmental studies accounted for most of the balance. The proportion labeled as "Analysis" stands for the cathode number used in our technical development. The pie chart on the right in Figure 4 illustrates the relative proportion of measured samples requested by users in JAEA to other users. Almost all of the samples were requested by JAEA users. The measurement of the other samples were performed under JAEA's common-use facility program for non-JAEA users [16]. This program started in 2006, in order to enlarge and expand the public use of JAEA's facilities. The study fields using the program were mostly in environmental science

of 14C, 10Be and 26Al sample cathodes are 76%, 20%, and 4%, respectively.

6 Geochronology - Methods and Case Studies

**Figure 3.** Evolution of the measurement time and sample number.

and archaeology.

#### **3.1. Overall features**

The AMS system is a versatile system based on the PelletronTM tandem accelerator (Model 15SDH-2, 5 MV terminal voltage) [17]. The same type (5 MV Pelletron) of the AMS system has been used in other facilities, for example, at the Micro Analysis Laboratory, Tandem accelerator (MALT) at the University of Tokyo, Japan [18], the AMS system at the National Institute for Environmental Studies (NIES) of Japan [19], at the Scottish Universities Environmental Research Center (SUERC) in the United Kingdom [20,21], and at the Uppsala 5 MV Pelletron tandem accelerator developed in the Uppsala University, Sweden [22,23].

This AMS system is designed for the AMS analysis with most radio-isotopes including 10Be, 14C, 26Al, 36Cl, and 129I. Although technological advances in recent years have enabled practical use of compact AMS systems below 1 MV allowing the measurement of 10Be, 14C, and 26Al [24– 26], the relatively wide range of high terminal voltage greater than several megavolts has, even now, been generally recognized to be beneficial to efficient suppression of signal background, resulting in further potential for expandability for a multi-nuclides measurement.

#### **3.2. System description [17]**

Figure 5 is a schematic of the AMS system layout. The system can be divided into five major subsystems: the ion sources, the sequential injection system, the tandem Pelletron accelerator, the post-accelerator beamline with the high-energy mass spectrometer components, and the heavy ion detection system by means of the ionization chamber. There are eight vacuum turbomolecular pumping systems attached along the beamline, where, several beam steerers and magnetic or electrostatic lenses are located, and the total length of the system is around 31 metres. Summary of the system configuration for rare isotopes are presented individually in Table 1.

The ion sources, the Multi-Cathode, Source of Negative Ions by Cesium Sputtering (MC-SNICS) for solid samples (40 cathodes) and the Multiple Gas Feed, SNICS (MGF-SNICS) for CO2-gas samples (12 cathodes) are connected to the main beamline through the 45° electrostatic spherical analyzer (ESA). The sources consist of a cesium oven generating Cs vapour, a heated

Quaternary Geochronology Using Accelerator Mass Spectrometry (AMS) – Current Status of the AMS System…

focus electrodes. Particles sputtered from the sample cathode by Cs+ bombardments pick up electrons as they pass through the cesium layer condensed on the sample; thus, negative ions are produced. To stabilize the cesium vapour feed to the source, we added a simple autocontrollable electrical heating subsystem to the cesium oven and its feeder pipe; the standard deviation of the temperature monitored during the routine measurement has been kept within a range of ±0.5C° [27]. This type of simple technical addition or modification is commonly used for the same purpose [28]. The acceleration voltage of the ion source is usually set to 55 kV. By using the beam-slit located at the image point of the ESA (before the injection magnet), the "tail" of the beam profile can be trimmed, where the tail is due to an energy spread in the sputtering process. This trimming assures open-aperture optical properties (often called "flat top transmission") on the downstream side of that slit. The combination of the ESA and the

MeV and *q* is the charge number, and the nominal radius is 0.457 m) limits the specific chargeto-mass ratio, *M* / *q*, for the transmittance of negative ions in the pre-accelerator region to remove unwanted particles from the ion beam. The mass resolution (*M* /*ΔM*) reaches 200 for the electrostatic and magnetic filters combination. The same concept is applied for the highenergy post-accelerator region containing the analyzing magnet and electrostatic cylindrical

The AMS system employs the sequential injection method for the precise measurement of the ratio of rare to abundant isotopes regardless of fluctuations of source conditions. This method, or the rapid switching of the masses (isotopes) to be injected toward the accelerator, (so called sequence "bounced" or "jumping" beams) is accomplished by applying an appropriate bias potential to the electrically insulated bent chamber inside the injection magnet. Most of the duration (~99%) in the sequential injection is allocated for the measurement of the rare isotope

In the tandem Pelletron accelerator (15-SDH), there are two parallel chains charging the highvoltage terminal with current up to 300 μA. The consequent maximum terminal potential of 5 MV leads to the suitable stripped ionization state of 4+ for carbon by using a gas stripper (the ion beam energy is up to 25 MeV). For the chlorine, the charge state is designed to be 7+ or 8+ by using a foil stripper and its energy would be lie in the range of 42-45 MeV. The pressure of the stripper gas is typically 10 μTorr, 9 μTorr, and 5 μTorr for 14C-, 10Be-, and 26Al-measurement,

The high-energy mass spectrometer in the post-accelerator region is composed of magnetic and electrostatic filters and detector systems. The analyzing magnet (produced by Danfysik A/S) is a double focusing 90° sector magnet with a nominal radius of 1.270 m, having param‐ eters of *ME* /*q*<sup>2</sup> ≤ 176 and *M* /*ΔM=*725. After the off-axis, multi-Faraday cup detector for the abundant isotopes, the 20° ECA with a nominal radius of 3.810 m is used to remove abundant

beam at the sample cathode, and extraction and

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9

≤ 15, where *M* is the mass number of the ion, *E* is the ion energy in

ionizer electrode producing a focused Cs+

injection magnet (*ME* / *q*<sup>2</sup>

analyzer (ECA) listed below.

(details in Table 1).

respectively.

**Figure 5.** AMS system at JAEA-AMS-TONO.


† Sample of BLK was made from a quantified standard sample for atomic absorption spectrometry supplied from Wa‐ ko Pure Chemical Industries, Ltd.

**Table 1.** Summary of current AMS specifications for the JAEA-AMS-TONO.

and magnetic or electrostatic lenses are located, and the total length of the system is around 31 metres. Summary of the system configuration for rare isotopes are presented individually

**Nuclide 14C 10Be 26Al**

) 2 μA (BeO-

Transmit. 58% (12C) 21% (9Be) 39% (27Al)

† Sample of BLK was made from a quantified standard sample for atomic absorption spectrometry supplied from Wa‐

(

4.8 MV (16.3 MeV)

BeO with Nb powder

Simultaneous

10Be16O, 9Be17O)

10Be3+/17O5+ (70 cps @S5-1)

< 7 x 10-15 @MITSUWA powder

> ΔE1-ERes (with gas cell)

4.3 MV (17.2 MeV)

Al2O3 with Ag powder

Sequential

26Al: 98 ms, 27Al: 1 ms)

26Al3+/ 27Al3+ (15 cps @S4-1)

> < 3 x 10-14 @Blank†

> > ΔE1-ETot

)

) 0.1 μA (Al-

(

4.5 MV (22.5 MeV)

powder

13C: 0.9 ms, 14C: 98.6 ms)

14C4+/12C4+, 13C4+/12C4+ (60 cps@ HOxII)

< 7x10-16 (< 0.06 pMC) @WAKO Powder

**Table 1.** Summary of current AMS specifications for the JAEA-AMS-TONO.

in Table 1.

8 Geochronology - Methods and Case Studies

**Figure 5.** AMS system at JAEA-AMS-TONO.

Target Graphite with Fe

Injection Sequential (12C: 0.3 ms ,

Current 20 μA (C-

Ionization chamber ΔE1-ERes

ko Pure Chemical Industries, Ltd.

Terminal volt. (Tot. Energy)

Meas. ratio (Count rate)

Background

The ion sources, the Multi-Cathode, Source of Negative Ions by Cesium Sputtering (MC-SNICS) for solid samples (40 cathodes) and the Multiple Gas Feed, SNICS (MGF-SNICS) for CO2-gas samples (12 cathodes) are connected to the main beamline through the 45° electrostatic spherical analyzer (ESA). The sources consist of a cesium oven generating Cs vapour, a heated ionizer electrode producing a focused Cs+ beam at the sample cathode, and extraction and focus electrodes. Particles sputtered from the sample cathode by Cs+ bombardments pick up electrons as they pass through the cesium layer condensed on the sample; thus, negative ions are produced. To stabilize the cesium vapour feed to the source, we added a simple autocontrollable electrical heating subsystem to the cesium oven and its feeder pipe; the standard deviation of the temperature monitored during the routine measurement has been kept within a range of ±0.5C° [27]. This type of simple technical addition or modification is commonly used for the same purpose [28]. The acceleration voltage of the ion source is usually set to 55 kV. By using the beam-slit located at the image point of the ESA (before the injection magnet), the "tail" of the beam profile can be trimmed, where the tail is due to an energy spread in the sputtering process. This trimming assures open-aperture optical properties (often called "flat top transmission") on the downstream side of that slit. The combination of the ESA and the injection magnet (*ME* / *q*<sup>2</sup> ≤ 15, where *M* is the mass number of the ion, *E* is the ion energy in MeV and *q* is the charge number, and the nominal radius is 0.457 m) limits the specific chargeto-mass ratio, *M* / *q*, for the transmittance of negative ions in the pre-accelerator region to remove unwanted particles from the ion beam. The mass resolution (*M* /*ΔM*) reaches 200 for the electrostatic and magnetic filters combination. The same concept is applied for the highenergy post-accelerator region containing the analyzing magnet and electrostatic cylindrical analyzer (ECA) listed below.

The AMS system employs the sequential injection method for the precise measurement of the ratio of rare to abundant isotopes regardless of fluctuations of source conditions. This method, or the rapid switching of the masses (isotopes) to be injected toward the accelerator, (so called sequence "bounced" or "jumping" beams) is accomplished by applying an appropriate bias potential to the electrically insulated bent chamber inside the injection magnet. Most of the duration (~99%) in the sequential injection is allocated for the measurement of the rare isotope (details in Table 1).

In the tandem Pelletron accelerator (15-SDH), there are two parallel chains charging the highvoltage terminal with current up to 300 μA. The consequent maximum terminal potential of 5 MV leads to the suitable stripped ionization state of 4+ for carbon by using a gas stripper (the ion beam energy is up to 25 MeV). For the chlorine, the charge state is designed to be 7+ or 8+ by using a foil stripper and its energy would be lie in the range of 42-45 MeV. The pressure of the stripper gas is typically 10 μTorr, 9 μTorr, and 5 μTorr for 14C-, 10Be-, and 26Al-measurement, respectively.

The high-energy mass spectrometer in the post-accelerator region is composed of magnetic and electrostatic filters and detector systems. The analyzing magnet (produced by Danfysik A/S) is a double focusing 90° sector magnet with a nominal radius of 1.270 m, having param‐ eters of *ME* /*q*<sup>2</sup> ≤ 176 and *M* /*ΔM=*725. After the off-axis, multi-Faraday cup detector for the abundant isotopes, the 20° ECA with a nominal radius of 3.810 m is used to remove abundant isotopes having the same mass energy product as the rare isotopes. The resolution attained by the ECA is *M* /*ΔM=*200.

procedure of data analysis, as well as 10Be and 26Al analyses has followed an algorithm in the software code "abc" available from the NEC Corporation [30]. It is noted that to obtain the net activity the 14C/12C ratio for all samples (except C1) is normally subtracted by the IAEA-C1 value as the chemical background due to the sample preparation, where C1 is made from marble, and its nominal value in the *percent Modern Carbon* (pMC) that is

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The left column of rectangles on Figure 6 shows the evolution of pMC for IAEA-C6, -C5, and -C1 in the year 2013. In some periods no 14C measurements were performed. The period from May to June was allotted for 10Be measurements. Intensive system maintenance was carried out (normally annually) in August, and then 10Be measurements were performed until the end of September. Almost all measured pMC-values for both C6 and C5 are in agreement with the nominal values within 3σ of each point (σ is basically the statistical uncertainty that is inversely proportional to the square root of 14C counts). A few irregular points in C6 could be due to surface roughness of the graphite sample. The roughness is reflected by unsuccessful graphite compression with an Arbor press (hammering with a press-pin). We continue to check the

**Figure 6.** Measurement quality in a 12-month period for 14C-AMS standards: C6 (a), C5 (b), and C1 (c). The left column shows evolutions of the measured value in percent Modern Carbon for the year 2013. The gray line and hatch for both left and right columns depicts respectively the nominal value and its range of uncertainty. The right column shows corresponding histograms for the left column measurement points. The dashed and dotted lines present respectively the arithmetic mean *m* and its standard deviation σ<sup>H</sup> on the histogram. The symbol c.n. stands for the number of sam‐ ple cathodes used in constructing the histogram, and σHrsd is the relative standard deviation given by σH/*m*× 100%.

equivalent to the 100%× *fraction modern* (described above) is 0.00 ± 0.02 [29].

surface condition and data related to such irregular results.

The final detector for counting the rare isotopes, the "heavy ion detector", is the gas ionization detector that contains multiple *ΔE* electrodes (five) arranged along the axis of the beamline (for details see Section 5.2). A nearly equivalent type of detector is used at the SUERC facility in the UK [21]. The *ΔE* electrodes determine the specific of energy loss arising in their respective regions. In the case for the ion energy over MeV/u, the rate of the energy loss (or stopping power) increases with increasing atomic number. Thus, the spectrum on the plane given by energy-loss 2D-coordinates for various combinations of Δ*E* electrodes shows its characteristic position on that plane, which makes it possible to distinguish the spectrum of desired nuclide from other spectra of interfering particles. Typical particles interfering with the rare isotopes of 10Be, 14C, and 36Cl are respectively10B, 7 Li, and 36S; their *M* / *q* ratios can be equivalent to those of the rare isotop es, thus allowing their entry into the detector. The problem with respect to the 10B and 36S is known as an isobar problem (different elements but same atomic weight). The *M* / *q* ratio of 14C4+ coincides with that of 7 Li2+; although the prime number of charge state (3+ for 14C in this case) is preferable to avoid the coincidence of the *M* / *q* ratio, we focus on the higher stripping yield for 4+ (59% at 4.5 MV [5]) rather than 3+ (55% at 2.9 MV [5]). Since the relative difference between atomic numbers for the case of "Be and B" or "Cl and S" is much smaller than the difference for C and Li, distinguishing them in the former case is harder than in the latter case. Therefore, discrimination techniques are crucial for measurements of 10Be and 36Cl. A gas absorber technique has been employed in the 10Be-AMS operation, which is described in Section 5.2. In addition, investigation of a pulse trace fluctuation caused by interfering particles in a preparation for the 36Cl measurement is presented in Section 7.

## **4. 14C measurement**

#### **4.1. Stability and reliability**

In the 14C-AMS operation, the stability and reliability of the routine measurements have been checked continuously against measuring standards. The typical standards are, IAEA-C1, -C5, and -C6 [29], and the oxalic acid HOxII (SRM-4990C) that is produced by the National Institute of Standards and Technology, NIST in the USA. Such checks have been performed simultaneously with routine measurements. In our AMS analysis, usually only the HOxII is used for obtaining the normalization constant that is given by the δ13C corrected activity divided by the *fraction modern* for the HOxII (1.3406). Here, the term *fraction modern* is a quantity defined as the ratio of a sample 14C activity *A*SN to a normalized sample 14C activity *A*ON, where *A*ON is equal to the 14C-decay corrected *absolute international standard specific activity A*abs that is intended to correspond to the hypothetical specific activity of atmospheric carbon of year 1950 (in detail see Ref.[30,31]). With respect to data quality of the HOxII standard, therefore, only the relative precision (or relative standard deviation, *rsd*) affects the normalization constant; in other words, the accuracy itself has no mean‐ ing. The value of *rsd* for the HOxII is around 0.22% in every routine-measurement. The procedure of data analysis, as well as 10Be and 26Al analyses has followed an algorithm in the software code "abc" available from the NEC Corporation [30]. It is noted that to obtain the net activity the 14C/12C ratio for all samples (except C1) is normally subtracted by the IAEA-C1 value as the chemical background due to the sample preparation, where C1 is made from marble, and its nominal value in the *percent Modern Carbon* (pMC) that is equivalent to the 100%× *fraction modern* (described above) is 0.00 ± 0.02 [29].

isotopes having the same mass energy product as the rare isotopes. The resolution attained by

The final detector for counting the rare isotopes, the "heavy ion detector", is the gas ionization detector that contains multiple *ΔE* electrodes (five) arranged along the axis of the beamline (for details see Section 5.2). A nearly equivalent type of detector is used at the SUERC facility in the UK [21]. The *ΔE* electrodes determine the specific of energy loss arising in their respective regions. In the case for the ion energy over MeV/u, the rate of the energy loss (or stopping power) increases with increasing atomic number. Thus, the spectrum on the plane given by energy-loss 2D-coordinates for various combinations of Δ*E* electrodes shows its characteristic position on that plane, which makes it possible to distinguish the spectrum of desired nuclide from other spectra of interfering particles. Typical particles interfering with the rare isotopes

of the rare isotop es, thus allowing their entry into the detector. The problem with respect to the 10B and 36S is known as an isobar problem (different elements but same atomic weight). The

for 14C in this case) is preferable to avoid the coincidence of the *M* / *q* ratio, we focus on the higher stripping yield for 4+ (59% at 4.5 MV [5]) rather than 3+ (55% at 2.9 MV [5]). Since the relative difference between atomic numbers for the case of "Be and B" or "Cl and S" is much smaller than the difference for C and Li, distinguishing them in the former case is harder than in the latter case. Therefore, discrimination techniques are crucial for measurements of 10Be and 36Cl. A gas absorber technique has been employed in the 10Be-AMS operation, which is described in Section 5.2. In addition, investigation of a pulse trace fluctuation caused by interfering particles in a preparation for the 36Cl measurement is presented in Section 7.

In the 14C-AMS operation, the stability and reliability of the routine measurements have been checked continuously against measuring standards. The typical standards are, IAEA-C1, -C5, and -C6 [29], and the oxalic acid HOxII (SRM-4990C) that is produced by the National Institute of Standards and Technology, NIST in the USA. Such checks have been performed simultaneously with routine measurements. In our AMS analysis, usually only the HOxII is used for obtaining the normalization constant that is given by the δ13C corrected activity divided by the *fraction modern* for the HOxII (1.3406). Here, the term *fraction modern* is a quantity defined as the ratio of a sample 14C activity *A*SN to a normalized sample 14C activity *A*ON, where *A*ON is equal to the 14C-decay corrected *absolute international standard specific activity A*abs that is intended to correspond to the hypothetical specific activity of atmospheric carbon of year 1950 (in detail see Ref.[30,31]). With respect to data quality of the HOxII standard, therefore, only the relative precision (or relative standard deviation, *rsd*) affects the normalization constant; in other words, the accuracy itself has no mean‐ ing. The value of *rsd* for the HOxII is around 0.22% in every routine-measurement. The

Li, and 36S; their *M* / *q* ratios can be equivalent to those

Li2+; although the prime number of charge state (3+

the ECA is *M* /*ΔM=*200.

10 Geochronology - Methods and Case Studies

of 10Be, 14C, and 36Cl are respectively10B, 7

*M* / *q* ratio of 14C4+ coincides with that of 7

**4. 14C measurement**

**4.1. Stability and reliability**

The left column of rectangles on Figure 6 shows the evolution of pMC for IAEA-C6, -C5, and -C1 in the year 2013. In some periods no 14C measurements were performed. The period from May to June was allotted for 10Be measurements. Intensive system maintenance was carried out (normally annually) in August, and then 10Be measurements were performed until the end of September. Almost all measured pMC-values for both C6 and C5 are in agreement with the nominal values within 3σ of each point (σ is basically the statistical uncertainty that is inversely proportional to the square root of 14C counts). A few irregular points in C6 could be due to surface roughness of the graphite sample. The roughness is reflected by unsuccessful graphite compression with an Arbor press (hammering with a press-pin). We continue to check the surface condition and data related to such irregular results.

**Figure 6.** Measurement quality in a 12-month period for 14C-AMS standards: C6 (a), C5 (b), and C1 (c). The left column shows evolutions of the measured value in percent Modern Carbon for the year 2013. The gray line and hatch for both left and right columns depicts respectively the nominal value and its range of uncertainty. The right column shows corresponding histograms for the left column measurement points. The dashed and dotted lines present respectively the arithmetic mean *m* and its standard deviation σ<sup>H</sup> on the histogram. The symbol c.n. stands for the number of sam‐ ple cathodes used in constructing the histogram, and σHrsd is the relative standard deviation given by σH/*m*× 100%.

The frames in the right-hand side of Figure 6 show histograms corresponding to temporal evolution for each standard in the left frames. It can be seen that the arithmetic mean of the histogram for C6 (labelled as *m*) is significantly lower than the range of its nominal value; in other words, a difference between them of 0.36% is quite a bit larger than "the standard deviation of *m"* of 0.10% given by 1*σ*H/(c.n.)0.5 where c.n. is the cathode number used in constructing the histogram. This fact, however, is very similar to results obtained in other facilities [32,33], suggesting to us to reconsider use of the nominal value for the purpose of comparisons with measured data. On the other hand, the arithmetic mean for C5 is consistent with its nominal value; the difference between them is within the standard deviation of *m* of 0.03%. The ±1*σ*H (0.18%) of the histogram for C5 is equivalent to ±100 years in its 14C age of 11,750 years, which can be a measure of the confidence interval for one-year. This performance is comparable with the other facilities, e.g., a well-known carbon dating lab in Japan, the Paleo Labo Co.,Ltd [33,34]. For C1, although the arithmetic mean is 0.15 pMC, sometimes values are below 0.1 pMC. This scatter implies possibility of contamination during the sample prepara‐ tion, which is supported by another fact described as follows. In contrast with C1 in the estimation of background, a high-purity synthetic graphite powder (No.072-03845 Wako Pure Chemical Industries, Ltd, Osaka, Japan) is used as a machine background samples. This powder is made from coke (14C-free), and is directly poured into the cathode hole without any chemical preparations; thus, 14C counts detected with the powder are due to machine back‐ ground. We have continuously observed around 0.06 pMC for the graphite powder, which shows potential for improvement for lowering the background due to chemical process.

and electrostatic analyzers. Consequently, the measurement of quite low concentration samples is suitable relative to use by the TONO. The detailed results and discussion for the

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**Figure 7.** Results of comparison test between JAEA-AMS-TONO (shaded circles) and JAEA-AMS-MUTSU (empty squares). This test was carried out in 2012. The samples were prepared in the MUTSU, distributed to the TONO, and measured in both facilities. The algorithm for the data analysis followed the software code used in the TONO.

The configuration for our 10Be-AMS operation is fundamentally standard, and is based on that used in the MALT [18,37,38]. Samples are made from the solid oxide of beryllium, BeO, for its positive electron affinity to produce negative ions. Since the amount of the rare isotope 10Be is distributed according to the abundance of oxygen isotope ratios, i.e., 16O : 17O : 18O=99.76: 0.04: 0.20, respectively, the 10Be16O is selected for injection into the accelera‐ tor to ensure high extraction efficiency of 10Be from the sample. The terminal voltage, usually set at 4.7 or 4.8 MV, is made preferably as high as possible within the range of around 8 MV so as to increase the stripping efficiency from negative ions to 3+ (in our AMS system, the terminal voltage is limited to the specification of 5 MV). In addition, the higher ion energy is also preferred for ensuring the good performance of the discrimination between 10Be and 10B for the heavy ion detector as described below (details in Section 5.2). One of the most significant features of 10Be measurement is the simultaneous injection of 9

with mass the same as 10Be16O for counting the abundance of 9Be isotopes. For this purpose the current of 17O5+ is measured to avoid uncertainty in the amount of beryllium hydrox‐

The mathematical formula used for obtaining the measured isotope ratio (*R*m=10Be/9

Faraday cup behind the analyzing magnet as shown in Figure 5.

Be16OH contamination in the sample [37]. The abundance isotope is detected with a

Be17O

Be) is:

**5. 10Be measurement**

ide 9

**5.1. System configuration and method**

series of comparison tests will be summarised in a JAEA report in the future.

#### **4.2. Inter-laboratory comparison testing**

Comparison of the results obtained in different laboratories on the same samples is funda‐ mental to objectively assessing accuracy and system performance. Comparison tests were carried out twice, in 2010 and 2012, with another AMS facility, the JAEA-AMS-MUTSU, of the Aomori Research and Development Center, JAEA [35,36]. This facility has provided highquality 129I-and 14C-AMSs for environmental science studies, especially for marine transport properties of radio-isotopes, as well as for radiocarbon dating. The typical properties of the AMS system are as follows: a 3 MV Tandetron Cockcroft–Walton accelerator manufactured by High Voltage Engineering Europa, and the simultaneous injection system with the sepa‐ rator-combiner.

In the comparison test performed in 2012, the samples of the HOxII, C5, and C1 were prepared in the MUTSU, distributed to the TONO, and measured in both facilities. The measurement condition such as the duration time or the beam current was taken as the normal condition in each facility. Figure 7 shows results obtained in 2012. For the data analysis, the algorithm used in the TONO was employed. It can be seen for the C5, that there was no significant difference between data obtained in both facilities. The results of the C1 analyzed in the TONO are much lower than that for the MUTSU. This is mostly due to the fact that during 14C counting, 13C ions for the simultaneous injection also entered the accelerator continuously, thus the counting rate (or its possibility) of 13C coming into the ionization detector is much higher for the simultaneous injection than for the sequential injection, in spite of filtering by the combination of magnetic and electrostatic analyzers. Consequently, the measurement of quite low concentration samples is suitable relative to use by the TONO. The detailed results and discussion for the series of comparison tests will be summarised in a JAEA report in the future.

**Figure 7.** Results of comparison test between JAEA-AMS-TONO (shaded circles) and JAEA-AMS-MUTSU (empty squares). This test was carried out in 2012. The samples were prepared in the MUTSU, distributed to the TONO, and measured in both facilities. The algorithm for the data analysis followed the software code used in the TONO.

## **5. 10Be measurement**

The frames in the right-hand side of Figure 6 show histograms corresponding to temporal evolution for each standard in the left frames. It can be seen that the arithmetic mean of the histogram for C6 (labelled as *m*) is significantly lower than the range of its nominal value; in other words, a difference between them of 0.36% is quite a bit larger than "the standard deviation of *m"* of 0.10% given by 1*σ*H/(c.n.)0.5 where c.n. is the cathode number used in constructing the histogram. This fact, however, is very similar to results obtained in other facilities [32,33], suggesting to us to reconsider use of the nominal value for the purpose of comparisons with measured data. On the other hand, the arithmetic mean for C5 is consistent with its nominal value; the difference between them is within the standard deviation of *m* of 0.03%. The ±1*σ*H (0.18%) of the histogram for C5 is equivalent to ±100 years in its 14C age of 11,750 years, which can be a measure of the confidence interval for one-year. This performance is comparable with the other facilities, e.g., a well-known carbon dating lab in Japan, the Paleo Labo Co.,Ltd [33,34]. For C1, although the arithmetic mean is 0.15 pMC, sometimes values are below 0.1 pMC. This scatter implies possibility of contamination during the sample prepara‐ tion, which is supported by another fact described as follows. In contrast with C1 in the estimation of background, a high-purity synthetic graphite powder (No.072-03845 Wako Pure Chemical Industries, Ltd, Osaka, Japan) is used as a machine background samples. This powder is made from coke (14C-free), and is directly poured into the cathode hole without any chemical preparations; thus, 14C counts detected with the powder are due to machine back‐ ground. We have continuously observed around 0.06 pMC for the graphite powder, which shows potential for improvement for lowering the background due to chemical process.

Comparison of the results obtained in different laboratories on the same samples is funda‐ mental to objectively assessing accuracy and system performance. Comparison tests were carried out twice, in 2010 and 2012, with another AMS facility, the JAEA-AMS-MUTSU, of the Aomori Research and Development Center, JAEA [35,36]. This facility has provided highquality 129I-and 14C-AMSs for environmental science studies, especially for marine transport properties of radio-isotopes, as well as for radiocarbon dating. The typical properties of the AMS system are as follows: a 3 MV Tandetron Cockcroft–Walton accelerator manufactured by High Voltage Engineering Europa, and the simultaneous injection system with the sepa‐

In the comparison test performed in 2012, the samples of the HOxII, C5, and C1 were prepared in the MUTSU, distributed to the TONO, and measured in both facilities. The measurement condition such as the duration time or the beam current was taken as the normal condition in each facility. Figure 7 shows results obtained in 2012. For the data analysis, the algorithm used in the TONO was employed. It can be seen for the C5, that there was no significant difference between data obtained in both facilities. The results of the C1 analyzed in the TONO are much lower than that for the MUTSU. This is mostly due to the fact that during 14C counting, 13C ions for the simultaneous injection also entered the accelerator continuously, thus the counting rate (or its possibility) of 13C coming into the ionization detector is much higher for the simultaneous injection than for the sequential injection, in spite of filtering by the combination of magnetic

**4.2. Inter-laboratory comparison testing**

12 Geochronology - Methods and Case Studies

rator-combiner.

#### **5.1. System configuration and method**

The configuration for our 10Be-AMS operation is fundamentally standard, and is based on that used in the MALT [18,37,38]. Samples are made from the solid oxide of beryllium, BeO, for its positive electron affinity to produce negative ions. Since the amount of the rare isotope 10Be is distributed according to the abundance of oxygen isotope ratios, i.e., 16O : 17O : 18O=99.76: 0.04: 0.20, respectively, the 10Be16O is selected for injection into the accelera‐ tor to ensure high extraction efficiency of 10Be from the sample. The terminal voltage, usually set at 4.7 or 4.8 MV, is made preferably as high as possible within the range of around 8 MV so as to increase the stripping efficiency from negative ions to 3+ (in our AMS system, the terminal voltage is limited to the specification of 5 MV). In addition, the higher ion energy is also preferred for ensuring the good performance of the discrimination between 10Be and 10B for the heavy ion detector as described below (details in Section 5.2). One of the most significant features of 10Be measurement is the simultaneous injection of 9 Be17O with mass the same as 10Be16O for counting the abundance of 9Be isotopes. For this purpose the current of 17O5+ is measured to avoid uncertainty in the amount of beryllium hydrox‐ ide 9 Be16OH contamination in the sample [37]. The abundance isotope is detected with a Faraday cup behind the analyzing magnet as shown in Figure 5.

The mathematical formula used for obtaining the measured isotope ratio (*R*m=10Be/9 Be) is:

$$R\_m = \frac{\mathbf{a}\_{10}}{\mathbf{a}\_{\theta}} \frac{T\_{1\mathcal{V}\_{O}^{5+}}}{T\_{10}} \frac{cps\_{10}}{cps\_{1\mathcal{V}\_{O}^{5+}}} \, ^{\prime} \tag{1}$$

**Figure 8.** Configuration of the rare isotope detector (heavy ion detector) for 10Be-AMS. The absorber gas-cell is attach‐ ed in front of ionization chamber. The anode electrode is separated into multiple (five) ΔE electrodes. ΔE<sup>1</sup> and ERes (that means *Residual energy*) indicate the energy loss arising in the region corresponding to the anode plate 1 and

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We investigated experimentally the discrimination function of the detector system through observation of the variation of the *ΔE*1-*E*Res spectrum by varying the gas pressure of the gas cell. Figure 9 shows the effect of gas pressure on the *ΔE*1-*E*Res spectrum. It can be seen that the horizontal width (*ΔE*1 component) is reduced significantly with increasing *P*C. It should be noted that the gas pressure of the ionization chamber *P*<sup>I</sup> is decreased with increasing *P*C, so as to balance between pulse heights for *ΔE*1 and *E*Res, because the peak position on the Bragg curve (that provides the distribution of energy-loss rate as a function of the distance along the beam axis) shifts toward the gas-cell, as *P*<sup>c</sup> is increased. There is a lower limit of 0.2 V in *E*Res for data points. This is because the both pulse heights corresponding to (*ΔE*1, *E*Res) coordinates of data points is acquired if the electrical voltage of the *E*Res-trace exceeds 0.2 V to discriminate between signal and noise. The middle and bottom frames in Figure 9 show the peak profiles in the *ΔE*<sup>1</sup> component for 10Be and 10B, respectively. These peak profiles are well fitted with the Gaussian

The width of the 10Be peak defined by *σ*10Be is approximately 1/6 smaller at *P*C=58 Torr than that at 32 Torr. As a measure of the discrimination function, the dimensionless normalized distance

> 2 2 22 10 , 10 , 10 , 10 , *Be H Be V B H B V*

D

ss

+ ++ (2)

 s

curve characterized by the standard deviation of *σ*10Be and *σ*10B.

s

between 10Be and 10B peaks given by

plates 2-4, respectively.

where *a* is the correction factor for the abundance ratio of the oxygen isotopes, *T* is transmittance for yielding positive ions from the negative ions, and *cps* is counts per second. The transmittance values for 17O5+ and 10Be3+ are substituted by the values for 16O5+ (=21%) and 10Be3+ (=6%), respectively. The absolute isotope ratio, *R*A, is obtained by correcting the measured ratio, as *R*A=*R*m/*Const*, where *Const* is the normalization constant that is given by the measured ratio of a standard divided by its nominal ratio. For the 10Be standard, usually so-called "ICN standard 01-5-1" (hereinafter simply S5-1) is used. The series of ICN standards has been prepared and distributed by Nishiizumi of U of C, Berkeley to worldwide AMS laboratories [39].

The boron ions 10B3+ has the same charge-to-mass ratios as the 10Be3+, thus it remains on the beamline regardless of the magnetic and electrostatic filters, and results in the entry into the ionization chamber, with respect to the isobar problem as mentioned in Section 3. Usually a gas or solid absorber technique has been used to discriminate between them. Therefore, optimization of the absorber is fundamental in the 10Be measurement.

#### **5.2. Optimization of the rare isotope detector**

For the development of the 10Be-AMS, discrimination between the 10Be and 10B isotopes was accomplished by optimization of gas pressures in the ionization chamber and the absorber gas cell (hereinafter simply the gas-cell) attached in front of gas ionization chamber. Figure 8 shows the configuration of the rare isotope detector for the 10Be-AMS in our system. Ionization chamber consists of the cathode electrode (plate), grid, and anodes that are multiple (five) *ΔE* electrodes arranged along the axis of the beamline. The grid so called *Frisch grid* is to remove the dependence of the pulse amplitude on position of ion pair generation by ionization due to an incident particles.

As mentioned in Section 3.2, the *ΔE* electrode provides the pulse signal equivalent to the energy loss arising in the area corresponding to the electrode. For incident ion energy over a million electron volts, the rate of energy loss depends largely on the atomic number. Thus, the spectrum peak on the plane given by, e.g., both the coordinates *ΔE*1 and *E*Res (where "Res" means *residual*, see Figure 8) takes a characteristic position on that plane. This individuality of the peak position helps to discriminate the 10B. However, the amount of 10B entering into the ionization chamber is enormous (over 106 times larger than that of the 10Be [40]), so that the 10Be signal is disturbed if there is no additional absorber for it. The gas-cell method is an absorbance technique for 10B based on the fact that the energy loss rate for the boron is expected to be larger than that for beryllium for the dependence of energy-loss rate on the atomic number. In our system, the gas-cell is prepared with 1-inch diameter pipe covered by the Havar and Mylar foils at both ends. The nitrogen gas pressure in the absorber significantly affects the shape of the 10Be spectrum, as described below.

10 17 5 10 3 9 10 3 17 5 <sup>a</sup> , <sup>a</sup> *Be O Be*

+ + + +

= (1)

*T cps <sup>R</sup>*

*Be Be O*

where *a* is the correction factor for the abundance ratio of the oxygen isotopes, *T* is transmittance for yielding positive ions from the negative ions, and *cps* is counts per second. The transmittance values for 17O5+ and 10Be3+ are substituted by the values for 16O5+ (=21%) and 10Be3+ (=6%), respectively. The absolute isotope ratio, *R*A, is obtained by correcting the measured ratio, as *R*A=*R*m/*Const*, where *Const* is the normalization constant that is given by the measured ratio of a standard divided by its nominal ratio. For the 10Be standard, usually so-called "ICN standard 01-5-1" (hereinafter simply S5-1) is used. The series of ICN standards has been prepared and distributed by Nishiizumi of U of C, Berkeley to

The boron ions 10B3+ has the same charge-to-mass ratios as the 10Be3+, thus it remains on the beamline regardless of the magnetic and electrostatic filters, and results in the entry into the ionization chamber, with respect to the isobar problem as mentioned in Section 3. Usually a gas or solid absorber technique has been used to discriminate between them. Therefore,

For the development of the 10Be-AMS, discrimination between the 10Be and 10B isotopes was accomplished by optimization of gas pressures in the ionization chamber and the absorber gas cell (hereinafter simply the gas-cell) attached in front of gas ionization chamber. Figure 8 shows the configuration of the rare isotope detector for the 10Be-AMS in our system. Ionization chamber consists of the cathode electrode (plate), grid, and anodes that are multiple (five) *ΔE* electrodes arranged along the axis of the beamline. The grid so called *Frisch grid* is to remove the dependence of the pulse amplitude on position of ion pair generation by ionization due to

As mentioned in Section 3.2, the *ΔE* electrode provides the pulse signal equivalent to the energy loss arising in the area corresponding to the electrode. For incident ion energy over a million electron volts, the rate of energy loss depends largely on the atomic number. Thus, the spectrum peak on the plane given by, e.g., both the coordinates *ΔE*1 and *E*Res (where "Res" means *residual*, see Figure 8) takes a characteristic position on that plane. This individuality of the peak position helps to discriminate the 10B. However, the amount of 10B entering into the

10Be signal is disturbed if there is no additional absorber for it. The gas-cell method is an absorbance technique for 10B based on the fact that the energy loss rate for the boron is expected to be larger than that for beryllium for the dependence of energy-loss rate on the atomic number. In our system, the gas-cell is prepared with 1-inch diameter pipe covered by the Havar and Mylar foils at both ends. The nitrogen gas pressure in the absorber significantly affects the

times larger than that of the 10Be [40]), so that the

*T cps*

*m*

optimization of the absorber is fundamental in the 10Be measurement.

worldwide AMS laboratories [39].

14 Geochronology - Methods and Case Studies

an incident particles.

**5.2. Optimization of the rare isotope detector**

ionization chamber is enormous (over 106

shape of the 10Be spectrum, as described below.

**Figure 8.** Configuration of the rare isotope detector (heavy ion detector) for 10Be-AMS. The absorber gas-cell is attach‐ ed in front of ionization chamber. The anode electrode is separated into multiple (five) ΔE electrodes. ΔE<sup>1</sup> and ERes (that means *Residual energy*) indicate the energy loss arising in the region corresponding to the anode plate 1 and plates 2-4, respectively.

We investigated experimentally the discrimination function of the detector system through observation of the variation of the *ΔE*1-*E*Res spectrum by varying the gas pressure of the gas cell. Figure 9 shows the effect of gas pressure on the *ΔE*1-*E*Res spectrum. It can be seen that the horizontal width (*ΔE*1 component) is reduced significantly with increasing *P*C. It should be noted that the gas pressure of the ionization chamber *P*<sup>I</sup> is decreased with increasing *P*C, so as to balance between pulse heights for *ΔE*1 and *E*Res, because the peak position on the Bragg curve (that provides the distribution of energy-loss rate as a function of the distance along the beam axis) shifts toward the gas-cell, as *P*<sup>c</sup> is increased. There is a lower limit of 0.2 V in *E*Res for data points. This is because the both pulse heights corresponding to (*ΔE*1, *E*Res) coordinates of data points is acquired if the electrical voltage of the *E*Res-trace exceeds 0.2 V to discriminate between signal and noise. The middle and bottom frames in Figure 9 show the peak profiles in the *ΔE*<sup>1</sup> component for 10Be and 10B, respectively. These peak profiles are well fitted with the Gaussian curve characterized by the standard deviation of *σ*10Be and *σ*10B.

The width of the 10Be peak defined by *σ*10Be is approximately 1/6 smaller at *P*C=58 Torr than that at 32 Torr. As a measure of the discrimination function, the dimensionless normalized distance between 10Be and 10B peaks given by

$$\frac{\Lambda}{\sqrt{\sigma\_{10\text{Re},H}^2 + \sigma\_{10\text{Re},V}^2 + \sigma\_{10\text{B},H}^2 + \sigma\_{10\text{B},V}^2}}\tag{2}$$

where, *Δ* is the distance between 10Be and 10B peaks, and subscripts H and V mean horizontal and vertical, respectively. Figure 10 shows the experimentally obtained normalized distance with equation (2) as a function of *P*C. In the calculation of Equation (2) the value of *σ*10Be,H is substituted in *σ*10B,H for the following two reasons. (i) The precision of *σ*10B,H is inadequate for higher *P*C, because the peak of boron becomes so small that the main portion of the peak hides under the lower limit of *E*Res (0.2 V) as shown in Figure 9. (ii) It has been experimentally observed that the relation *σ*10Be,H ≈ *σ*10B,H holds independent of *P*C within the range where *σ*10Be,H and *σ*10B,H vary significantly in the same way with *P*C. It is found that the values of normalized distance peak at *P*C=57 Torr. This is due to the fact that as *P*C increases over 55 Torr, the reduction of peak widths becomes saturated, while the distance *Δ* going into decline because of the enhancement in the energy loss in the gas-cell. It can be said that the optimum *P*<sup>C</sup> is around 57 Torr providing the best discrimination. The nature of the *P*C-dependence on the peak-width is attributed to the baseline fluctuation of the pulse trace detected in the ionization chamber, which is briefly discussed in Section 7.

**Figure 10.** Variation of the normalized distance between 10Be and 10B spectrum peaks as a function of the *P*C. The

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We completed the development of the 10Be measurement technique last fiscal year (2012), confirming high stability and reliability of the 10Be/Be ratios in numerous test measurements. Even after we started to perform requested 10Be measurements in 2013, data quality has been continuously checked for every routine measurement using standards. Three typical standards are the ICN standards mention in the Section 5.1, S5-1, S5-2, S6-2, and a blank sample. The blank sample (hereafter BLK) is made from a quantified standard for atomic absorption spectrometry (No.020-07481) produced by Wako Pure Chemical Industries for which a 10Be/Be ratio of ~2×10-14 is expected [41]. As shown in Section 5.1, S5-1 is used to obtain the normali‐ zation constant given by the measured ratio of S5-1 divided by its nominal ratio. With respect to the data quality of the S5-1 standard, the relative precision, in terms of the relative standard deviation *rsd* is important, since only the *rsd* affects the normalization constant, where the role of the S5-1 standard is similar to that of HOxII standard in the 14C measurement, as described

range of *P*C indicated by the shaded region is an optimized range, and is usually employed.

in Section 4.1. The value of *rsd* is around 0.50% for each routine measurement.

Figure 11 shows the quality of measurements from October 2011 to December 2013. The left column shows the evolution of the 10Be/Be ratios using the S5-2, S6-2, and BLK standards. The 10Be measurements have been conducted at intervals ranging from around a few months to half a year. All 10Be/Be ratios for both S5-2 and S6-2 agree with the nominal values within 3*σ* of each point, where the *σ* is the combined uncertainty of the counting and the nominal ratio of the S5-1 standard. On the other hand, although the ratio of BLKs lies around its expected range, it seems to vary systematically up and down. This variation can be related to at least two factors: one is beam slit condition, and the other is the problem of the sample preparation. As shown in Figure 11, the time when the BLK ratio is low, in December 2012, coincides with the time when we began to narrow the beam slit behind the ECA to improve the ECA resolu‐

**5.3. Test measurements, Long-term reliability**

**Figure 9.** The variation of ΔE1-ERes spectrum for two gas pressures of the gas-cell, *P*<sup>C</sup> (upper frames). Corresponding peak profiles of 10Be and 10B are also depicted for the ΔE1 component in the middle and bottom frames, respectively. The peak is fitted by a Gaussian distribution characterized with a standard deviation of σ. The gas pressure for the ionization chamber for left and right cases are 114 Torr and 56 Torr, respectively.

**Figure 10.** Variation of the normalized distance between 10Be and 10B spectrum peaks as a function of the *P*C. The range of *P*C indicated by the shaded region is an optimized range, and is usually employed.

#### **5.3. Test measurements, Long-term reliability**

where, *Δ* is the distance between 10Be and 10B peaks, and subscripts H and V mean horizontal and vertical, respectively. Figure 10 shows the experimentally obtained normalized distance with equation (2) as a function of *P*C. In the calculation of Equation (2) the value of *σ*10Be,H is substituted in *σ*10B,H for the following two reasons. (i) The precision of *σ*10B,H is inadequate for higher *P*C, because the peak of boron becomes so small that the main portion of the peak hides under the lower limit of *E*Res (0.2 V) as shown in Figure 9. (ii) It has been experimentally observed that the relation *σ*10Be,H ≈ *σ*10B,H holds independent of *P*C within the range where *σ*10Be,H and *σ*10B,H vary significantly in the same way with *P*C. It is found that the values of normalized distance peak at *P*C=57 Torr. This is due to the fact that as *P*C increases over 55 Torr, the reduction of peak widths becomes saturated, while the distance *Δ* going into decline because of the enhancement in the energy loss in the gas-cell. It can be said that the optimum *P*<sup>C</sup> is around 57 Torr providing the best discrimination. The nature of the *P*C-dependence on the peak-width is attributed to the baseline fluctuation of the pulse trace detected in the ionization chamber,

**Figure 9.** The variation of ΔE1-ERes spectrum for two gas pressures of the gas-cell, *P*<sup>C</sup> (upper frames). Corresponding peak profiles of 10Be and 10B are also depicted for the ΔE1 component in the middle and bottom frames, respectively. The peak is fitted by a Gaussian distribution characterized with a standard deviation of σ. The gas pressure for the

ionization chamber for left and right cases are 114 Torr and 56 Torr, respectively.

which is briefly discussed in Section 7.

16 Geochronology - Methods and Case Studies

We completed the development of the 10Be measurement technique last fiscal year (2012), confirming high stability and reliability of the 10Be/Be ratios in numerous test measurements. Even after we started to perform requested 10Be measurements in 2013, data quality has been continuously checked for every routine measurement using standards. Three typical standards are the ICN standards mention in the Section 5.1, S5-1, S5-2, S6-2, and a blank sample. The blank sample (hereafter BLK) is made from a quantified standard for atomic absorption spectrometry (No.020-07481) produced by Wako Pure Chemical Industries for which a 10Be/Be ratio of ~2×10-14 is expected [41]. As shown in Section 5.1, S5-1 is used to obtain the normali‐ zation constant given by the measured ratio of S5-1 divided by its nominal ratio. With respect to the data quality of the S5-1 standard, the relative precision, in terms of the relative standard deviation *rsd* is important, since only the *rsd* affects the normalization constant, where the role of the S5-1 standard is similar to that of HOxII standard in the 14C measurement, as described in Section 4.1. The value of *rsd* is around 0.50% for each routine measurement.

Figure 11 shows the quality of measurements from October 2011 to December 2013. The left column shows the evolution of the 10Be/Be ratios using the S5-2, S6-2, and BLK standards. The 10Be measurements have been conducted at intervals ranging from around a few months to half a year. All 10Be/Be ratios for both S5-2 and S6-2 agree with the nominal values within 3*σ* of each point, where the *σ* is the combined uncertainty of the counting and the nominal ratio of the S5-1 standard. On the other hand, although the ratio of BLKs lies around its expected range, it seems to vary systematically up and down. This variation can be related to at least two factors: one is beam slit condition, and the other is the problem of the sample preparation. As shown in Figure 11, the time when the BLK ratio is low, in December 2012, coincides with the time when we began to narrow the beam slit behind the ECA to improve the ECA resolu‐ tion. The rise in the ratio after December 2012 implies that there could still remain a somewhat unidentified route for contamination in the sample preparation. This interpretation is sup‐ ported by the observation that the ratio for the commercial, high-purity chemical reagent BeO powder produced by the Mitsuwa Chemical Co., Ltd. stabilized below 10-14 after Dec. 2012.

for the 10Be measurements that had already been performed in the MALT accelerator in 2010. In this test, therefore, the samples measured at both facilities are produced by the same process

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Figure 12 and Table 2 show the results of the comparison test. Almost all of the measured 10Be/ Be ratios are consistent with the values obtained by the MALT AMS system. There is a significant difference between samples B and H (indicated by the arrow) taking into consid‐ eration their uncertainties, which could be due to unknown systematic errors. For the results of the 10Be/Be ratio measured in the MALT AMS system, two data sets are depicted for different data processing methods: one method is used at Hirosaki Univ. Japan, and the other is our method. The algorithms used for drawing the 10Be/Be ratios for both methods are almost the

, but the uncertainty has different interpretations, depending on which method is chosen. The uncertainty in the Hirosaki Univ., unc1, represents the standard deviation obtained by repeated measurements, whereas in our method, the uncertainty, unc2, is drawn basically by combining both the counting uncertainty of 10Be and the uncertainty in the nominal value of S5-1 (1.1% for 1σ [39]). The uncertainty in the nominal value affects that in the normalized constant, but is generally not always considered. This is because a lot number of standards does not change until the sample bottle is renewed, and thus the true value of the normalization constant rarely shifts. However, even when one assesses the data quality over a long time frame or performs an inter-laboratory comparison test, one should consider the difference in the lot number in order to accurately compare the isotope ratio values. Despite this fact, as shown in Table 2, there seems to be no systematic difference between unc1 and unc2, as expected. It is

TONO MALT\*

A 3.278 4.4 3.273 6.4 3.253 6.1 B 6.252 8.0 6.637 14.4 6.617 9.9 C 4.067 5.8 4.095 7.1 4.077 7.4 D 4.406 5.6 4.379 9.3 4.362 7.2 E 2.869 4.1 2.970 9.2 2.951 5.6 F 3.846 5.0 3.967 4.8 3.945 6.1 G 4.501 6.0 4.585 10.0 4.561 7.0 H 4.414 5.6 4.643 5.6 4.620 7.1 I 4.148 5.5 4.266 4.7 4.244 6.8 J 3.618 4.8 3.688 5.7 3.666 6.2

1 In the calculation in the normalized constant, S5-1 and/or S5-2 were used atHirosaki Univ., but only S5-1 was used in

Ratio unc Ratio1 unc1 Ratio2 unc2 x10-12 x10-14 x10-12 x10-14 x10-12 x10-14

noted that the counting uncertainty itself is around 70% of unc1.

**Table 2.** Results of the comparison test corresponding to the points in Figure 12.

Sample

\*Ratio1:HIROSAKI Univ., Ratio2: TONO.

the TONO.

for the comparison.

same1

**Figure 11.** Measurement quality from October 2011 to December 2013 using 10Be-AMS standards: S5-2 (a), S6-2 (b), and blank BLK (c). The left column shows evolution of the measurement results. The gray line and hatching in both the left and right columns depicts respectively the nominal values and their uncertainty. The uncertainty range is based on a nominal uncertainty of 1.1% (1σ) [39]. The right column depicts the corresponding histograms based on the left col‐ umn measurement points. The dashed and dotted lines represent respectively the mean values (*m*) and their uncer‐ tainty (1σH). The symbol c.n. stands for the number of sampling cathodes used in building the histograms, and σHrsd is the relative standard deviation of the σH.

The frames on the right-hand side of Figure 11 are histograms corresponding to the left-hand frames. It can be seen that the value of *m* (the arithmetic mean of the histogram) for the S5-2 and S6-2 standards are fairly consistent with corresponding nominal values; each difference between measured and nominal value is within the standard deviation of *m,* given by 1*σ*H/ (c.n.)0.5, i.e., 1.8 × 10-14 for S5-2, and 0.34 × 10-14 for S6-2.

#### **5.4. Comparison test**

We performed a comparison test with the AMS system in the MALT using beryllium samples made from an ice core. The samples measured in our system were originally prepared as spares for the 10Be measurements that had already been performed in the MALT accelerator in 2010. In this test, therefore, the samples measured at both facilities are produced by the same process for the comparison.

Figure 12 and Table 2 show the results of the comparison test. Almost all of the measured 10Be/ Be ratios are consistent with the values obtained by the MALT AMS system. There is a significant difference between samples B and H (indicated by the arrow) taking into consid‐ eration their uncertainties, which could be due to unknown systematic errors. For the results of the 10Be/Be ratio measured in the MALT AMS system, two data sets are depicted for different data processing methods: one method is used at Hirosaki Univ. Japan, and the other is our method. The algorithms used for drawing the 10Be/Be ratios for both methods are almost the same1 , but the uncertainty has different interpretations, depending on which method is chosen. The uncertainty in the Hirosaki Univ., unc1, represents the standard deviation obtained by repeated measurements, whereas in our method, the uncertainty, unc2, is drawn basically by combining both the counting uncertainty of 10Be and the uncertainty in the nominal value of S5-1 (1.1% for 1σ [39]). The uncertainty in the nominal value affects that in the normalized constant, but is generally not always considered. This is because a lot number of standards does not change until the sample bottle is renewed, and thus the true value of the normalization constant rarely shifts. However, even when one assesses the data quality over a long time frame or performs an inter-laboratory comparison test, one should consider the difference in the lot number in order to accurately compare the isotope ratio values. Despite this fact, as shown in Table 2, there seems to be no systematic difference between unc1 and unc2, as expected. It is noted that the counting uncertainty itself is around 70% of unc1.


\*Ratio1:HIROSAKI Univ., Ratio2: TONO.

tion. The rise in the ratio after December 2012 implies that there could still remain a somewhat unidentified route for contamination in the sample preparation. This interpretation is sup‐ ported by the observation that the ratio for the commercial, high-purity chemical reagent BeO powder produced by the Mitsuwa Chemical Co., Ltd. stabilized below 10-14 after Dec. 2012.

**Figure 11.** Measurement quality from October 2011 to December 2013 using 10Be-AMS standards: S5-2 (a), S6-2 (b), and blank BLK (c). The left column shows evolution of the measurement results. The gray line and hatching in both the left and right columns depicts respectively the nominal values and their uncertainty. The uncertainty range is based on a nominal uncertainty of 1.1% (1σ) [39]. The right column depicts the corresponding histograms based on the left col‐ umn measurement points. The dashed and dotted lines represent respectively the mean values (*m*) and their uncer‐ tainty (1σH). The symbol c.n. stands for the number of sampling cathodes used in building the histograms, and σHrsd is

The frames on the right-hand side of Figure 11 are histograms corresponding to the left-hand frames. It can be seen that the value of *m* (the arithmetic mean of the histogram) for the S5-2 and S6-2 standards are fairly consistent with corresponding nominal values; each difference between measured and nominal value is within the standard deviation of *m,* given by 1*σ*H/

We performed a comparison test with the AMS system in the MALT using beryllium samples made from an ice core. The samples measured in our system were originally prepared as spares

the relative standard deviation of the σH.

18 Geochronology - Methods and Case Studies

**5.4. Comparison test**

(c.n.)0.5, i.e., 1.8 × 10-14 for S5-2, and 0.34 × 10-14 for S6-2.

**Table 2.** Results of the comparison test corresponding to the points in Figure 12.

<sup>1</sup> In the calculation in the normalized constant, S5-1 and/or S5-2 were used atHirosaki Univ., but only S5-1 was used in the TONO.

**Figure 12.** Comparison test using unknown samples.

On the other hand, the average uncertainty for the TONO (unc) is approximately 78% of that for the MALT (unc2). This is due to the fact that the total counts of the 10Be signal for the TONO were three times larger than that for the MALT because of longer measurement time at the TONO. It should be mentioned that the average count rate is 70 cps at the TONO but is 250 cps at the MALT AMS system, a reflection of the different specifications of their respective ion sources. Actually, the ion current for the MALT AMS system can provide a few times larger current than for the TONO.

We have performed test measurements using 26Al standards, in order to investigate measure‐ ment stability. For the test measurements, the series of standards prepared and distributed by Nishiizumi have been used [42], as well as the 10Be measurement mentioned in Section 5.3. Typical standards, 01-4-1 and 01-5-1 (S4-1 and S5-1, respectively), especially the former have been employed to compute the normalization constant that is given by the measured ratio of a standard S4-1 divided by its nominal ratio. The blank sample (BLK) was made from a quantified standard for atomic absorption spectrometry (No.016-15471) supplied by Wako

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**Figure 13.** Example of the observed 26Al3+ peak in the *E*Tot-ΔE1 spectrum.

The left column on Figure 14 shows the results of the two test measurements carried out between routine 14C- and 10Be-AMS operations. Concerning the isotope ratio for the S4-1 standard, only the precision is meaningful (in other words, the accuracy has little meaning), since the arithmetic mean of the points obtained in the same batch (or on same date) is already normalized to the nominal value. A long-term precision can be drawn from the statistical dispersion of the data points displayed on the same figure, which will be mentioned in the below description related to the histogram. All data points for the S5-1 standard are consistent with the nominal value within 3*σ*, where *σ* (indicated as error bars) is combined uncertainty from the statistical uncertainty and the uncertainty normalized constant (originated from the uncertainty of the nominal ratio of the S4-1 standard). On the other hand, the result of the BLK implies a decrease in the isotope ratio. This can be due to improvement of the energy resolution of the ECA accomplished by narrowing the beam slit located behind the ECA. It is believed that the contamination to the beamline originates in the ion source or in the high energy beamline where unwanted ions can exist because of the dissociation of molecular ions [5]. Although the expected isotope ratio for the BLK is not yet known, the observed range might

be acceptable considering the range of the BLK ratio for 10Be [see Section 5.3].

Pure Chemical Industries.

## **6. 26Al measurement**

#### **6.1. Development and test measurements**

Tuning up of the system and the test measurements for the routine 26Al-AMS operation started in March of 2013, after the development of routine 10Be measurements was finished. We plan to complete the development of the routine 26Al-AMS operation in the middle of fiscal year 2014, confirming the long term stability and reliability of the operation through statistical analysis of accumulated data.

For the 26Al measurements, Al2O3 powder is chosen as the sample material (mixed with silver powder), and fundamentally no isobar problems occur because 26Mg does not form negative ions. Therefore, the development of the 26Al-AMS procedure is more straightforward than the 10Be-AMS procedure. The configuration for 26Al-AMS is listed in Table 1.

Figure 13 shows the observed 26Al3+ peak in the *E*Tot-*ΔE*<sup>1</sup> spectrum. As described above, there is no other peak. The data points outside the counting gate (the green square) can be attributed to some incident ions reaching the electrode without complete volumetric energy-loss due to large-angle scattering at the window of the ionization chamber.

**Figure 13.** Example of the observed 26Al3+ peak in the *E*Tot-ΔE1 spectrum.

**Figure 12.** Comparison test using unknown samples.

20 Geochronology - Methods and Case Studies

**6.1. Development and test measurements**

current than for the TONO.

**6. 26Al measurement**

analysis of accumulated data.

On the other hand, the average uncertainty for the TONO (unc) is approximately 78% of that for the MALT (unc2). This is due to the fact that the total counts of the 10Be signal for the TONO were three times larger than that for the MALT because of longer measurement time at the TONO. It should be mentioned that the average count rate is 70 cps at the TONO but is 250 cps at the MALT AMS system, a reflection of the different specifications of their respective ion sources. Actually, the ion current for the MALT AMS system can provide a few times larger

Tuning up of the system and the test measurements for the routine 26Al-AMS operation started in March of 2013, after the development of routine 10Be measurements was finished. We plan to complete the development of the routine 26Al-AMS operation in the middle of fiscal year 2014, confirming the long term stability and reliability of the operation through statistical

For the 26Al measurements, Al2O3 powder is chosen as the sample material (mixed with silver powder), and fundamentally no isobar problems occur because 26Mg does not form negative ions. Therefore, the development of the 26Al-AMS procedure is more straightforward than

Figure 13 shows the observed 26Al3+ peak in the *E*Tot-*ΔE*<sup>1</sup> spectrum. As described above, there is no other peak. The data points outside the counting gate (the green square) can be attributed to some incident ions reaching the electrode without complete volumetric energy-loss due to

the 10Be-AMS procedure. The configuration for 26Al-AMS is listed in Table 1.

large-angle scattering at the window of the ionization chamber.

We have performed test measurements using 26Al standards, in order to investigate measure‐ ment stability. For the test measurements, the series of standards prepared and distributed by Nishiizumi have been used [42], as well as the 10Be measurement mentioned in Section 5.3. Typical standards, 01-4-1 and 01-5-1 (S4-1 and S5-1, respectively), especially the former have been employed to compute the normalization constant that is given by the measured ratio of a standard S4-1 divided by its nominal ratio. The blank sample (BLK) was made from a quantified standard for atomic absorption spectrometry (No.016-15471) supplied by Wako Pure Chemical Industries.

The left column on Figure 14 shows the results of the two test measurements carried out between routine 14C- and 10Be-AMS operations. Concerning the isotope ratio for the S4-1 standard, only the precision is meaningful (in other words, the accuracy has little meaning), since the arithmetic mean of the points obtained in the same batch (or on same date) is already normalized to the nominal value. A long-term precision can be drawn from the statistical dispersion of the data points displayed on the same figure, which will be mentioned in the below description related to the histogram. All data points for the S5-1 standard are consistent with the nominal value within 3*σ*, where *σ* (indicated as error bars) is combined uncertainty from the statistical uncertainty and the uncertainty normalized constant (originated from the uncertainty of the nominal ratio of the S4-1 standard). On the other hand, the result of the BLK implies a decrease in the isotope ratio. This can be due to improvement of the energy resolution of the ECA accomplished by narrowing the beam slit located behind the ECA. It is believed that the contamination to the beamline originates in the ion source or in the high energy beamline where unwanted ions can exist because of the dissociation of molecular ions [5]. Although the expected isotope ratio for the BLK is not yet known, the observed range might be acceptable considering the range of the BLK ratio for 10Be [see Section 5.3].

**7. Research and development: Baseline fluctuation of the 10Be pulse trace**

Quaternary Geochronology Using Accelerator Mass Spectrometry (AMS) – Current Status of the AMS System…

As described in Section 5.2, we saw that discrimination between 10Be and 10B in the 10Be-AMS is strongly dependent on the gas pressure of the gas-cell (*P*C) located in front of gas ionization chamber. The *ΔE*<sup>1</sup> component of the 10Be peak on the *ΔE*1-*E*Res spectrum shrinks significantly with decreased *P*<sup>C</sup> value. This shrinking, it should be emphasized, accompanies the reduction of the *ΔE*<sup>1</sup> component of the 10B (see Figure 9), implying the shape of the 10Be peak is closely

The peak width is a reflection of the statistical dispersion of the pulse height of signal traces detected from the ionization chamber. Figure 15 shows pulse traces observed by the *ΔE*1 electrode for the two *P*C values corresponding to Figure 9. One can see that the fluctuation of the baseline of the 10Be signal is greater for the lower *P*C. It is noted that there was no remarkable fluctuation in the pulse trace observed by the *E*Res-electrode; this indicates the dependence of the fluctuation on the distance from the inlet of ionization chamber. In addition, the fluctuation contains frequency components of around a few tenths of a kilohertz. It was observed that the

Figure 16 shows both the variation of the standard deviation of the fluctuation (σ in Figure

**Figure 15.** Pulse traces detected by the ΔE1-electrode in the ionization chamber for two values of *P*C corresponding to those shown in Figure 9. The trigger for acquiring the pulse trace is taken from the pulse detected simultaneously by

In general, if the incident frequency can no longer be ignored comparing the reciprocal of the time scale for the pulse width, a high counting rate induces pulse pile-up, and deteriorates the time resolution of the ionization chamber. Our investigation, by measuring the 10B current shows that the average frequency of the 10B incident is on the order of a megahertz, which is comparable to the reciprocal of the pulse width. In fact, the amount of 10B entering toward the

for the baseline fluctuation, however, is independent of the signal pile-up, but can be due to

the Eres-electrode at *t=*0.5 ms. The dashed line shows the standard deviation of the fluctuation.

10B (evaluated using SRIM [43]) as a function

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times larger than that of 10Be [40]. The mechanism

10B decrease in a similar way with increasing *P*C, indicating that

related to the occurrence of incident 10B, an interfering particle, is briefly discussed.

frequency of the major component seems to decrease as *P*C increases.

the fluctuation can be closely related to the incident energy of the 10B.

15) and the energy loss of 10B in the area of *ΔE*1, *ΔE*<sup>1</sup>

ionization chamber is expected to be over 106

of *P*C. The values of σ and *ΔE*<sup>1</sup>

**Figure 14.** Data quality in test measurements implemented from March 2013 for 26Al standards: S4-1 (a), S5-1 (b), and BLK (c). The left column shows time series plots. The gray line and hatching in (b) for left and right frames indicate the nominal value and the range of its uncertainty, respectively. This range is based on the uncertainty of the absolute value of the nominal value (0.37% for 1σ [42]). The right side shows corresponding histograms representing the left measurements. The dashed and dotted lines stand for arithmetical mean (*m*) and uncertainty (σH), respectively, for the histogram. The symbol c.n. denotes the number of cathodes used for building the histogram, and σHrsd is the relative standard deviation of σH.

The histograms in the right-side frames in Figure 14 present the data distributions shown as data points in the left-side frames. The long-term precision of the S4-1 standard can be indicated by the statistical dispersion of their points in Figure 14a labelled as *σ*H. The relative uncertainty of *σ*H, or *σ*Hrsd (=0.74%) is less than 0.9%. This magnitude relation, or *σ*Hrsd < 0.9%, provides a necessary condition that the precision of the S5-1 standard would be less than 4% where the value itself is comparable to the precision in the 26Al measurements in the MALT AMS system [18]. Here we assume that the 10Be counts are simply proportional to the nominal ratio, and the S5-1 standard is regarded as representative of 26Al standards except the S4-1 standard. Actually, the nominal 26Al/Al ratio of the S5-1 standard is nearest to a log-average (or a geometric mean) for all the 26Al-AMS standards. As shown in the histogram of the S5-1 standard, the precision is, indeed, less than 4%. Furthermore, the difference between *m* and the nominal value is less than the standard deviation of the mean given by *σ*H/(c.n.)0.5, indicating there is consistency between them. For all the results shown in this figure, data points are inadequate for a statistical discussion; thus we will need to acquire additional data for evaluation of the measurement stability, in order to develop 26Al routine measurements.

## **7. Research and development: Baseline fluctuation of the 10Be pulse trace**

As described in Section 5.2, we saw that discrimination between 10Be and 10B in the 10Be-AMS is strongly dependent on the gas pressure of the gas-cell (*P*C) located in front of gas ionization chamber. The *ΔE*<sup>1</sup> component of the 10Be peak on the *ΔE*1-*E*Res spectrum shrinks significantly with decreased *P*<sup>C</sup> value. This shrinking, it should be emphasized, accompanies the reduction of the *ΔE*<sup>1</sup> component of the 10B (see Figure 9), implying the shape of the 10Be peak is closely related to the occurrence of incident 10B, an interfering particle, is briefly discussed.

The peak width is a reflection of the statistical dispersion of the pulse height of signal traces detected from the ionization chamber. Figure 15 shows pulse traces observed by the *ΔE*1 electrode for the two *P*C values corresponding to Figure 9. One can see that the fluctuation of the baseline of the 10Be signal is greater for the lower *P*C. It is noted that there was no remarkable fluctuation in the pulse trace observed by the *E*Res-electrode; this indicates the dependence of the fluctuation on the distance from the inlet of ionization chamber. In addition, the fluctuation contains frequency components of around a few tenths of a kilohertz. It was observed that the frequency of the major component seems to decrease as *P*C increases.

Figure 16 shows both the variation of the standard deviation of the fluctuation (σ in Figure 15) and the energy loss of 10B in the area of *ΔE*1, *ΔE*<sup>1</sup> 10B (evaluated using SRIM [43]) as a function of *P*C. The values of σ and *ΔE*<sup>1</sup> 10B decrease in a similar way with increasing *P*C, indicating that the fluctuation can be closely related to the incident energy of the 10B.

**Figure 14.** Data quality in test measurements implemented from March 2013 for 26Al standards: S4-1 (a), S5-1 (b), and BLK (c). The left column shows time series plots. The gray line and hatching in (b) for left and right frames indicate the nominal value and the range of its uncertainty, respectively. This range is based on the uncertainty of the absolute value of the nominal value (0.37% for 1σ [42]). The right side shows corresponding histograms representing the left measurements. The dashed and dotted lines stand for arithmetical mean (*m*) and uncertainty (σH), respectively, for the histogram. The symbol c.n. denotes the number of cathodes used for building the histogram, and σHrsd is the relative

The histograms in the right-side frames in Figure 14 present the data distributions shown as data points in the left-side frames. The long-term precision of the S4-1 standard can be indicated by the statistical dispersion of their points in Figure 14a labelled as *σ*H. The relative uncertainty of *σ*H, or *σ*Hrsd (=0.74%) is less than 0.9%. This magnitude relation, or *σ*Hrsd < 0.9%, provides a necessary condition that the precision of the S5-1 standard would be less than 4% where the value itself is comparable to the precision in the 26Al measurements in the MALT AMS system [18]. Here we assume that the 10Be counts are simply proportional to the nominal ratio, and the S5-1 standard is regarded as representative of 26Al standards except the S4-1 standard. Actually, the nominal 26Al/Al ratio of the S5-1 standard is nearest to a log-average (or a geometric mean) for all the 26Al-AMS standards. As shown in the histogram of the S5-1 standard, the precision is, indeed, less than 4%. Furthermore, the difference between *m* and the nominal value is less than the standard deviation of the mean given by *σ*H/(c.n.)0.5, indicating there is consistency between them. For all the results shown in this figure, data points are inadequate for a statistical discussion; thus we will need to acquire additional data for evaluation of the measurement stability, in order to develop 26Al routine measurements.

standard deviation of σH.

22 Geochronology - Methods and Case Studies

**Figure 15.** Pulse traces detected by the ΔE1-electrode in the ionization chamber for two values of *P*C corresponding to those shown in Figure 9. The trigger for acquiring the pulse trace is taken from the pulse detected simultaneously by the Eres-electrode at *t=*0.5 ms. The dashed line shows the standard deviation of the fluctuation.

In general, if the incident frequency can no longer be ignored comparing the reciprocal of the time scale for the pulse width, a high counting rate induces pulse pile-up, and deteriorates the time resolution of the ionization chamber. Our investigation, by measuring the 10B current shows that the average frequency of the 10B incident is on the order of a megahertz, which is comparable to the reciprocal of the pulse width. In fact, the amount of 10B entering toward the ionization chamber is expected to be over 106 times larger than that of 10Be [40]. The mechanism for the baseline fluctuation, however, is independent of the signal pile-up, but can be due to

multi-nuclide AMS in order to provide geochronological dating methods applicable to the

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Our versatile AMS system, based on the 5 MV PelletronTM tandem accelerator, is designed for AMS analysis of most radio-isotopes including 10Be, 14C, 26Al, 36Cl, and 129I. The AMS system is in good condition after fifteen years of operation, ensured by regularly scheduled maintenance. Total measurement time has been increasing for the last 15 years, and reached 15,000 hours this year. The average annual number of samples measured is 800, and the grand total number

In the 14C-AMS operation, the long-term reliability of routine measurements has been contin‐ uously verified by measuring standard samples such as C1, C5, and so on, produced by the IAEA, and HOxII produced by the NIST and by comparative testing with other AMS facilities. Almost all the relative standard deviations of the isotope ratios of HOxII in percent modern carbon are less than 0.25% for each measurement, and the average isotope ratio of C1 lies around 0.15 pMC. The comparison tests were carried out twice, in 2010 and in 2012, with the AMS facility at the JAEA-AMS-MUTSU. The results showed that there was no significant

With respect to the 10Be-AMS operation, we completed the development of 10Be measurement capability last year, confirming both high stability and reliability of the 10Be/Be ratios obtained from numerous test measurements. Then, routine measurements started since the beginning of fiscal year 2013. The detection limit of the isotope ratio can be less than 7 × 10-15, estimated by using samples made of commercial high-purity BeO powders. For the development of 10Be-AMS, discrimination of 10Be from 10B was accomplished by optimization of gas pressure in the gas cell located in front of gas ionization chamber. We also performed a comparison test with the AMS system at the MALT in the University of Tokyo using beryllium samples taken from an ice core. Measured 10Be/Be ratios were consistent with the values obtained by the MALT

We have now entered into the development of 26Al-AMS. This development and the test measurement have progressed and have shown satisfactory results. We have performed system tuning and test measurements using 26Al standard samples. Almost all measured ratios of 26Al/Al are consistent with nominal values, within the range of their uncertainty and routine

We have also conducted investigations for improving the heavy ion detection system based on the *ΔE-E*Res type-gas ionization chamber with multi-anodes. It has been observed that the high incident ion rate of the stable isobars into the gas ionization chamber disturbs the baseline of the pulse trace for the measured rare nuclide. This can be related to the fact that the remaining positive charge produced by isobars makes the anode signal fluctuate, which will be one of key factors that should be resolved for achieving the 36Cl-AMS with good discrimi‐ nation of 36S, a stable isobar. The development for the 36Cl-AMS analysis will be one focus for

entire Quaternary timescale.

of samples will exceed 12,500 within a few months.

difference in the data obtained from the facilities.

group, confirming the reliability of our measurements.

measurements of 26Al will start in the near future.

development work in the next few years.

**Figure 16.** PC-dependence on the standard deviation of the fluctuation σ. The energy loss of 10B in the ΔE<sup>1</sup> region, ΔE<sup>1</sup> 10B, deduced by using the SRIM is also shown. The shaded hutch depicts the *P*C-range normally used.

the effect of the charge accumulation in the ionization chamber, as qualitatively described below.

In ordinary cases, electrons (negative charges) and ions (positive charges) produced by the ionization caused by the incident ion colliding with atoms in the ionization chamber drift toward the anode and cathode, respectively, in the applied electric field, and finally lose their charge at the electrodes. In the present case, it should be noted that the time interval of the 10B incident is much shorter than the time scale for the ions-loss on the order of milliseconds for the ordinary condition as mentioned above. This can lead to the charge accumulation in the space; the positive charge reaches a certain level so as to provide a balance between production rate and loss rate. The substantial positive charge lowers the anode potential through the inefficiency of the Frisch-grid playing a role in shielding the charges [44,45]. If some instability exists inherently in the relationship between the enhancement of the charge and the ion loss system, the anode potential, therefore, can fluctuate around its equilibrium value. Volumetric ion-electron recombination would be a candidate system for causing an instability so as to enhance the fluctuation of positive charge. This kind of degradation of the performance of the ionization chamber caused by the residual positive charge is not just related to the 10Be measurement, but to more general measurements of rare isotopes accompanied by the isobar problem. Indeed, an effect of remaining charge was mentioned in a paper for improving the discrimination of 36S in 36Cl-AMS [46]. Therefore, it can be said that the investigation of the nature of pulse trace presented here has been conducted as a preparatory activity in the development of the 36Cl-AMS operation.

#### **8. Summary**

The AMS system operating at the Tono Geoscience Center (TGC) has not only continued to contribute reliable routine AMS measurements, but also made steady progress in developing multi-nuclide AMS in order to provide geochronological dating methods applicable to the entire Quaternary timescale.

Our versatile AMS system, based on the 5 MV PelletronTM tandem accelerator, is designed for AMS analysis of most radio-isotopes including 10Be, 14C, 26Al, 36Cl, and 129I. The AMS system is in good condition after fifteen years of operation, ensured by regularly scheduled maintenance. Total measurement time has been increasing for the last 15 years, and reached 15,000 hours this year. The average annual number of samples measured is 800, and the grand total number of samples will exceed 12,500 within a few months.

In the 14C-AMS operation, the long-term reliability of routine measurements has been contin‐ uously verified by measuring standard samples such as C1, C5, and so on, produced by the IAEA, and HOxII produced by the NIST and by comparative testing with other AMS facilities. Almost all the relative standard deviations of the isotope ratios of HOxII in percent modern carbon are less than 0.25% for each measurement, and the average isotope ratio of C1 lies around 0.15 pMC. The comparison tests were carried out twice, in 2010 and in 2012, with the AMS facility at the JAEA-AMS-MUTSU. The results showed that there was no significant difference in the data obtained from the facilities.

the effect of the charge accumulation in the ionization chamber, as qualitatively described

**Figure 16.** PC-dependence on the standard deviation of the fluctuation σ. The energy loss of 10B in the ΔE<sup>1</sup> region,

10B, deduced by using the SRIM is also shown. The shaded hutch depicts the *P*C-range normally used.

In ordinary cases, electrons (negative charges) and ions (positive charges) produced by the ionization caused by the incident ion colliding with atoms in the ionization chamber drift toward the anode and cathode, respectively, in the applied electric field, and finally lose their charge at the electrodes. In the present case, it should be noted that the time interval of the 10B incident is much shorter than the time scale for the ions-loss on the order of milliseconds for the ordinary condition as mentioned above. This can lead to the charge accumulation in the space; the positive charge reaches a certain level so as to provide a balance between production rate and loss rate. The substantial positive charge lowers the anode potential through the inefficiency of the Frisch-grid playing a role in shielding the charges [44,45]. If some instability exists inherently in the relationship between the enhancement of the charge and the ion loss system, the anode potential, therefore, can fluctuate around its equilibrium value. Volumetric ion-electron recombination would be a candidate system for causing an instability so as to enhance the fluctuation of positive charge. This kind of degradation of the performance of the ionization chamber caused by the residual positive charge is not just related to the 10Be measurement, but to more general measurements of rare isotopes accompanied by the isobar problem. Indeed, an effect of remaining charge was mentioned in a paper for improving the discrimination of 36S in 36Cl-AMS [46]. Therefore, it can be said that the investigation of the nature of pulse trace presented here has been conducted as a preparatory activity in the

The AMS system operating at the Tono Geoscience Center (TGC) has not only continued to contribute reliable routine AMS measurements, but also made steady progress in developing

below.

24 Geochronology - Methods and Case Studies

ΔE<sup>1</sup>

development of the 36Cl-AMS operation.

**8. Summary**

With respect to the 10Be-AMS operation, we completed the development of 10Be measurement capability last year, confirming both high stability and reliability of the 10Be/Be ratios obtained from numerous test measurements. Then, routine measurements started since the beginning of fiscal year 2013. The detection limit of the isotope ratio can be less than 7 × 10-15, estimated by using samples made of commercial high-purity BeO powders. For the development of 10Be-AMS, discrimination of 10Be from 10B was accomplished by optimization of gas pressure in the gas cell located in front of gas ionization chamber. We also performed a comparison test with the AMS system at the MALT in the University of Tokyo using beryllium samples taken from an ice core. Measured 10Be/Be ratios were consistent with the values obtained by the MALT group, confirming the reliability of our measurements.

We have now entered into the development of 26Al-AMS. This development and the test measurement have progressed and have shown satisfactory results. We have performed system tuning and test measurements using 26Al standard samples. Almost all measured ratios of 26Al/Al are consistent with nominal values, within the range of their uncertainty and routine measurements of 26Al will start in the near future.

We have also conducted investigations for improving the heavy ion detection system based on the *ΔE-E*Res type-gas ionization chamber with multi-anodes. It has been observed that the high incident ion rate of the stable isobars into the gas ionization chamber disturbs the baseline of the pulse trace for the measured rare nuclide. This can be related to the fact that the remaining positive charge produced by isobars makes the anode signal fluctuate, which will be one of key factors that should be resolved for achieving the 36Cl-AMS with good discrimi‐ nation of 36S, a stable isobar. The development for the 36Cl-AMS analysis will be one focus for development work in the next few years.

## **Acknowledgements**

We would like to express our gratitude to Prof. Matsuzaki of the University of Tokyo for his continuous academic and practical advice. We would like to offer our special thanks also to Dr. Horiuchi of Hirosaki University for providing unknown beryllium samples and measure‐ ment data. Special thanks to Mr. Hanaki of the facility administrator for his management support and constant encouragement. We also thank the staff members of the AMS laboratory for their help and support.

[7] IVY-OCHS S, KOBER F. Surface exposure dating with cosmogenic nuclides. Quater‐

Quaternary Geochronology Using Accelerator Mass Spectrometry (AMS) – Current Status of the AMS System…

http://dx.doi.org/10.5772/58549

27

[8] Asamori K, Niwa M, Hanamuro T, Yamada K, Kusano T, et al. Annual Report for Research on Geosphere Stability for Long-Term Isolation of Radioactive Waste in Fis‐

[9] Xu S, Ito S, Iwatsuki T, Abe M, Watanabe M. A new AMS facility at the JNC Tono Geoscience Center, Japan. Nuclear Instruments and Methods in Physics Research

[10] Itoh S, Abe M, Watanabe M, Nakai S, Touyama H, et al. Present status of the JNC Tono Geoscience Center AMS system. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 2004; 223-224: 100–

[11] Saito-Kokubu Y, Nishizawa A, Suzuki M, Ohwaki Y, Nishio T, et al. Current status of the AMS facility at the Tono Geoscience Center of the Japan Atomic Energy Agency. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions

[12] Matsubara A, Kokubu Y, Nishizawa A, Owaki Y, Nishio T, et al. Current status of JAEA-AMS-TONO, the Japan Atomic Energy Agency (2011). 4th East Asian Sympo‐

[13] Matsubara A, Saito-Kokubu Y, Nishizawa A, Miyake M, Ishimaru T. System technol‐ ogy for 10Be and 26Al measurements in the JAEA-AMS-TONO (2013). 5th East Asia

[14] Saito-Kokubu Y, Ishimaru T, Matsubara A, Nishizawa A, Miyake M, et al. Present status of JAEA-AMS-TONO; Progress of multi nuclide AMS (2013). 5th East Asia

[15] Matsubara A, Nishizawa A, Miyake M, Saito-Kokubu Y, Ishimaru T. Development of System Techonology for Routine 10Be Measurement in the JAEA-AMS-TONO (in Jap‐ anese) (2013). Proceedings of the 15th Japaneses Symposium on Accelerator Mass

[16] JAEA. Procedure of shared use of JAEA facilities. Available: http://sanga‐ ku.jaea.go.jp/3-facility/05-support/jaea-facilities-eng.html. Accessed 8 January 2013.

[17] Sundquist M. 15SDH-2 AMS SYSTEM MANUAL. National Electrostatics Corp.; 1998.

[18] Matsuzaki H, Nakano C, Tsuchiya Y (Sunohara), Kato K, Maejima Y, et al. Multi-nu‐ clide AMS performances at MALT. Nuclear Instruments and Methods in Physics Re‐ search Section B: Beam Interactions with Materials and Atoms 2007; 259: 36–40.

[19] Kume H, Shibata Y, Tanaka A, Yoneda M, Kumamoto Y, et al. The AMS facility at the National Institute for Environmental Studies (NIES), Japan. Nuclear Instruments

sium on Accelerator Mass Spectrometry (EA-AMS-4). Tokyo. pp. 147–150.

nary Science Journal (Eiszeitalter und Gegenwart) 2008; 57: 179–209.

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cal Years 2012. JAEA-Research 2014; in press.

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AMS Symposium (EA-AMS-5). Daejeon. p. 53.

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Spectrometry. Nagoya. pp. 25–28.

103.

## **Author details**

Akihiro Matsubara1\*, Yoko Saito-Kokubu1 , Akimitsu Nishizawa2 , Masayasu Miyake2 , Tsuneari Ishimaru1 and Koji Umeda1

\*Address all correspondence to: matsubara.akihiro@jaea.go.jp

1 Tono Geoscience Center, Japan Atomic Energy Agency, Jorinji, Izumi, Toki, Japan

2 Pesco Corp., Ltd., Tokiguchiminami, Toki, Japan

## **References**


[7] IVY-OCHS S, KOBER F. Surface exposure dating with cosmogenic nuclides. Quater‐ nary Science Journal (Eiszeitalter und Gegenwart) 2008; 57: 179–209.

**Acknowledgements**

26 Geochronology - Methods and Case Studies

for their help and support.

Akihiro Matsubara1\*, Yoko Saito-Kokubu1

and Koji Umeda1

2 Pesco Corp., Ltd., Tokiguchiminami, Toki, Japan

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\*Address all correspondence to: matsubara.akihiro@jaea.go.jp

1 Tono Geoscience Center, Japan Atomic Energy Agency, Jorinji, Izumi, Toki, Japan

[1] GIRDD. Geological Isolation Research and Development Directorate (GIRDD). Avail‐ able: http://www.jaea.go.jp/04/tisou/english/index/e-index.html. Accessed 5 January

[2] Asamori K, Niwa M, Hanamuro T, Yamada K, Kusano T, et al. Annual Report for Research on Geosphere Stability for Long-Term Isolation of Radioactive Waste in Fis‐

[3] Yasue K, Asamori K, Niwa M, Hanamuro T, Saito-Kokubu Y, et al. Research Plan on Geosphere Stability for Long-Term Isolation of Radioactive Waste (Scientific Pro‐

[4] Finkel RC, Suter M. AMS in the Earth Sciences: Technique and Applications. Advan‐

[5] Tuniz C, Bird JR, Fink D, Gregory F H. Accelerator Mass Spectrometry: Ultrasensi‐

[6] Muzikar P, Elmore D, Granger DE. Accelerator mass spectrometry in geologic re‐

search. Geological Society of America Bulletin 2003; 115: 643–654.

**Author details**

Tsuneari Ishimaru1

**References**

2014.

We would like to express our gratitude to Prof. Matsuzaki of the University of Tokyo for his continuous academic and practical advice. We would like to offer our special thanks also to Dr. Horiuchi of Hirosaki University for providing unknown beryllium samples and measure‐ ment data. Special thanks to Mr. Hanaki of the facility administrator for his management support and constant encouragement. We also thank the staff members of the AMS laboratory

, Akimitsu Nishizawa2

, Masayasu Miyake2

,


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28 Geochronology - Methods and Case Studies

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[46] Martschini M, Andersson P, Forstner O, Golser R, Hanstorp D, et al. AMS of 36Cl with the VERA 3MV tandem accelerator. Nuclear Instruments and Methods in Phys‐ ics Research Section B: Beam Interactions with Materials and Atoms 2013; 294: 115– 120.

**Chapter 2**

**Luminescence Chronology**

Additional information is available at the end of the chapter

Luminescence dating is a collective term for dating methods that encompass thermolumines‐ cence (TL) and optically stimulated luminescence (OSL) dating techniques. OSL is also less commonly referred to as optical dating [1], photon stimulated luminescence dating or photoluminescence dating [2]. Luminescence dating methods are based on the ability of some mineral grains to absorb and store energy from environmental ionizing radiation emanating from the immediate surroundings of the mineral grains as well as from cosmic radiation. When stimulated these minerals, generally referred to as dosimeters [3], will release the stored energy in the form of visible light; hence the term luminescence. Measuring the energy and deter‐ mining the rate at which the energy accumulated allows an age representing the time that has elapsed since the energy began accumulating to be determined. Stimulation of energy release using heat is termed TL while stimulation using light is referred to as OSL. The age range of luminescence methods generally spans from a few decades to about 100,000 years, though ages exceeding several hundred thousand years have been reported in some studies [for example, 4, 5]. In addition, there are dating protocols that are currently under investigation that, if successful, could extend the range even further [6]. Thus, the method is useful for dating Late Quaternary events and, not only does it provide chronology beyond the range that can be attained using radiocarbon methods, but it offers an alternative chronometer in settings where

This chapter aims to acquaint readers who are not familiar with luminescence dating methods with the basics of the techniques. It is not intended to be used as a manual but rather as an introductory primer that brings awareness about the principles behind the dating methods, their practical aspects, as well as their applications. Accordingly, the chapter comprises nine sections. Following the introduction in the first section which briefly lays out the historical development of luminescence dating, the second section examines the principles of the dating methods. This is followed by a discussion of sample stimulation mechanisms and basic

> © 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Ken Munyikwa

**1. Introduction**

http://dx.doi.org/10.5772/58554

no carbon bearing material can be found.

## **Chapter 2**

## **Luminescence Chronology**

Ken Munyikwa

[46] Martschini M, Andersson P, Forstner O, Golser R, Hanstorp D, et al. AMS of 36Cl with the VERA 3MV tandem accelerator. Nuclear Instruments and Methods in Phys‐ ics Research Section B: Beam Interactions with Materials and Atoms 2013; 294: 115–

120.

30 Geochronology - Methods and Case Studies

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/58554

## **1. Introduction**

Luminescence dating is a collective term for dating methods that encompass thermolumines‐ cence (TL) and optically stimulated luminescence (OSL) dating techniques. OSL is also less commonly referred to as optical dating [1], photon stimulated luminescence dating or photoluminescence dating [2]. Luminescence dating methods are based on the ability of some mineral grains to absorb and store energy from environmental ionizing radiation emanating from the immediate surroundings of the mineral grains as well as from cosmic radiation. When stimulated these minerals, generally referred to as dosimeters [3], will release the stored energy in the form of visible light; hence the term luminescence. Measuring the energy and deter‐ mining the rate at which the energy accumulated allows an age representing the time that has elapsed since the energy began accumulating to be determined. Stimulation of energy release using heat is termed TL while stimulation using light is referred to as OSL. The age range of luminescence methods generally spans from a few decades to about 100,000 years, though ages exceeding several hundred thousand years have been reported in some studies [for example, 4, 5]. In addition, there are dating protocols that are currently under investigation that, if successful, could extend the range even further [6]. Thus, the method is useful for dating Late Quaternary events and, not only does it provide chronology beyond the range that can be attained using radiocarbon methods, but it offers an alternative chronometer in settings where no carbon bearing material can be found.

This chapter aims to acquaint readers who are not familiar with luminescence dating methods with the basics of the techniques. It is not intended to be used as a manual but rather as an introductory primer that brings awareness about the principles behind the dating methods, their practical aspects, as well as their applications. Accordingly, the chapter comprises nine sections. Following the introduction in the first section which briefly lays out the historical development of luminescence dating, the second section examines the principles of the dating methods. This is followed by a discussion of sample stimulation mechanisms and basic

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

measurement equipment used in luminescence dating in the third section. Luminescence properties of different minerals are examined in the fourth section. In the fifth section, methods used to determine the energy stored within mineral grains (paleodose) as well as the rate at which the energy accumulates (dose rate) are explored. In the sixth section, practical aspects pertaining to sample collection and laboratory preparation for analysis are discussed after which the types of materials that can be dated using luminescence materials are examined in the seventh section. To illustrate the multifaceted character of some luminescence dating studies, the eighth section presents a case study that uses the chronology of postglacial eolian dune deposition in western Canada to constrain the timing of Late Pleistocene deglaciation in the region. The chapter concludes in the ninth section with a look at current and potential future developments in luminescence dating.

[1] that light could also be used to stimulate energy release from sediments during measure‐ ment as opposed to heating. This led to the birth of OSL dating and throughout the rest of the 1980s [19, 20], the 1990s [21-25] and the early part of the following decade, improved protocols were introduced [26, 27]) and OSL dating equipment refined. All these developments saw luminescence dating emerge as a robust method for dating clastic Quaternary sediments, especially eolian deposits. A comprehensive historical account of the first 50 yrs of lumines‐

Some dielectric materials (insulators) that include many minerals such as quartz, feldspar, zircon and calcite have the ability to store energy in their crystal lattices that emanates from ionizing radiation. In natural geological settings, such ionizing radiation (for example, alpha, beta, and gamma radiation) occurs naturally within the immediate surroundings of the geological materials while a small component is also contributed by cosmic radiation. When stimulated, the minerals will exhibit luminescence which essentially represents a release of the stored energy. Luminescence dating employs this phenomenon by measuring the energy stored in the mineral, called the paleodose, and dividing it by the rate at which the energy was received by the mineral in question. Hence, the basic age equation for luminescence dating is:

*Luminescence Age* <sup>=</sup> *Paleodose*

The exact mechanisms through which luminescence energy accumulates in the minerals are complex. However, it is thought that the energy is stored when electrons in the mineral crystal lattices are displaced from the valence band of their parent nuclei. Once detached, the electrons diffuse into the surroundings of lattice defects that act as electron traps. Such defects include a missing atom in the crystal lattice of the mineral, an atom out of its rightful place or the occurrence of impurity atoms in the lattice [29]. Importantly for dating purposes, the number of trapped electrons increases with the duration of exposure of the mineral to the ionising radiation. Fig. 1 is a depiction of an energy level diagram that is commonly used to visualise the trapping mechanism involved in luminescence. The depth of the trap (T) below the conduction band, indicated by 'E' [Fig. 1], is an indication of the efficacy of a given trap. Stable traps are those that can withstand perturbations such as lattice vibrations that could dislodge the electrons from their traps. Stimulation of the crystal lattice structure by heating to an appropriate temperature or by optical means using a suitable wavelength will excite the electrons out of the traps. Once expelled from the traps, the electrons diffuse within the crystal lattice until they come across another site that is attractive to electrons and these are referred to as recombination centres [29]. When electrons reach some recombination centres, energy is

*Dose rate* (1)

Luminescence Chronology http://dx.doi.org/10.5772/58554 33

cence dating is provided by Wintle [28].

**2.1. Luminescent materials**

**2.2. Electron trapping mechanisms**

**2. Principles of luminescence dating**

#### **1.1. The historical development of luminescence dating**

The history of the development of luminescence dating spans the last six decades and it began with experimental applications of the phenomenon of thermoluminescence, which is the emission of light when materials are heated to temperatures below those of incandescence. One of the earliest documented suggestions of the possibility of using thermoluminescence to measure time in archaeology was by Daniels et al. [7] who in 1953 proposed using thermolu‐ minescence observed from ancient pottery artifacts that had previously been fired as a measure of their age [8]. A few years later, the application of thermoluminescence to date pottery was discussed by Kennedy and Knopff [9] and technical aspects of measurements that would be employed for dating were described by Grögler et al. [10]. Notably the application of thermo‐ luminescence in geology to study ages of carbonates [11] and lava flows [12] was already being discussed. An early study that tested the application of thermoluminescence dating on pottery was reported by Aitken et al. [8] when in 1964 they applied the method on pottery sherds ranging in age from around 1,000–8,000 years and collected from sites spread over a large area. Results indicated that the luminescence ages were linearly proportional to radiocarbon chronology from contemporaneous materials. Subsequently, developments throughout the rest of the 1960s and the 1970s saw improvements in dating procedures that used thermolu‐ minescence in archaeological applications [3].

Incidentally, parallel developments in the former Soviet Union during the late 1960s and early 1970s began seeing the tentative application of thermoluminescence dating on unburnt Quaternary sediments [13, 14] when it was noted that older sediments returned higher TL signals than younger ones. Dating of unburnt sediments was based on the recognition that stored energy in sediment grains could also be depleted by exposure to sunlight as opposed to heating that occurs in fired artifacts. In the West, attempts to apply thermoluminescence on unburnt sediments appear to have begun around the end of the 1970s [15] including attempts by Wintle and Huntley [16, 17] to apply the technique on deep sea sediments. Accurate sediment ages from TL dating, however, remained elusive since the optimal conditions for solar resetting were not yet fully understood [18]. Huntley [18] conducted some of the earliest studies investigating the most appropriate conditions for solar bleaching. In a related devel‐ opment, sediment dating using luminescence methods progressed rapidly when it was noted [1] that light could also be used to stimulate energy release from sediments during measure‐ ment as opposed to heating. This led to the birth of OSL dating and throughout the rest of the 1980s [19, 20], the 1990s [21-25] and the early part of the following decade, improved protocols were introduced [26, 27]) and OSL dating equipment refined. All these developments saw luminescence dating emerge as a robust method for dating clastic Quaternary sediments, especially eolian deposits. A comprehensive historical account of the first 50 yrs of lumines‐ cence dating is provided by Wintle [28].

## **2. Principles of luminescence dating**

#### **2.1. Luminescent materials**

measurement equipment used in luminescence dating in the third section. Luminescence properties of different minerals are examined in the fourth section. In the fifth section, methods used to determine the energy stored within mineral grains (paleodose) as well as the rate at which the energy accumulates (dose rate) are explored. In the sixth section, practical aspects pertaining to sample collection and laboratory preparation for analysis are discussed after which the types of materials that can be dated using luminescence materials are examined in the seventh section. To illustrate the multifaceted character of some luminescence dating studies, the eighth section presents a case study that uses the chronology of postglacial eolian dune deposition in western Canada to constrain the timing of Late Pleistocene deglaciation in the region. The chapter concludes in the ninth section with a look at current and potential

The history of the development of luminescence dating spans the last six decades and it began with experimental applications of the phenomenon of thermoluminescence, which is the emission of light when materials are heated to temperatures below those of incandescence. One of the earliest documented suggestions of the possibility of using thermoluminescence to measure time in archaeology was by Daniels et al. [7] who in 1953 proposed using thermolu‐ minescence observed from ancient pottery artifacts that had previously been fired as a measure of their age [8]. A few years later, the application of thermoluminescence to date pottery was discussed by Kennedy and Knopff [9] and technical aspects of measurements that would be employed for dating were described by Grögler et al. [10]. Notably the application of thermo‐ luminescence in geology to study ages of carbonates [11] and lava flows [12] was already being discussed. An early study that tested the application of thermoluminescence dating on pottery was reported by Aitken et al. [8] when in 1964 they applied the method on pottery sherds ranging in age from around 1,000–8,000 years and collected from sites spread over a large area. Results indicated that the luminescence ages were linearly proportional to radiocarbon chronology from contemporaneous materials. Subsequently, developments throughout the rest of the 1960s and the 1970s saw improvements in dating procedures that used thermolu‐

Incidentally, parallel developments in the former Soviet Union during the late 1960s and early 1970s began seeing the tentative application of thermoluminescence dating on unburnt Quaternary sediments [13, 14] when it was noted that older sediments returned higher TL signals than younger ones. Dating of unburnt sediments was based on the recognition that stored energy in sediment grains could also be depleted by exposure to sunlight as opposed to heating that occurs in fired artifacts. In the West, attempts to apply thermoluminescence on unburnt sediments appear to have begun around the end of the 1970s [15] including attempts by Wintle and Huntley [16, 17] to apply the technique on deep sea sediments. Accurate sediment ages from TL dating, however, remained elusive since the optimal conditions for solar resetting were not yet fully understood [18]. Huntley [18] conducted some of the earliest studies investigating the most appropriate conditions for solar bleaching. In a related devel‐ opment, sediment dating using luminescence methods progressed rapidly when it was noted

future developments in luminescence dating.

32 Geochronology - Methods and Case Studies

minescence in archaeological applications [3].

**1.1. The historical development of luminescence dating**

Some dielectric materials (insulators) that include many minerals such as quartz, feldspar, zircon and calcite have the ability to store energy in their crystal lattices that emanates from ionizing radiation. In natural geological settings, such ionizing radiation (for example, alpha, beta, and gamma radiation) occurs naturally within the immediate surroundings of the geological materials while a small component is also contributed by cosmic radiation. When stimulated, the minerals will exhibit luminescence which essentially represents a release of the stored energy. Luminescence dating employs this phenomenon by measuring the energy stored in the mineral, called the paleodose, and dividing it by the rate at which the energy was received by the mineral in question. Hence, the basic age equation for luminescence dating is:

$$\text{Luminesscence } Age = \frac{\text{Palecodge}}{\text{Dose rate}} \tag{1}$$

#### **2.2. Electron trapping mechanisms**

The exact mechanisms through which luminescence energy accumulates in the minerals are complex. However, it is thought that the energy is stored when electrons in the mineral crystal lattices are displaced from the valence band of their parent nuclei. Once detached, the electrons diffuse into the surroundings of lattice defects that act as electron traps. Such defects include a missing atom in the crystal lattice of the mineral, an atom out of its rightful place or the occurrence of impurity atoms in the lattice [29]. Importantly for dating purposes, the number of trapped electrons increases with the duration of exposure of the mineral to the ionising radiation. Fig. 1 is a depiction of an energy level diagram that is commonly used to visualise the trapping mechanism involved in luminescence. The depth of the trap (T) below the conduction band, indicated by 'E' [Fig. 1], is an indication of the efficacy of a given trap. Stable traps are those that can withstand perturbations such as lattice vibrations that could dislodge the electrons from their traps. Stimulation of the crystal lattice structure by heating to an appropriate temperature or by optical means using a suitable wavelength will excite the electrons out of the traps. Once expelled from the traps, the electrons diffuse within the crystal lattice until they come across another site that is attractive to electrons and these are referred to as recombination centres [29]. When electrons reach some recombination centres, energy is emitted in the form of light and these are described as luminescence centres. Thermal stimu‐ lation would yield TL whereas optical stimulation would produce OSL. Importantly, the diffusion process is very rapid such that the time between stimulation and recombination can be treated as instantaneous. For a recombination centre to be effective, an electron must be missing from the site in the lattice, creating what is termed a hole. Holes are created in materials by ionising radiation. The intensity of the luminescence given out following stimulation is proportional to the number of trapped electrons and this is assumed to be proportional to the energy absorbed from the nuclear radiation [29]. Significant for dating applications, however, though the energy storage mechanisms might be the same for a given mineral, the sensitivity to radiation will vary between samples; a consideration that has huge implications for methodological approaches as will be shown later.

than 5% for potassium, where 40K is one part in 10,000). However, collectively, their radioactive isotopes emit enough radiation to cause detectable luminescence for dating purposes. The radiation emitted includes alpha and beta particles as well as gamma radiation. Alpha particles have penetration ranges of 0.02 mm while beta and gamma rays have ranges of around 0.02 cm and 20 cm respectively [29]. The shorter distance travelled by alpha particles is because they are heavier and much more ionizing, which results in more rapid loss of energy as they knock atoms of materials through which they are traveling out of the way. Beta particles and

Luminescence Chronology http://dx.doi.org/10.5772/58554 35

Besides the radiation from radioactive isotopes, an additional, albeit smaller, component to the energy received by the mineral grains is contributed by cosmic rays from outer space. The cosmic radiation comprises a soft and a hard component and once it reaches the earth's surface, the soft component is absorbed by the upper 50 cm of the near-surface substrate. Only the hard component can penetrate deeper and is of interest to luminescence dating. This hard component comprises muons mostly and at sea level it varies slightly with latitude, increasing in intensity by about 7% from the equator to latitude 40°. Above 40° latitude however, it remains constant up to the poles [29]. At altitudes higher than 1 km, the contribution from the hard component also increases significantly with both latitude and altitude. For dating purposes, special formulae have been developed for evaluating

**3. Basic luminescence measurement equipment and sample stimulation**

The primary objective of TL or OSL measurements in dating studies is to ascertain the amount of energy that has accumulated in the mineral grains since the start of the event being dated. This energy is determined by stimulating the mineral grains using an appropriate mechanism and measuring the amount of light released. As already outlined, trapped electrons in luminescence dating can be evicted from their traps by heating, as is done in TL, or using light, in OSL methods. Fig. 2 illustrates the basic layout of equipment used to measure the lumines‐ cence. Typically, samples are placed on discs about 1 cm in diameter and then introduced into the machine in multiples on an appropriate sample holder. A servo-control mechanism moves the sample to the appropriate position for stimulation and measurement. Most modern luminescence measurement systems possess both thermal and optical stimulation capabilities. The luminescence signal from the sample is captured by the photomultiplier tube after going

For TL measurements, the filters help exclude infrared signals from the heating while allowing blue or violet signals. For OSL measurements, the filters reject wavelengths used for stimula‐ tion while usually allowing violet and near-ultraviolet wavelengths. The end of the photo‐ multiplier tube closest to the sample is fitted with a photocathode that emits electrons when struck by light photons as a result of the photoelectric effect. Potential differences allow emitted electrons to be attracted by the first dynode in the photomultiplier. For each electron arriving

gamma rays, on the other hand, tend to get scattered.

cosmic ray contribution to the dose rate [30].

**mechanisms**

through optical filters.

**Figure 1.** Energy Level diagram illustrating the creation of luminescence centres in crystal lattices through exposure to ionising radiation (redrawn from [29]). (a) Irradiation leads to electrons being expelled from their original site and dif‐ fuse within the lattice. (b) Electrons become trapped while holes of missing electrons become localised at particular centres. (c) Stimulation (by heat or light) results in electrons being evicted from the traps and diffusing until they meet a recombination centre. Light (luminescence) is emitted when luminescence centres are encountered by the electrons to give TL or OSL.

It is pertinent to note that the number of traps within the lattice of any dosimeter is not infinite and, hence, they will be exhausted following extended exposure to radiation, beyond which energy will not be stored efficiently. This is referred to as saturation. For dating purposes this is what defines the upper limit on the age beyond which samples cannot be dated using luminescence methods [29]. The exact age representing this upper limit will ultimately depend on the dose rate, with samples experiencing high dose rates having lower age limits.

#### **2.3. Natural sources of ionizing radiation**

For purposes of luminescence dating, natural sources of ionising radiation that contribute to the trapped energy in mineral grains are isotopes of uranium (238U and 235U) and thorium (232Th) decay chains, potassium (40K) and rubidium (87Rb). These elements occur in natural materials in very low levels (around 3-10 parts per million for uranium and thorium and generally less than 5% for potassium, where 40K is one part in 10,000). However, collectively, their radioactive isotopes emit enough radiation to cause detectable luminescence for dating purposes. The radiation emitted includes alpha and beta particles as well as gamma radiation. Alpha particles have penetration ranges of 0.02 mm while beta and gamma rays have ranges of around 0.02 cm and 20 cm respectively [29]. The shorter distance travelled by alpha particles is because they are heavier and much more ionizing, which results in more rapid loss of energy as they knock atoms of materials through which they are traveling out of the way. Beta particles and gamma rays, on the other hand, tend to get scattered.

emitted in the form of light and these are described as luminescence centres. Thermal stimu‐ lation would yield TL whereas optical stimulation would produce OSL. Importantly, the diffusion process is very rapid such that the time between stimulation and recombination can be treated as instantaneous. For a recombination centre to be effective, an electron must be missing from the site in the lattice, creating what is termed a hole. Holes are created in materials by ionising radiation. The intensity of the luminescence given out following stimulation is proportional to the number of trapped electrons and this is assumed to be proportional to the energy absorbed from the nuclear radiation [29]. Significant for dating applications, however, though the energy storage mechanisms might be the same for a given mineral, the sensitivity to radiation will vary between samples; a consideration that has huge implications for

**Figure 1.** Energy Level diagram illustrating the creation of luminescence centres in crystal lattices through exposure to ionising radiation (redrawn from [29]). (a) Irradiation leads to electrons being expelled from their original site and dif‐ fuse within the lattice. (b) Electrons become trapped while holes of missing electrons become localised at particular centres. (c) Stimulation (by heat or light) results in electrons being evicted from the traps and diffusing until they meet a recombination centre. Light (luminescence) is emitted when luminescence centres are encountered by the electrons

It is pertinent to note that the number of traps within the lattice of any dosimeter is not infinite and, hence, they will be exhausted following extended exposure to radiation, beyond which energy will not be stored efficiently. This is referred to as saturation. For dating purposes this is what defines the upper limit on the age beyond which samples cannot be dated using luminescence methods [29]. The exact age representing this upper limit will ultimately depend

For purposes of luminescence dating, natural sources of ionising radiation that contribute to the trapped energy in mineral grains are isotopes of uranium (238U and 235U) and thorium (232Th) decay chains, potassium (40K) and rubidium (87Rb). These elements occur in natural materials in very low levels (around 3-10 parts per million for uranium and thorium and generally less

on the dose rate, with samples experiencing high dose rates having lower age limits.

methodological approaches as will be shown later.

34 Geochronology - Methods and Case Studies

to give TL or OSL.

**2.3. Natural sources of ionizing radiation**

Besides the radiation from radioactive isotopes, an additional, albeit smaller, component to the energy received by the mineral grains is contributed by cosmic rays from outer space. The cosmic radiation comprises a soft and a hard component and once it reaches the earth's surface, the soft component is absorbed by the upper 50 cm of the near-surface substrate. Only the hard component can penetrate deeper and is of interest to luminescence dating. This hard component comprises muons mostly and at sea level it varies slightly with latitude, increasing in intensity by about 7% from the equator to latitude 40°. Above 40° latitude however, it remains constant up to the poles [29]. At altitudes higher than 1 km, the contribution from the hard component also increases significantly with both latitude and altitude. For dating purposes, special formulae have been developed for evaluating cosmic ray contribution to the dose rate [30].

## **3. Basic luminescence measurement equipment and sample stimulation mechanisms**

The primary objective of TL or OSL measurements in dating studies is to ascertain the amount of energy that has accumulated in the mineral grains since the start of the event being dated. This energy is determined by stimulating the mineral grains using an appropriate mechanism and measuring the amount of light released. As already outlined, trapped electrons in luminescence dating can be evicted from their traps by heating, as is done in TL, or using light, in OSL methods. Fig. 2 illustrates the basic layout of equipment used to measure the lumines‐ cence. Typically, samples are placed on discs about 1 cm in diameter and then introduced into the machine in multiples on an appropriate sample holder. A servo-control mechanism moves the sample to the appropriate position for stimulation and measurement. Most modern luminescence measurement systems possess both thermal and optical stimulation capabilities. The luminescence signal from the sample is captured by the photomultiplier tube after going through optical filters.

For TL measurements, the filters help exclude infrared signals from the heating while allowing blue or violet signals. For OSL measurements, the filters reject wavelengths used for stimula‐ tion while usually allowing violet and near-ultraviolet wavelengths. The end of the photo‐ multiplier tube closest to the sample is fitted with a photocathode that emits electrons when struck by light photons as a result of the photoelectric effect. Potential differences allow emitted electrons to be attracted by the first dynode in the photomultiplier. For each electron arriving at the dynode, several are emitted and the process is repeated through the photomultiplier many times such that several millions of electrons reach the anode at the other end of the photomultiplier for each electron leaving the photocathode. These electrons give an easily detectable current pulse at the anode that is commensurate with the arrival of photons at the photocathode. The current pulses are then amplified and the output is presented as photon counts, representing the luminescence signal. As will be shown below, TL output is distinctly different from OSL output.

material when heated to a temperature high enough. Absent from this second heating will be the luminescence emanating from ionizing radiation that accrued since the last event that

Luminescence Chronology http://dx.doi.org/10.5772/58554 37

**Figure 3.** Examples of TL glow-curves showing (a) natural signal obtained from a mineral sample (quartz) during the first heating and (b) red-hot glow-curve obtained from a second heating (incandescence). Note that in curve (a), in‐ candescence is also obtained during the first heating when the sample is heated above 400° C (redrawn from [31].

Optical stimulation of luminescence uses light of a particular wavelength (for example, blue, green, or near-infrared) to expel electrons from their traps. Notably, the rate at which these electrons are evicted directly depends on the rate at which the stimulating photons are received. The sensitivity of a given trap-type to photostimulation is also an important factor influencing the rate at which electrons leave their traps. In essence, the curve that depicts the emission follows an exponential decay (often referred to as a shine-down curve), with high emission rates in the beginning that gradually fall with continued stimulation (Fig. 4). If the process is continued, at a certain point, all electron traps that are sensitive to optical stimulation become exhausted. Integrating the number of photons released over the period of stimulation quantifies the luminescence of the sample and this should be commensurate with the sample's

Factors that influence the sensitivity of a trap type to electron eviction by light include characteristics of the trap as well as the wavelength of the stimulating light. Generally, shorter wavelengths are associated with faster eviction rates. Stable traps may require more energy than that available from some optical stimulation wavelengths and in such cases, thermal assistance can be used to bridge the gap. This enables longer wavelengths to be used for

When selecting a stimulation wavelength for OSL measurements, it is important to select a wavelength that effectively allows separation to be made between the wavelength of the stimulating source and that of the emitted signal. As described earlier, filters are used to assist this process. Quartz and feldspar for instance, have strong emissions in the near-ultraviolet (365 nm) and violet (410 nm) respectively and filters used in either case are selected because

stimulation in cases where they would not be able to unassisted [29].

emptied the electrons from the traps.

**3.2. Optical Stimulation (OSL)**

age [29].

**Figure 2.** Basic features of a TL/OSL reader that can be used to measure luminescence signals using either heat or optical stimulation (modified after [31]).

#### **3.1. Stimulation by heat (TL)**

When stimulating using heat, a sample is heated rapidly at rates in the range of 20°C /s. Once a temperature commensurate with 'E' in Fig.1 and characteristic of a particular trap type is reached, electrons are rapidly evicted from the traps [29]. The temperature is represented by a peak in emission on a plot of the luminescence versus temperature referred to as the TL glowcurve. Continued heating will empty the traps and the luminescence given out will be proportional to the number of electrons trapped in the mineral grains since the beginning of the event being dated. Fig. 3 shows a glow-curve of a sample observed after the first heating. If a sample is heated for a second time immediately after the first heating, a different curve is observed. This second curve is the red-hot glow that is the incandescence given out by any material when heated to a temperature high enough. Absent from this second heating will be the luminescence emanating from ionizing radiation that accrued since the last event that emptied the electrons from the traps.

**Figure 3.** Examples of TL glow-curves showing (a) natural signal obtained from a mineral sample (quartz) during the first heating and (b) red-hot glow-curve obtained from a second heating (incandescence). Note that in curve (a), in‐ candescence is also obtained during the first heating when the sample is heated above 400° C (redrawn from [31].

#### **3.2. Optical Stimulation (OSL)**

at the dynode, several are emitted and the process is repeated through the photomultiplier many times such that several millions of electrons reach the anode at the other end of the photomultiplier for each electron leaving the photocathode. These electrons give an easily detectable current pulse at the anode that is commensurate with the arrival of photons at the photocathode. The current pulses are then amplified and the output is presented as photon counts, representing the luminescence signal. As will be shown below, TL output is distinctly

**Figure 2.** Basic features of a TL/OSL reader that can be used to measure luminescence signals using either heat or

When stimulating using heat, a sample is heated rapidly at rates in the range of 20°C /s. Once a temperature commensurate with 'E' in Fig.1 and characteristic of a particular trap type is reached, electrons are rapidly evicted from the traps [29]. The temperature is represented by a peak in emission on a plot of the luminescence versus temperature referred to as the TL glowcurve. Continued heating will empty the traps and the luminescence given out will be proportional to the number of electrons trapped in the mineral grains since the beginning of the event being dated. Fig. 3 shows a glow-curve of a sample observed after the first heating. If a sample is heated for a second time immediately after the first heating, a different curve is observed. This second curve is the red-hot glow that is the incandescence given out by any

different from OSL output.

36 Geochronology - Methods and Case Studies

optical stimulation (modified after [31]).

**3.1. Stimulation by heat (TL)**

Optical stimulation of luminescence uses light of a particular wavelength (for example, blue, green, or near-infrared) to expel electrons from their traps. Notably, the rate at which these electrons are evicted directly depends on the rate at which the stimulating photons are received. The sensitivity of a given trap-type to photostimulation is also an important factor influencing the rate at which electrons leave their traps. In essence, the curve that depicts the emission follows an exponential decay (often referred to as a shine-down curve), with high emission rates in the beginning that gradually fall with continued stimulation (Fig. 4). If the process is continued, at a certain point, all electron traps that are sensitive to optical stimulation become exhausted. Integrating the number of photons released over the period of stimulation quantifies the luminescence of the sample and this should be commensurate with the sample's age [29].

Factors that influence the sensitivity of a trap type to electron eviction by light include characteristics of the trap as well as the wavelength of the stimulating light. Generally, shorter wavelengths are associated with faster eviction rates. Stable traps may require more energy than that available from some optical stimulation wavelengths and in such cases, thermal assistance can be used to bridge the gap. This enables longer wavelengths to be used for stimulation in cases where they would not be able to unassisted [29].

When selecting a stimulation wavelength for OSL measurements, it is important to select a wavelength that effectively allows separation to be made between the wavelength of the stimulating source and that of the emitted signal. As described earlier, filters are used to assist this process. Quartz and feldspar for instance, have strong emissions in the near-ultraviolet (365 nm) and violet (410 nm) respectively and filters used in either case are selected because they have windows in the respective wavelength range. Wavelengths used for stimulation, therefore, should be excluded by the filters [29].

In a study examining the bleaching rates of quartz and feldspar (Fig. 5), it was shown [19] that after 20 hrs of exposure to sunlight, samples of both minerals had less than 0.1% of their original OSL signal remaining (Fig. 5). Conversely, similar samples had TL signals that were at least 10 times higher remaining after an equal time under solar radiation. This means that the optical signal is zeroed much more rapidly by the sun and the hard to bleach TL traps generally result in a much higher residual signal following solar bleaching compared to the OSL signal. Consequently, it is often difficult to date very young samples using TL [1, 29]), with OSL being preferred for dating sediments in general. However, dating of fired artifacts and baked sediments using TL remains an appropriate methodology since the zeroing mechanism in

Luminescence Chronology http://dx.doi.org/10.5772/58554 39

The discovery of luminescence in minerals is not a recent event. Aitken [2, 29, 32] has often cited the case of Robert Boyle who in 1663 reported holding a diamond close to his body and noting that it 'shined in the dark'. This is because many minerals are capable of luminescing in the dark when appropriately irradiated and stimulated. For dating purposes using lumi‐ nescence methods, however, quartz and feldspar are the dosimeters that have received the most focus. Attempts have also been made to use zircon and calcite but these are not commonly used for dating because of a number of drawbacks. In this section, the luminescence properties

Quartz is a widely used mineral in luminescence dating because of the advantages it offers compared to the alternatives. It is one of the most abundant minerals on the earth's surface, making it ubiquitous in most depositional environments, a feature arising from its high stability at the earth's surface and resistance to abrasion. It also has very stable luminescence properties. Because of its chemistry, quartz itself has no internal source of radiation that is a major element of its composition. As a result, the radiation quartz grains receive in nature usually originates from outside the grain. In some settings, however, quartz may contain some

For TL analysis, natural quartz signals normally display two peaks above 300 °C [33]. One peak is at 325 °C and another at 375 °C and it is the latter that is usually used for dating. Laboratory irradiated quartz also shows a peak at 110 °C. In terms of emissions, when heated above 300 °C, quartz has a TL emission band in the range 460-480 nm (blue) and another in the range 610-630 nm (orange) [34]. Below 300 °C, quartz has an emission in the range 360-420 nm (near UV to violet) when irradiated with a laboratory dose. The 375 °C peak is thought to be from AlO4 impurities in the quartz lattice serving as holes [35].The peak has a high thermal

nature will be similar to the stimulation mechanism at measurement.

**4. Luminescence properties of some common minerals**

of these minerals are briefly examined.

trace amounts of uranium [32].

*4.1.1. Quartz TL properties*

**4.1. Quartz**

**Figure 4.** OSL shine-down curve for a hypothetical mineral stimulated using light for about 100 s (modified from [29]).

#### **3.3. Advantages of OSL over TL**

As will be shown later, advantages of OSL dating versus TL dating mainly apply to dating of sediments that have been zeroed by solar bleaching. Studies have demonstrated that solar bleaching of natural TL occurs much more slowly than OSL [29].

**Figure 5.** Comparison of bleaching rates of natural TL and OSL (green light) signals of quartz (q) and feldspar (f) con‐ ducted by Godfrey-Smith et al. [19] (redrawn from [29]). The slower bleaching curves are from the TL signals.

In a study examining the bleaching rates of quartz and feldspar (Fig. 5), it was shown [19] that after 20 hrs of exposure to sunlight, samples of both minerals had less than 0.1% of their original OSL signal remaining (Fig. 5). Conversely, similar samples had TL signals that were at least 10 times higher remaining after an equal time under solar radiation. This means that the optical signal is zeroed much more rapidly by the sun and the hard to bleach TL traps generally result in a much higher residual signal following solar bleaching compared to the OSL signal. Consequently, it is often difficult to date very young samples using TL [1, 29]), with OSL being preferred for dating sediments in general. However, dating of fired artifacts and baked sediments using TL remains an appropriate methodology since the zeroing mechanism in nature will be similar to the stimulation mechanism at measurement.

## **4. Luminescence properties of some common minerals**

The discovery of luminescence in minerals is not a recent event. Aitken [2, 29, 32] has often cited the case of Robert Boyle who in 1663 reported holding a diamond close to his body and noting that it 'shined in the dark'. This is because many minerals are capable of luminescing in the dark when appropriately irradiated and stimulated. For dating purposes using lumi‐ nescence methods, however, quartz and feldspar are the dosimeters that have received the most focus. Attempts have also been made to use zircon and calcite but these are not commonly used for dating because of a number of drawbacks. In this section, the luminescence properties of these minerals are briefly examined.

### **4.1. Quartz**

they have windows in the respective wavelength range. Wavelengths used for stimulation,

**Figure 4.** OSL shine-down curve for a hypothetical mineral stimulated using light for about 100 s (modified from [29]).

As will be shown later, advantages of OSL dating versus TL dating mainly apply to dating of sediments that have been zeroed by solar bleaching. Studies have demonstrated that solar

**Figure 5.** Comparison of bleaching rates of natural TL and OSL (green light) signals of quartz (q) and feldspar (f) con‐ ducted by Godfrey-Smith et al. [19] (redrawn from [29]). The slower bleaching curves are from the TL signals.

bleaching of natural TL occurs much more slowly than OSL [29].

therefore, should be excluded by the filters [29].

38 Geochronology - Methods and Case Studies

**3.3. Advantages of OSL over TL**

Quartz is a widely used mineral in luminescence dating because of the advantages it offers compared to the alternatives. It is one of the most abundant minerals on the earth's surface, making it ubiquitous in most depositional environments, a feature arising from its high stability at the earth's surface and resistance to abrasion. It also has very stable luminescence properties. Because of its chemistry, quartz itself has no internal source of radiation that is a major element of its composition. As a result, the radiation quartz grains receive in nature usually originates from outside the grain. In some settings, however, quartz may contain some trace amounts of uranium [32].

#### *4.1.1. Quartz TL properties*

For TL analysis, natural quartz signals normally display two peaks above 300 °C [33]. One peak is at 325 °C and another at 375 °C and it is the latter that is usually used for dating. Laboratory irradiated quartz also shows a peak at 110 °C. In terms of emissions, when heated above 300 °C, quartz has a TL emission band in the range 460-480 nm (blue) and another in the range 610-630 nm (orange) [34]. Below 300 °C, quartz has an emission in the range 360-420 nm (near UV to violet) when irradiated with a laboratory dose. The 375 °C peak is thought to be from AlO4 impurities in the quartz lattice serving as holes [35].The peak has a high thermal stability but it tends to saturate at low doses such that it is of limited use beyond 50-100 ka unless very low dose rates are involved [36].

drawback for feldspar, however, which delayed its application in routine dating, is that it is afflicted by a phenomenon called anomalous fading [42]. In anomalous fading, the measured luminescence intensity decays with increasing time from the time of irradiation because some electrons have much shorter residence times in their traps than predicted by physical models [29, 43]. The ultimate result is that most feldspar grains will return equivalent doses slightly lower than they would if the dose were stable over time. To address this phenomenon in

Luminescence Chronology http://dx.doi.org/10.5772/58554 41

Many K-feldspars of sedimentary origin have been shown to display natural signals with TL peaks at 280 °C and 330 °C [36]. With regards to emissions, some studies [for example, 34] have reported emissions from K-rich feldspars in the 390-440 nm range (violet to blue) while plagioclase feldspar emissions have been reported in the 550-560 nm range (blue-green).

Luminescence from feldspars has been investigated using visible light stimulation. Earlier investigations used lasers (for example, 514.5 nm (green) from argon and 633 nm (red) from krypton) and observed the emissions at shorter wavelengths [1, 19]. For plagioclase feldspars, results showed that the spectra observed were similar to those from TL. Another study [48] that used a stimulation wavelength of 633 nm also showed that the emission was centred at 400 nm. OSL applications for dating using green light stimulation have been very limited and this has largely been because infrared stimulation (IRSL), as discussed below, was found to be a much better alternative. However, a study [49] that compared green light stimulation luminescence (GLSL) and IRSL data from feldspars from alluvial sediment showed results that suggested the signals had different thermal stabilities, with GLSL signals being more stable than IRSL signals at 10 °C. Stimulation of feldspars using a wide range of wavelengths in the

range 380-1020 nm, apart from green and red, has also been demonstrated by [50].

Feldspars can also be stimulated using the near infrared part of the electromagnetic spectrum (around 880 nm). Since the discovery of this stimulation peak [20], most dating research that uses optical stimulation of feldspars for sediment dating has been focussed on IRSL. A major advantage of this, as stated earlier, is that it leaves the rest of the visible part of the spectrum open for use in emission detection. Other investigations using IRSL include studies on finegrained sedimentary samples containing both plagioclase and K-feldspar that have also shown a major stimulation peak at 854 nm (1.45 eV) and another weaker one at 775 nm (1.6 eV) at room temperature [51]. Overall, these characteristics allow feldspars to be stimulated by light emitting diodes that have emission peaks at around 880±40 nm and these are widely available and cheap. Emission spectra of the K-feldspars stimulated using IRSL were reported by Huntley et al. [52] to show a dominant peak at 410 nm and another minor peak between 300 and 350 nm. Plagioclase feldspars, on the other hand, showed an emission peak at 570 nm.

feldspar dating, correction methods for the fading have been devised [43, 44].

However, results from other studies [47] suggest a more complex pattern.

*4.2.1. Feldspar TL properties*

*4.2.2. Feldspar OSL properties*

*4.2.3. Feldspar IRSL properties*

The 325 °C TL peak bleaches more rapidly than that at 375 °C [36] and its emission peaks at about 380 nm [37]. Hence, by using appropriate filters, the emission can be separated from that of the 375 °C peak. Studies suggest that, because of its relative stability, the 325 °C peak could in theory be used for dating up to 1 million years, which is much older than ages that can be measured using the 375 °C peak [36].

### *4.1.2. Quartz OSL properties*

Irradiated quartz has been shown to emit OSL when stimulated by light from any part of the visible spectrum. For dating purposes using current methods, however, blue light is preferred in most applications because, as indicated earlier, OSL yield is proportional to the wavelength used for stimulation, with higher energies yielding higher OSL intensities [38]. The OSL signal from sedimentary quartz has been demonstrated to comprise at least three or four components and these are referred to as fast medium and slow components with respect to the rates at which they decay [39]. Others have reported up to seven components [40]. These components can only be separated when stimulation uses a constantly increasing stimulating power, referred to as linearly modulated OSL (LM-OSL) [41]. A stimulation source with a constant power (continuous wave), as is used in most regular dating, cannot resolve the components. However by using heat treatments (to eliminate unstable signals) or stimulating for controlled times (to exclude dominant slow components) the appropriate signal can be focused on when using continuous wave stimulation.

#### **4.2. Feldspar**

Feldspar has been used extensively in OSL dating. Like quartz, it is a mineral that is widely available at the earth's surface, though it weathers more rapidly. The chemistry of feldspars has important implications with respect to how they are used in luminescence dating. They are aluminosilicates that have potassium (K), calcium (Ca) or sodium (Na) as end members. The presence of potassium in some of the feldspars is critical in that 40K isotopes that form part of the potassium constitute an internal source of radiation, in addition to any external radiation the grains may receive. Feldspars that do not have high potassium as part of their chemistry (for example, Ca and Na-feldspars), however, would not have this additional internal dose. Hence, for dating purposes, as will be discussed later, K-feldspars are normally separated from other feldspars prior to conducting analysis.

For dating applications, feldspar has a number of attractive characteristics. One is that, in terms of emissions, feldspars have a higher brightness compared to quartz which means that it gives strong signals, allowing smaller doses to be measured. Secondly, the internal dose in feldspars that have a high potassium content constitutes a reliable radiation source that is immune from environmental changes that would affect external sources (for example, interstitial water). As a result, dose rates can be determined more accurately. The third advantage of feldspar, which will be discussed below, is that it can be stimulated using infrared stimulation. A major drawback for feldspar, however, which delayed its application in routine dating, is that it is afflicted by a phenomenon called anomalous fading [42]. In anomalous fading, the measured luminescence intensity decays with increasing time from the time of irradiation because some electrons have much shorter residence times in their traps than predicted by physical models [29, 43]. The ultimate result is that most feldspar grains will return equivalent doses slightly lower than they would if the dose were stable over time. To address this phenomenon in feldspar dating, correction methods for the fading have been devised [43, 44].

## *4.2.1. Feldspar TL properties*

stability but it tends to saturate at low doses such that it is of limited use beyond 50-100 ka

The 325 °C TL peak bleaches more rapidly than that at 375 °C [36] and its emission peaks at about 380 nm [37]. Hence, by using appropriate filters, the emission can be separated from that of the 375 °C peak. Studies suggest that, because of its relative stability, the 325 °C peak could in theory be used for dating up to 1 million years, which is much older than ages that can be

Irradiated quartz has been shown to emit OSL when stimulated by light from any part of the visible spectrum. For dating purposes using current methods, however, blue light is preferred in most applications because, as indicated earlier, OSL yield is proportional to the wavelength used for stimulation, with higher energies yielding higher OSL intensities [38]. The OSL signal from sedimentary quartz has been demonstrated to comprise at least three or four components and these are referred to as fast medium and slow components with respect to the rates at which they decay [39]. Others have reported up to seven components [40]. These components can only be separated when stimulation uses a constantly increasing stimulating power, referred to as linearly modulated OSL (LM-OSL) [41]. A stimulation source with a constant power (continuous wave), as is used in most regular dating, cannot resolve the components. However by using heat treatments (to eliminate unstable signals) or stimulating for controlled times (to exclude dominant slow components) the appropriate signal can be focused on when

Feldspar has been used extensively in OSL dating. Like quartz, it is a mineral that is widely available at the earth's surface, though it weathers more rapidly. The chemistry of feldspars has important implications with respect to how they are used in luminescence dating. They are aluminosilicates that have potassium (K), calcium (Ca) or sodium (Na) as end members. The presence of potassium in some of the feldspars is critical in that 40K isotopes that form part of the potassium constitute an internal source of radiation, in addition to any external radiation the grains may receive. Feldspars that do not have high potassium as part of their chemistry (for example, Ca and Na-feldspars), however, would not have this additional internal dose. Hence, for dating purposes, as will be discussed later, K-feldspars are normally separated from

For dating applications, feldspar has a number of attractive characteristics. One is that, in terms of emissions, feldspars have a higher brightness compared to quartz which means that it gives strong signals, allowing smaller doses to be measured. Secondly, the internal dose in feldspars that have a high potassium content constitutes a reliable radiation source that is immune from environmental changes that would affect external sources (for example, interstitial water). As a result, dose rates can be determined more accurately. The third advantage of feldspar, which will be discussed below, is that it can be stimulated using infrared stimulation. A major

unless very low dose rates are involved [36].

measured using the 375 °C peak [36].

using continuous wave stimulation.

other feldspars prior to conducting analysis.

**4.2. Feldspar**

*4.1.2. Quartz OSL properties*

40 Geochronology - Methods and Case Studies

Many K-feldspars of sedimentary origin have been shown to display natural signals with TL peaks at 280 °C and 330 °C [36]. With regards to emissions, some studies [for example, 34] have reported emissions from K-rich feldspars in the 390-440 nm range (violet to blue) while plagioclase feldspar emissions have been reported in the 550-560 nm range (blue-green). However, results from other studies [47] suggest a more complex pattern.

## *4.2.2. Feldspar OSL properties*

Luminescence from feldspars has been investigated using visible light stimulation. Earlier investigations used lasers (for example, 514.5 nm (green) from argon and 633 nm (red) from krypton) and observed the emissions at shorter wavelengths [1, 19]. For plagioclase feldspars, results showed that the spectra observed were similar to those from TL. Another study [48] that used a stimulation wavelength of 633 nm also showed that the emission was centred at 400 nm. OSL applications for dating using green light stimulation have been very limited and this has largely been because infrared stimulation (IRSL), as discussed below, was found to be a much better alternative. However, a study [49] that compared green light stimulation luminescence (GLSL) and IRSL data from feldspars from alluvial sediment showed results that suggested the signals had different thermal stabilities, with GLSL signals being more stable than IRSL signals at 10 °C. Stimulation of feldspars using a wide range of wavelengths in the range 380-1020 nm, apart from green and red, has also been demonstrated by [50].

### *4.2.3. Feldspar IRSL properties*

Feldspars can also be stimulated using the near infrared part of the electromagnetic spectrum (around 880 nm). Since the discovery of this stimulation peak [20], most dating research that uses optical stimulation of feldspars for sediment dating has been focussed on IRSL. A major advantage of this, as stated earlier, is that it leaves the rest of the visible part of the spectrum open for use in emission detection. Other investigations using IRSL include studies on finegrained sedimentary samples containing both plagioclase and K-feldspar that have also shown a major stimulation peak at 854 nm (1.45 eV) and another weaker one at 775 nm (1.6 eV) at room temperature [51]. Overall, these characteristics allow feldspars to be stimulated by light emitting diodes that have emission peaks at around 880±40 nm and these are widely available and cheap. Emission spectra of the K-feldspars stimulated using IRSL were reported by Huntley et al. [52] to show a dominant peak at 410 nm and another minor peak between 300 and 350 nm. Plagioclase feldspars, on the other hand, showed an emission peak at 570 nm. Krbetschek et al., [53] reported additional natural emission peaks for K-feldspar at 560 nm as well as at 280 and 700 nm.

**5. Paleodose and dose rate determination**

methods used to determine these variables are discussed.

**5.1. Paleodose determination**

*5.1.1. Additive dose method*

As outlined earlier, in order to calculate an age, the basic luminescence age equation divides the dose that has accumulated since the beginning of the event being dated (paleodose) by the rate at which the energy was accumulated (dose rate) (Equation 1). Hence, the two basic parameters that have to be determined are the paleodose and the dose rate. In this section

Luminescence Chronology http://dx.doi.org/10.5772/58554 43

Paleodose determination aims to ascertain the amount of energy that has accumulated in a dosimeter since the event being dated occurred. In luminescence dating this is generally the period that coincides with the time when the mineral grains were emptied of any previously accumulated energy (or zeroed). The dose is generally defined as the energy absorbed per kilogram of material and the unit used to measure it is the gray (Gy) where 1 Gy=1 joule per kg. For fired (or heated) materials, the period corresponding to zeroing would be the time when the samples were last heated to the appropriate temperature whereas, for unheated sediments, it would commonly be the time when they were last adequately exposed to the bleaching effects of the sun. The luminescence signal obtained from a sample from the field is referred to as the natural signal. In principle, in order to determine the natural signal in a given sample in Gy, artificial irradiation that is well calibrated is used to induce luminescence in the sample in a laboratory setting after which the natural signal is compared to the signals from the artificial irradiation. This allows the magnitude of the laboratory dose that induces a signal equivalent to that produced by the natural dose to be ascertained. That laboratory dose is referred to as the equivalent dose (De). As depicted in Fig. 6, there are two main methods that have been developed for determining De and these are the additive dose and the regenerative dose (or regeneration) methods [3, 29]. Other methods that have been used in the past include the partial bleach method [for example, 17]. However, these are no longer widely used.

As initially developed, to determine the equivalent dose using the additive dose method, samples from the field are typically separated into multiple aliquots. One set of aliquots would have the natural signal measured after which the other aliquots are irradiated with well calibrated incremental doses and then measured, with multiple aliquots being used for each dose level. The acquired signals are then plotted to give a dose-response curve that shows the luminescence signal against the laboratory irradiation (Fig. 6a). This is referred to as a growth curve and it is essentially a simulation of the evolution of the total dose had the sample experienced similar dose levels in its natural setting over time. Since the method employs multiple aliquots, signal normalisation is performed to correct for inter-aliquot variations by giving the aliquots a small test dose afterwards and then measuring the response. Such variations arise from differences in mass and grain sensitivity. To determine De using the additive dose method, the curve is extrapolated backwards to zero signal intensity and the De will be where the curve intercepts the horizontal (dose) axis (Fig. 6a). In TL dating, the

Feldspar samples irradiated with a laboratory dose have been shown to display an additional emission peak at 290 nm. For feldspar samples with a natural signal however, this 290 nm peak is absent. In dating studies, the 290 nm peak can be eliminated by preheating [36].

## **4.3. Calcite**

The mineral calcite has been demonstrated to have a TL signal with an emission at 570 nm [36] and attempts have been made to use the mineral for dating. However, calcite which often occurs in carbonate cave deposits has a limited environmental occurrence, which constrains its applicability. The luminescence from calcite is also complicated by the tendency of calcite to preferentially concentrate uranium. Thus, evaluation of the dose rate has to account for the disequilibrium of the decay chain of uranium. Notably, calcite dating using the uranium disequilibrium can be used to establish better chronologies than would be attainable using luminescence methods [36]. Hence, overall, the incentive to use calcite in luminescence studies has been low. Published attempts to use OSL emissions from calcite for dating include studies by Ugumori and Ikeya [54]. Nonetheless, as with the TL efforts, these have not translated into widespread applications.

#### **4.4. Zircon**

Zircon also has luminescence properties. Its properties as a dosimeter are particularly inter‐ esting because zircons naturally have a high concentration of uranium such that the internal dose that they receive is usually far greater than any radiation originating from the grain's exterior. As a result, the dose rate is very constant because it is not susceptible to variations that may be induced by changes in interstitial water content or burial depth [36]. A major methodological drawback, however, is that the uranium content is variable between grains and, as a result, measurements have to be made on individual grains. Zircon studies using TL include investigations by Huntley at al. [46] and Templer and Smith [55]. OSL analyses on zircons include studies by Smith [56].

In addition to variations in the uranium content between individual grains, zircon grains also have inhomogeneities in their crystal structures that develop during formation. Hence, for dating purposes, the luminescence from the internal dose is not easily comparable to the signal from the artificial dose administered in the laboratory as would be done with quartz or feldspar. To circumvent this problem, zircon dating often employs the autoregeneration method whereby the natural signal of the zircon is measured after which the grains are stored for several months [55, 56]. At the end of this storage period, the grains are measured again to determine the signal that has accrued from the internal dose since the initial measurement. This storage signal is then used to calibrate the natural signal from antiquity to determine an age [2].

## **5. Paleodose and dose rate determination**

As outlined earlier, in order to calculate an age, the basic luminescence age equation divides the dose that has accumulated since the beginning of the event being dated (paleodose) by the rate at which the energy was accumulated (dose rate) (Equation 1). Hence, the two basic parameters that have to be determined are the paleodose and the dose rate. In this section methods used to determine these variables are discussed.

#### **5.1. Paleodose determination**

Krbetschek et al., [53] reported additional natural emission peaks for K-feldspar at 560 nm as

Feldspar samples irradiated with a laboratory dose have been shown to display an additional emission peak at 290 nm. For feldspar samples with a natural signal however, this 290 nm peak

The mineral calcite has been demonstrated to have a TL signal with an emission at 570 nm [36] and attempts have been made to use the mineral for dating. However, calcite which often occurs in carbonate cave deposits has a limited environmental occurrence, which constrains its applicability. The luminescence from calcite is also complicated by the tendency of calcite to preferentially concentrate uranium. Thus, evaluation of the dose rate has to account for the disequilibrium of the decay chain of uranium. Notably, calcite dating using the uranium disequilibrium can be used to establish better chronologies than would be attainable using luminescence methods [36]. Hence, overall, the incentive to use calcite in luminescence studies has been low. Published attempts to use OSL emissions from calcite for dating include studies by Ugumori and Ikeya [54]. Nonetheless, as with the TL efforts, these have not translated into

Zircon also has luminescence properties. Its properties as a dosimeter are particularly inter‐ esting because zircons naturally have a high concentration of uranium such that the internal dose that they receive is usually far greater than any radiation originating from the grain's exterior. As a result, the dose rate is very constant because it is not susceptible to variations that may be induced by changes in interstitial water content or burial depth [36]. A major methodological drawback, however, is that the uranium content is variable between grains and, as a result, measurements have to be made on individual grains. Zircon studies using TL include investigations by Huntley at al. [46] and Templer and Smith [55]. OSL analyses on

In addition to variations in the uranium content between individual grains, zircon grains also have inhomogeneities in their crystal structures that develop during formation. Hence, for dating purposes, the luminescence from the internal dose is not easily comparable to the signal from the artificial dose administered in the laboratory as would be done with quartz or feldspar. To circumvent this problem, zircon dating often employs the autoregeneration method whereby the natural signal of the zircon is measured after which the grains are stored for several months [55, 56]. At the end of this storage period, the grains are measured again to determine the signal that has accrued from the internal dose since the initial measurement. This storage signal is then used to calibrate the natural signal from antiquity to determine an

is absent. In dating studies, the 290 nm peak can be eliminated by preheating [36].

well as at 280 and 700 nm.

42 Geochronology - Methods and Case Studies

widespread applications.

zircons include studies by Smith [56].

**4.4. Zircon**

age [2].

**4.3. Calcite**

Paleodose determination aims to ascertain the amount of energy that has accumulated in a dosimeter since the event being dated occurred. In luminescence dating this is generally the period that coincides with the time when the mineral grains were emptied of any previously accumulated energy (or zeroed). The dose is generally defined as the energy absorbed per kilogram of material and the unit used to measure it is the gray (Gy) where 1 Gy=1 joule per kg. For fired (or heated) materials, the period corresponding to zeroing would be the time when the samples were last heated to the appropriate temperature whereas, for unheated sediments, it would commonly be the time when they were last adequately exposed to the bleaching effects of the sun. The luminescence signal obtained from a sample from the field is referred to as the natural signal. In principle, in order to determine the natural signal in a given sample in Gy, artificial irradiation that is well calibrated is used to induce luminescence in the sample in a laboratory setting after which the natural signal is compared to the signals from the artificial irradiation. This allows the magnitude of the laboratory dose that induces a signal equivalent to that produced by the natural dose to be ascertained. That laboratory dose is referred to as the equivalent dose (De). As depicted in Fig. 6, there are two main methods that have been developed for determining De and these are the additive dose and the regenerative dose (or regeneration) methods [3, 29]. Other methods that have been used in the past include the partial bleach method [for example, 17]. However, these are no longer widely used.

#### *5.1.1. Additive dose method*

As initially developed, to determine the equivalent dose using the additive dose method, samples from the field are typically separated into multiple aliquots. One set of aliquots would have the natural signal measured after which the other aliquots are irradiated with well calibrated incremental doses and then measured, with multiple aliquots being used for each dose level. The acquired signals are then plotted to give a dose-response curve that shows the luminescence signal against the laboratory irradiation (Fig. 6a). This is referred to as a growth curve and it is essentially a simulation of the evolution of the total dose had the sample experienced similar dose levels in its natural setting over time. Since the method employs multiple aliquots, signal normalisation is performed to correct for inter-aliquot variations by giving the aliquots a small test dose afterwards and then measuring the response. Such variations arise from differences in mass and grain sensitivity. To determine De using the additive dose method, the curve is extrapolated backwards to zero signal intensity and the De will be where the curve intercepts the horizontal (dose) axis (Fig. 6a). In TL dating, the residual signal remaining after solar bleaching, would have to be taken into account as well. It is important to note that, for both feldspar and quartz, growth curves usually show a linear relationship between the luminescence signal and the dose in the early part of the curve. If high enough doses are administered, however, the signal tends to level off, indicating saturation or an exhaustion of the luminescence traps.

inter-aliquot signal variations. Signals from the doses are then plotted to give a regenerative growth curve. The equivalent dose would be obtained by interpolating the natural signal or

Luminescence Chronology http://dx.doi.org/10.5772/58554 45

As outlined above, when initially developed for TL dating, both the additive dose and regener‐ ative dose methods involved the use of multiple aliquots. With the introduction of OSL dating, though the possibility of using single aliquots was contemplated early [1], the practice of using multiple aliquots was adopted too [36]. An inherent assumption when using multiple ali‐ quots is that all the aliquots of a given sample respond similarly to the dose received. Howev‐ er, this is not what is observed; inter-aliqout variations arise from a number of sources and this necessitates the implementation of normalisation to try and address the differences. Quartz in particular appears susceptible to sensitivity changes. The different grains within any particu‐ lar aliquot would have experienced dissimilar histories within the natural environment and these can include differences in erosion and deposition cycles, episodes of heating from wildfires or other extreme conditions [3, 57]. The heating administered in the laboratory as part of the analysis (see further below) also results in sensitivity modifications that are dissimilar be‐ tween the grains, and ultimately between the aliquots. Variations in sample mass between aliquots may also be a reason for differences in behavior between aliquots. The net result of these disparities is that they give rise to differences in sensitivity that contribute to uncertainties in the calculated ages, even in cases where normalisation is used. Hence, to help address these aspects, there was a desire to develop a method that only utilised a single aliquot. There are a number of advantages associated with the use of single aliquots and these include [36]:

**i.** when using a single aliquot, to obtain De, the natural signal obtained will be compared

**ii.** only a very small amount of sample material is required. This is particularly impor‐

**iii.** normalisation to correct for inter-aliquot variations in numbers of grains on different

**iv.** the measurement protocol employed with single aliquots which entails preheating,

The introduction of single aliquot methods began with investigations that employed the additive dose approach. Efforts to use single aliquots for dating quartz had observed that there were sensitivity changes associated with repeated preheating of the aliquot that was required after every successive laboratory dose prior to conducting the luminescence measurements.

eliminated resulting in much higher precision for De.

tant for archeological samples that may be of limited size.

The following section examines procedures used in single aliquot methods.

*5.1.4. Single-Aliquot Regenerative-dose (SAR) protocol*

to the dose response curve of the same aliquot. Hence, inter-aliquot variations are

sample discs or variations in sensitivity is not necessary when using a single aliquot.

bleaching, and irradiation can all be conducted within most modern readers which are automated and it increases precision in addition to reducing analysis times.

the unknown signal into the curve (Fig. 6b).

*5.1.3. Single aliquot methods versus multiple aliquot methods*

**Figure 6.** Methods used to determine the equivalent dose (De). In the additive dose method (a), incremental doses are given to unbleached samples and measured. In the regenerative dose method (b), on the other hand, the samples are zeroed first before incremental doses are administered and measured (modified after [3]).

#### *5.1.2. Regenerative dose method*

The regeneration method differs from the additive dose method in that the samples are zeroed first before any laboratory dose is applied. For TL dating, previously fired artifacts, for example archeological materials or baked sediments, are zeroed by heating. For OSL samples, on the other hand, zeroing is achieved by exposure to sunlight. The zeroed aliquots are then given incremental doses as with the additive dose method, preferably with the doses being chosen to lie above and below the natural signal. Normalisation can also be conducted to correct for inter-aliquot signal variations. Signals from the doses are then plotted to give a regenerative growth curve. The equivalent dose would be obtained by interpolating the natural signal or the unknown signal into the curve (Fig. 6b).

### *5.1.3. Single aliquot methods versus multiple aliquot methods*

residual signal remaining after solar bleaching, would have to be taken into account as well. It is important to note that, for both feldspar and quartz, growth curves usually show a linear relationship between the luminescence signal and the dose in the early part of the curve. If high enough doses are administered, however, the signal tends to level off, indicating

**Figure 6.** Methods used to determine the equivalent dose (De). In the additive dose method (a), incremental doses are given to unbleached samples and measured. In the regenerative dose method (b), on the other hand, the samples are

The regeneration method differs from the additive dose method in that the samples are zeroed first before any laboratory dose is applied. For TL dating, previously fired artifacts, for example archeological materials or baked sediments, are zeroed by heating. For OSL samples, on the other hand, zeroing is achieved by exposure to sunlight. The zeroed aliquots are then given incremental doses as with the additive dose method, preferably with the doses being chosen to lie above and below the natural signal. Normalisation can also be conducted to correct for

zeroed first before incremental doses are administered and measured (modified after [3]).

*5.1.2. Regenerative dose method*

saturation or an exhaustion of the luminescence traps.

44 Geochronology - Methods and Case Studies

As outlined above, when initially developed for TL dating, both the additive dose and regener‐ ative dose methods involved the use of multiple aliquots. With the introduction of OSL dating, though the possibility of using single aliquots was contemplated early [1], the practice of using multiple aliquots was adopted too [36]. An inherent assumption when using multiple ali‐ quots is that all the aliquots of a given sample respond similarly to the dose received. Howev‐ er, this is not what is observed; inter-aliqout variations arise from a number of sources and this necessitates the implementation of normalisation to try and address the differences. Quartz in particular appears susceptible to sensitivity changes. The different grains within any particu‐ lar aliquot would have experienced dissimilar histories within the natural environment and these can include differences in erosion and deposition cycles, episodes of heating from wildfires or other extreme conditions [3, 57]. The heating administered in the laboratory as part of the analysis (see further below) also results in sensitivity modifications that are dissimilar be‐ tween the grains, and ultimately between the aliquots. Variations in sample mass between aliquots may also be a reason for differences in behavior between aliquots. The net result of these disparities is that they give rise to differences in sensitivity that contribute to uncertainties in the calculated ages, even in cases where normalisation is used. Hence, to help address these aspects, there was a desire to develop a method that only utilised a single aliquot. There are a number of advantages associated with the use of single aliquots and these include [36]:


The following section examines procedures used in single aliquot methods.

#### *5.1.4. Single-Aliquot Regenerative-dose (SAR) protocol*

The introduction of single aliquot methods began with investigations that employed the additive dose approach. Efforts to use single aliquots for dating quartz had observed that there were sensitivity changes associated with repeated preheating of the aliquot that was required after every successive laboratory dose prior to conducting the luminescence measurements. Preheating is necessary because it ensures that the distribution of trapped electrons after administering the laboratory dose is similar to that resulting from the natural dose [58]. To correct for the sensitivity changes using the single aliquot additive dose method on feldspars, Duller [21] monitored an additional aliquot for sensitivity changes. Galloway [22] improved on Duller's [21] approach by correcting the luminescence signals using a least squares fitting approach using measurements made on the same feldspar aliquot subsequent to the additive dose irradiation and measurements. This modification by Galloway [22] essentially trans‐ formed the method into a bona-fide single aliquot protocol. Alternatively, using the additive dose method on quartz, Murray et al. [27] devised a correction procedure that included additional preheat and stimulation cycles without any additional dose being given, the results of which were used to formulate a decay constant. The constant was then used to correct the data from the additive dose measurements.

Since its original introduction, the SAR protocol has undergone some minor modifications (Table 1) [26, 27] and, over the last 10-15 years, the protocol has emerged as the preferred method for routine dating of both sedimentary and fired materials using both quartz and

Luminescence Chronology http://dx.doi.org/10.5772/58554 47

Measurement protocols used for analyzing single aliquots can also be adapted for determining paleodoses using individual mineral grains. Special equipment for loading and analyzing single grains have been developed that allow multiple sand-sized grains (up to 250 μm) to be mounted individually in a regular array that permits automated measurement [25]. In this format, thousands of grains can be analysed in a relatively short period of time. Analyzing single grains makes it possible to recognize differences in behaviour between grains from a given sample. For instance, it enables the identification of grains that have been bleached to different levels prior to burial because such grains will yield different paleodoses. For that reason, single grain analysis is commonly used to identify partial bleaching in sediments,

Once determined, paleodose results can be presented as growth curves as in Fig. 6, with the horizontal axis showing the dose given and the vertical axis showing the lumines‐ cence signal (or nor normalised signal). However, for SAR procedures where multiple determinations can be made on the same sample that yield a range of equivalent dose values, individual growth curves do not convey all pertinent statistical information. Alternative means that can be used to provide some statistical information include frequency histograms. However, histograms do not provide information on precision [64]. To address that aspect, paleodose data in luminescence dating are now commonly presented as radial plots that show both the number of De determinations made as well as the relative

In the example of a radial plot given in Fig. 7, each dot represents an equivalent dose that was determined for a single grain using the SAR approach for a total of 204 grains. Had multiple grain aliquots been used, each data point would denote an aliquot. Any straight line that passes radially through the origin represents a line of constant dose. The horizontal axis at the bottom shows the relative error associated with each paleodose calculation, with the precision increasing from left to right. The shaded area in the plot [Fig. 7] denotes the 2 sigma error band centred at the equivalent dose of 25 Gy, representing the weighted-mean of all 204 data points. Thus, the band represents a 95% confidence level on all aliquots that were analysed. The number of data points that lie outside the shaded area are reflected by the overdispersion of the data and can be calculated [65]. The equivalent dose scale on the right is a logarithmic scale. Overall, radial plots enable investigators to visualize the dose distributions, allowing appropriate data to be targeted for further analysis. For instance, they permit investigators to differentiate between variations in equivalent dose that arise from the bleaching history and local dose rates from those that are caused by intrinsic differ‐

feldspar. Further methodological details can be found in [26, 27, 62].

*5.1.5. Single grain analysis*

especially in fluvial deposits [63].

*5.1.6. Presentation of luminescence paleodose data*

precision associated with each determination [65].

Attempts to use single aliquots with the regeneration method on feldspar [21] and on quartz [58] had initially concluded that it would not be possible because of sensitivity changes [58]. However, to address those sensitivity changes, Mejdahl and Bøtter-Jensen [60] proposed the single-aliquot/ regeneration–added dose protocol (SARA) to date previously heated materials and Murray [61] later used the same method on unheated sediments. However, with SARA, at least two aliquots are required [58]. Subsequently, a truly single aliquot regenerative dose (SAR) protocol was introduced by Murray and Roberts [58] using sedimentary quartz from Australia and, with that SAR method, corrections for the sensitivity changes were made by monitoring the 110° C TL signal measured immediately after administering a regeneration dose. A major methodological breakthrough was made when a streamlined version of the method proposed by Murray and Roberts [58] was put forward by Murray and Wintle [26] whereby sensitivity changes were monitored using a test dose whose signal was also meas‐ ured. As initially proposed by Murray and Wintle [26], this SAR protocol essentially entailed the following sequence:


*D*i is the regeneration dose which gives signal *L*<sup>i</sup> whereas *D*t is the test dose which gives signal *T*<sup>i</sup> . The observed signals, *L*<sup>i</sup> and *T*<sup>i</sup> are then used to plot a curve of Li /Ti vs. the regeneration dose, *D*.

a For the natural sample, 1=0 and D0=0 Gy.

bLater modifications stimulate the sample for 40s (instead of 100s) and an additional step to optically stimulate the sample for 40s at a temperature above the preheat temperature is conducted after step 6 to reduce recuperation [see 26, 62].

**Table 1.** Steps in the SAR protocol as originally proposed by Murray and Wintle [26].

Since its original introduction, the SAR protocol has undergone some minor modifications (Table 1) [26, 27] and, over the last 10-15 years, the protocol has emerged as the preferred method for routine dating of both sedimentary and fired materials using both quartz and feldspar. Further methodological details can be found in [26, 27, 62].

## *5.1.5. Single grain analysis*

Preheating is necessary because it ensures that the distribution of trapped electrons after administering the laboratory dose is similar to that resulting from the natural dose [58]. To correct for the sensitivity changes using the single aliquot additive dose method on feldspars, Duller [21] monitored an additional aliquot for sensitivity changes. Galloway [22] improved on Duller's [21] approach by correcting the luminescence signals using a least squares fitting approach using measurements made on the same feldspar aliquot subsequent to the additive dose irradiation and measurements. This modification by Galloway [22] essentially trans‐ formed the method into a bona-fide single aliquot protocol. Alternatively, using the additive dose method on quartz, Murray et al. [27] devised a correction procedure that included additional preheat and stimulation cycles without any additional dose being given, the results of which were used to formulate a decay constant. The constant was then used to correct the

Attempts to use single aliquots with the regeneration method on feldspar [21] and on quartz [58] had initially concluded that it would not be possible because of sensitivity changes [58]. However, to address those sensitivity changes, Mejdahl and Bøtter-Jensen [60] proposed the single-aliquot/ regeneration–added dose protocol (SARA) to date previously heated materials and Murray [61] later used the same method on unheated sediments. However, with SARA, at least two aliquots are required [58]. Subsequently, a truly single aliquot regenerative dose (SAR) protocol was introduced by Murray and Roberts [58] using sedimentary quartz from Australia and, with that SAR method, corrections for the sensitivity changes were made by monitoring the 110° C TL signal measured immediately after administering a regeneration dose. A major methodological breakthrough was made when a streamlined version of the method proposed by Murray and Roberts [58] was put forward by Murray and Wintle [26] whereby sensitivity changes were monitored using a test dose whose signal was also meas‐ ured. As initially proposed by Murray and Wintle [26], this SAR protocol essentially entailed

**Step Treatmenta Observed**

vs. the regeneration dose, *D*.

**Table 1.** Steps in the SAR protocol as originally proposed by Murray and Wintle [26].

whereas *D*t is the test dose which gives signal *T*<sup>i</sup>

bLater modifications stimulate the sample for 40s (instead of 100s) and an additional step to optically stimulate the sample for

40s at a temperature above the preheat temperature is conducted after step 6 to reduce recuperation [see 26, 62].

. The observed signals, *L*<sup>i</sup>

and *T*<sup>i</sup>

 Irradiate sample with dose, *Di* - Preheat sample (160-300°) for 10s - Stimulate sample for 100s at 125 °C *Li* Irradiate sample with test dose, *Dt* - Heat to 160°C - Stimulate for 100 s at 125°C *Ti* bReturn to 1 and repeat sequence -

data from the additive dose measurements.

46 Geochronology - Methods and Case Studies

the following sequence:

is the regeneration dose which gives signal *L*<sup>i</sup>

For the natural sample, 1=0 and D0=0 Gy.

/Ti

are then used to plot a curve of Li

*D*i

a

Measurement protocols used for analyzing single aliquots can also be adapted for determining paleodoses using individual mineral grains. Special equipment for loading and analyzing single grains have been developed that allow multiple sand-sized grains (up to 250 μm) to be mounted individually in a regular array that permits automated measurement [25]. In this format, thousands of grains can be analysed in a relatively short period of time. Analyzing single grains makes it possible to recognize differences in behaviour between grains from a given sample. For instance, it enables the identification of grains that have been bleached to different levels prior to burial because such grains will yield different paleodoses. For that reason, single grain analysis is commonly used to identify partial bleaching in sediments, especially in fluvial deposits [63].

#### *5.1.6. Presentation of luminescence paleodose data*

Once determined, paleodose results can be presented as growth curves as in Fig. 6, with the horizontal axis showing the dose given and the vertical axis showing the lumines‐ cence signal (or nor normalised signal). However, for SAR procedures where multiple determinations can be made on the same sample that yield a range of equivalent dose values, individual growth curves do not convey all pertinent statistical information. Alternative means that can be used to provide some statistical information include frequency histograms. However, histograms do not provide information on precision [64]. To address that aspect, paleodose data in luminescence dating are now commonly presented as radial plots that show both the number of De determinations made as well as the relative precision associated with each determination [65].

In the example of a radial plot given in Fig. 7, each dot represents an equivalent dose that was determined for a single grain using the SAR approach for a total of 204 grains. Had multiple grain aliquots been used, each data point would denote an aliquot. Any straight line that passes radially through the origin represents a line of constant dose. The horizontal axis at the bottom shows the relative error associated with each paleodose calculation, with the precision increasing from left to right. The shaded area in the plot [Fig. 7] denotes the 2 sigma error band centred at the equivalent dose of 25 Gy, representing the weighted-mean of all 204 data points. Thus, the band represents a 95% confidence level on all aliquots that were analysed. The number of data points that lie outside the shaded area are reflected by the overdispersion of the data and can be calculated [65]. The equivalent dose scale on the right is a logarithmic scale. Overall, radial plots enable investigators to visualize the dose distributions, allowing appropriate data to be targeted for further analysis. For instance, they permit investigators to differentiate between variations in equivalent dose that arise from the bleaching history and local dose rates from those that are caused by intrinsic differ‐

High-resolution gamma ray spectrometry and alpha spectrometry are alternative methods that can be used to measure activities of individual radionuclides, including those from the thorium and uranium decay chains. Hence, they can be used to measure dose rates whether there is disequilibrium or not. However, extended measurement times could be required and the

Luminescence Chronology http://dx.doi.org/10.5772/58554 49

Another approach that has been employed to minimise effects of disequilibrium of the thorium and uranium chains is to measure the contribution of uranium and thorium using thick source alpha counting (TSAC) and then use AAS, XRF, FPD or ICP-MS to measure potassium. An alternative procedure is to measure the alpha contribution only using TSAC and determine the beta contribution using a beta particle counter. Whenever possible, the gamma dose rate should be measured on site, especially in settings where there is uncertainty about the uniformity of the dose within a 30 cm radius of the sample. The recent development of powerful portable gamma–ray spectrometers has made such field measurements relatively practical [67]. Other field measurement options include the use of very sensitive synthetic dosimeters such as α-Al2O3:C that only need to be buried in the field for a few weeks at the

As described earlier, cosmic-rays also contribute to the dose received by the mineral grains. This contribution is usually minor but in settings where the contributions from the radionu‐ clides are low, cosmic contribution can be significant. Formulae for evaluating cosmic ray

The effect of moisture content in the natural setting of the material being dated is to absorb part of the dose that should normally reach the grain. Consequently, when calculating dose rates, the levels of moisture content have to be noted and factored into the determination. In

Improvements in luminescence dating instrumentation and dating protocols have reached a stage where current OSL methods can be used to date samples deposited as recently as the last few decades [for example, 68]. The single grain dating method in particular can yield dates with very high precision. Prerequisites for dating such young samples include appropriate bleaching to remove all previously acquired luminescence energy prior to burial as well as the availability of grains that have high luminescence sensitivity [28]. To optimize the measure‐ ments and increase precision, thermal charge transfer is minimized in order to increase the signal size. Such advances mean that luminescence methods can now produce ages from the last 300 years that are more reliable than those attainable using radiocarbon methods. Cali‐ brated radiocarbon ages from the same period have comparatively larger uncertainties due to

With regards to the upper age limit attainable using luminescence methods, empirical studies have demonstrated that the storage of luminescence energy through the trapping of electrons is not a process that can continue indefinitely within any given material because, eventually, the traps do get exhausted [29, 31]. For that reason, luminescence growth curves are often

most, as opposed to earlier dosimeters that required burial for up to a year [67].

essence, dry sediment will experience a higher dose rate than moist sediment.

contribution have been provided by Prescott and Hutton [30].

**5.3. Lower and upper limits for luminescence ages**

fluctuations in 14C production [69].

equipment can be costly [29].

**Figure 7.** Radial plot of equivalent dose (De) estimates for 204 single grains from Sample M3T 61.2 m from Lake Mun‐ go, Australia, obtained using the SAR protocol (redrawn from [66]). Each data point represents a single grain. Preci‐ sion, shown on the x-axis, is simply the reciprocal of the standard error. Hence, as relative error decreases as one moves to the right, the precision increases.

ences in luminescence sensitivity of the measured aliquots [64]. Accordingly, radial plots have often been used to identify poorly bleached samples, especially when used to analyze individual grain paleodoses. Once the paleodose is determined, statistical models used to calculate the age include the central age model, which calculates the weighted mean equiva‐ lent dose from a set of data points taking into account the overdispersion above that associ‐ ated with measurement errors. Similarly, the minimum age model gives the equivalent dose associated with the population of aliquots (or grains for single grain dating) with the lowest dose. Details on these models can be found in [65].

#### **5.2. Dose rate determination**

Apart from the paleodose, an additional element that has to be determined before a lumines‐ cence age can be calculated is the dose rate. As outlined earlier, the main isotopes responsible for the accumulation of luminescence energy in natural setting are the isotopes of the uranium and thorium decay chains, potassium and rubidium as well as cosmic radiation. Hence, the total contribution of these effects have to be evaluated. There are several methods that can be used to determine the total dose rate. One is the concentration approach and with this method, concentrations of thorium, uranium, potassium and rubidium in representative samples are measured using an appropriate technique such as neutron activation, atomic absorption spectroscopy (AAS), X-ray fluorescence (XRF), flame photometric detection (FPD) or induc‐ tively coupled plasma mass spectrometry (ICP-MS). However, measurement of uranium and thorium using this approach can be inaccurate if there is disequilibrium [29].

High-resolution gamma ray spectrometry and alpha spectrometry are alternative methods that can be used to measure activities of individual radionuclides, including those from the thorium and uranium decay chains. Hence, they can be used to measure dose rates whether there is disequilibrium or not. However, extended measurement times could be required and the equipment can be costly [29].

Another approach that has been employed to minimise effects of disequilibrium of the thorium and uranium chains is to measure the contribution of uranium and thorium using thick source alpha counting (TSAC) and then use AAS, XRF, FPD or ICP-MS to measure potassium. An alternative procedure is to measure the alpha contribution only using TSAC and determine the beta contribution using a beta particle counter. Whenever possible, the gamma dose rate should be measured on site, especially in settings where there is uncertainty about the uniformity of the dose within a 30 cm radius of the sample. The recent development of powerful portable gamma–ray spectrometers has made such field measurements relatively practical [67]. Other field measurement options include the use of very sensitive synthetic dosimeters such as α-Al2O3:C that only need to be buried in the field for a few weeks at the most, as opposed to earlier dosimeters that required burial for up to a year [67].

As described earlier, cosmic-rays also contribute to the dose received by the mineral grains. This contribution is usually minor but in settings where the contributions from the radionu‐ clides are low, cosmic contribution can be significant. Formulae for evaluating cosmic ray contribution have been provided by Prescott and Hutton [30].

The effect of moisture content in the natural setting of the material being dated is to absorb part of the dose that should normally reach the grain. Consequently, when calculating dose rates, the levels of moisture content have to be noted and factored into the determination. In essence, dry sediment will experience a higher dose rate than moist sediment.

#### **5.3. Lower and upper limits for luminescence ages**

ences in luminescence sensitivity of the measured aliquots [64]. Accordingly, radial plots have often been used to identify poorly bleached samples, especially when used to analyze individual grain paleodoses. Once the paleodose is determined, statistical models used to calculate the age include the central age model, which calculates the weighted mean equiva‐ lent dose from a set of data points taking into account the overdispersion above that associ‐ ated with measurement errors. Similarly, the minimum age model gives the equivalent dose associated with the population of aliquots (or grains for single grain dating) with the lowest

**Figure 7.** Radial plot of equivalent dose (De) estimates for 204 single grains from Sample M3T 61.2 m from Lake Mun‐ go, Australia, obtained using the SAR protocol (redrawn from [66]). Each data point represents a single grain. Preci‐ sion, shown on the x-axis, is simply the reciprocal of the standard error. Hence, as relative error decreases as one

Apart from the paleodose, an additional element that has to be determined before a lumines‐ cence age can be calculated is the dose rate. As outlined earlier, the main isotopes responsible for the accumulation of luminescence energy in natural setting are the isotopes of the uranium and thorium decay chains, potassium and rubidium as well as cosmic radiation. Hence, the total contribution of these effects have to be evaluated. There are several methods that can be used to determine the total dose rate. One is the concentration approach and with this method, concentrations of thorium, uranium, potassium and rubidium in representative samples are measured using an appropriate technique such as neutron activation, atomic absorption spectroscopy (AAS), X-ray fluorescence (XRF), flame photometric detection (FPD) or induc‐ tively coupled plasma mass spectrometry (ICP-MS). However, measurement of uranium and

thorium using this approach can be inaccurate if there is disequilibrium [29].

dose. Details on these models can be found in [65].

**5.2. Dose rate determination**

moves to the right, the precision increases.

48 Geochronology - Methods and Case Studies

Improvements in luminescence dating instrumentation and dating protocols have reached a stage where current OSL methods can be used to date samples deposited as recently as the last few decades [for example, 68]. The single grain dating method in particular can yield dates with very high precision. Prerequisites for dating such young samples include appropriate bleaching to remove all previously acquired luminescence energy prior to burial as well as the availability of grains that have high luminescence sensitivity [28]. To optimize the measure‐ ments and increase precision, thermal charge transfer is minimized in order to increase the signal size. Such advances mean that luminescence methods can now produce ages from the last 300 years that are more reliable than those attainable using radiocarbon methods. Cali‐ brated radiocarbon ages from the same period have comparatively larger uncertainties due to fluctuations in 14C production [69].

With regards to the upper age limit attainable using luminescence methods, empirical studies have demonstrated that the storage of luminescence energy through the trapping of electrons is not a process that can continue indefinitely within any given material because, eventually, the traps do get exhausted [29, 31]. For that reason, luminescence growth curves are often

has an upper limit beyond which the traps become saturated. This dose level places a limit beyond which the method cannot be applied. For quartz this dose is about 100-150 Gy. The solution adopted in some studies when working with older ages [4, 5] is to model the lumi‐ nescence growth using a combination of the saturating function and a linear function to give a curve as in Fig. 8b [28]. Ages in excess of 200-400 ka have been reported from quartz using this method [for example, 5]. Feldspar ages in excess of 100 ka obtained using IRSL signals have also been reported in a number of studies [71]. However, at such dose levels (>100-200 Gy) the growth curve is no longer linear such that correction for anomalous fading using

Overall, however, the indication is that for both feldspars and quartz, there is an upper age limit beyond which accurate OSL ages cannot be obtained. The exact limit will ultimately

Having looked at the principles of luminescence dating and the methodological aspects in the preceding sections, a topic that needs to be discussed are the types of materials that can be dated using luminescence methods. Prior to exploring that topic, however, it is imperative to examine aspects of the dating method that have a bearing on the types of materials one can date using luminescence techniques. Accordingly, this section examines the importance of

For technical reasons, luminescence dating is usually conducted on mineral grains in two broad grain size categories: coarse grains and fine grains. In the coarse grain method, grains in the fine sand category are separated and analyzed. Such grains would normally receive dose from alpha, beta, and gamma radiation. However, because alpha particles can only penetrate the outer 25 μm of the grain, the coarse grains are typically etched using hydrofluoric acid (HF) to remove the outer ring of the grain that experienced alpha radiation. As a result, methods that date coarse grains are often referred to as inclusion dating methods [33]. For quartz inclusion dating of pottery, particles in the size range 90-125 μm are usually extracted [2]. For sediment dating, on the other hand, coarse grains representing the modal grain size are normally extracted. For eolian dunes for instance, grains in the size range 150-180 μm are usually preferred [3]. In feldspar inclusion dating, similar procedures are used to extract grains in the fine to medium sand size range and etching can also be used to remove alpha particle effects. As a result, when dating coarse grains, the annual dose is calculated by evaluating contributions from beta, gamma and cosmic radiation and the age equation is modified to:

**6. Sample collection and preparation: some practical aspects**

grain size after which sample collection and preparation methods are discussed.

*Luminescence Age* <sup>=</sup> *Paleodose*

0.90*D<sup>β</sup>* + *D<sup>γ</sup>* + *DC*

(2)

Luminescence Chronology http://dx.doi.org/10.5772/58554 51

standard procedures [43] becomes problematic.

depend on the dose rate.

**6.1. Importance of grain size**

**Figure 8.** Illustration of quartz growth curves constructed using the SAR protocol (redrawn from [3]). In (a) the curve is constructed using a saturating exponential of the form *I* (*D*)= *<sup>I</sup>*<sup>0</sup> (*<sup>I</sup>* - *exp*-*D*/*D*0) where I(D) denotes the OSL signal com‐ mensurate with the dose D and I0 is the maximum OSL intensity that can be obtained. The variable D0 determines the shape of the curve and in curve (a), D0 is 55 Gy. Because curve (a) appeared to underestimate some older ages, in (b) the saturating function from (a) is combined with a linear function [3].

represented by saturating exponential functions and the form of the curve determines the maximum luminescence signal that can be stored in the mineral grain beyond which no dose can accumulate efficiently. For quartz for instance, Fig. 8a shows the fast component expressed as a single saturating exponential and, beyond a certain dose, the curve flattens. Dating of quartz samples close to saturation (>100 Gy) has been demonstrated in some studies to yield OSL ages that underestimate the true age by up to 10% [for example, 70]. In reality, the absolute age limit will be determined by the dose rate, with low dose rates having higher age limits. The curve in Fig. 8a also shows that the growth curve is relatively linear for low doses, and ages from the linear part of the growth curve have been shown in numerous studies to be comparable to those obtained using other dating methods, for example, radiocarbon. Gener‐ ally, however, these results indicate that the storage of luminescence dose in mineral grains has an upper limit beyond which the traps become saturated. This dose level places a limit beyond which the method cannot be applied. For quartz this dose is about 100-150 Gy. The solution adopted in some studies when working with older ages [4, 5] is to model the lumi‐ nescence growth using a combination of the saturating function and a linear function to give a curve as in Fig. 8b [28]. Ages in excess of 200-400 ka have been reported from quartz using this method [for example, 5]. Feldspar ages in excess of 100 ka obtained using IRSL signals have also been reported in a number of studies [71]. However, at such dose levels (>100-200 Gy) the growth curve is no longer linear such that correction for anomalous fading using standard procedures [43] becomes problematic.

Overall, however, the indication is that for both feldspars and quartz, there is an upper age limit beyond which accurate OSL ages cannot be obtained. The exact limit will ultimately depend on the dose rate.

## **6. Sample collection and preparation: some practical aspects**

Having looked at the principles of luminescence dating and the methodological aspects in the preceding sections, a topic that needs to be discussed are the types of materials that can be dated using luminescence methods. Prior to exploring that topic, however, it is imperative to examine aspects of the dating method that have a bearing on the types of materials one can date using luminescence techniques. Accordingly, this section examines the importance of grain size after which sample collection and preparation methods are discussed.

#### **6.1. Importance of grain size**

represented by saturating exponential functions and the form of the curve determines the maximum luminescence signal that can be stored in the mineral grain beyond which no dose can accumulate efficiently. For quartz for instance, Fig. 8a shows the fast component expressed as a single saturating exponential and, beyond a certain dose, the curve flattens. Dating of quartz samples close to saturation (>100 Gy) has been demonstrated in some studies to yield OSL ages that underestimate the true age by up to 10% [for example, 70]. In reality, the absolute age limit will be determined by the dose rate, with low dose rates having higher age limits. The curve in Fig. 8a also shows that the growth curve is relatively linear for low doses, and ages from the linear part of the growth curve have been shown in numerous studies to be comparable to those obtained using other dating methods, for example, radiocarbon. Gener‐ ally, however, these results indicate that the storage of luminescence dose in mineral grains

the saturating function from (a) is combined with a linear function [3].

50 Geochronology - Methods and Case Studies

**Figure 8.** Illustration of quartz growth curves constructed using the SAR protocol (redrawn from [3]). In (a) the curve is constructed using a saturating exponential of the form *I* (*D*)= *<sup>I</sup>*<sup>0</sup> (*<sup>I</sup>* - *exp*-*D*/*D*0) where I(D) denotes the OSL signal com‐ mensurate with the dose D and I0 is the maximum OSL intensity that can be obtained. The variable D0 determines the shape of the curve and in curve (a), D0 is 55 Gy. Because curve (a) appeared to underestimate some older ages, in (b) For technical reasons, luminescence dating is usually conducted on mineral grains in two broad grain size categories: coarse grains and fine grains. In the coarse grain method, grains in the fine sand category are separated and analyzed. Such grains would normally receive dose from alpha, beta, and gamma radiation. However, because alpha particles can only penetrate the outer 25 μm of the grain, the coarse grains are typically etched using hydrofluoric acid (HF) to remove the outer ring of the grain that experienced alpha radiation. As a result, methods that date coarse grains are often referred to as inclusion dating methods [33]. For quartz inclusion dating of pottery, particles in the size range 90-125 μm are usually extracted [2]. For sediment dating, on the other hand, coarse grains representing the modal grain size are normally extracted. For eolian dunes for instance, grains in the size range 150-180 μm are usually preferred [3]. In feldspar inclusion dating, similar procedures are used to extract grains in the fine to medium sand size range and etching can also be used to remove alpha particle effects. As a result, when dating coarse grains, the annual dose is calculated by evaluating contributions from beta, gamma and cosmic radiation and the age equation is modified to:

$$\text{LuminousScore } Age = \frac{\text{Paledose}}{0.90D\_{\text{f}} + D\_{\text{y}} + D\_{\text{C}}} \tag{2}$$

where Dβ, Dγ and Dc refer to the beta, gamma and cosmic ray dose contributions respectively [29, 31]. When the dose rates are expressed as annual rates, the age will be given in years. The beta contribution in Equation 2 is factored by 0.9 to account for the attenuation due to grain size as well as the etching that removes some parts of the grain that received the beta dose [29, 31]. Exceptions to Equation 2 are only those cases where the quartz itself has some uranium and thorium within it. Also for feldspar dating, where K-feldspars are usually isolated from Na-feldspars and dated separately, an additional parameter would also have to be included in the denominator in Equation 2 to account for the internal dose from potassium.

In fine-grain dating procedures, dated materials are often not separated into mineral specific concentrations. Rather, polymineral grains with diameter in the range 4-11μm (fine silt) are extracted and analysed. Because of their size, alpha particles can penetrate these particles completely and alpha contribution has to be taken into account when calculating the dose rate. As a result, general age equation is modified to:

$$\text{Luminousence } Age = \frac{\text{Paleodose}}{\text{k}D\_a + D\_\beta + D\_\gamma + D\_C} \tag{3}$$

preferably airtight seal. At the laboratory, sediment at the ends of the pipe is removed and the sample for luminescence measurements is taken from the central portion of the pipe. In places where depositional units are not directly accessible, drilling has also been used to reach targeted units and methods that can be used for such sampling are reviewed in [73].

Luminescence Chronology http://dx.doi.org/10.5772/58554 53

Once the collected sample has reached the laboratory, it has to be prepared for analysis and a number of procedures have been established depending on the material targeted for analysis. As outlined in Section 6.1, samples for luminescence analysis are either measured as fine or coarse grains. It was also indicated that for coarse grains, analysis is usually made on pure mineral separates (for example quartz, or K-feldspars) whereas for fine grains, either poly‐ mineral fractions or fine grained quartz extracts can be used. When dating fine grained sediments such as loess, for instance it would be preferable to use the fine grain procedure whereas for eolian dune sands, the coarse grain approach would be more appropriate. In sediments that have equal components of fine grains and coarse grains, dating both fractions

For coarse grained materials, quartz or feldspar are typically separated using a heavy liquid such as sodium polytungstate solution. Fig. 9 is a flowchart of a separating procedure for quartz and feldspar using heavy liquids with successively lighter or heavier specific densities. Prior to separating minerals using the heavy liquid, carbonates and organic materials that often occur in sediments, and are usually introduced during the postdepositional phase of the deposit, are removed using hydrochloric acid and hydrogen peroxide respectively. Additional

For fine grains, carbonates and organic materials are also removed using dilute hydrochloric acid and hydrogen peroxide respectively. Dilute sodium oxalate solution is then added to prevent flocculation of the particles after which appropriate grain sizes are separated using a sedimentation column. Sedimentation columns employ Stoke's law which states that the velocity of a particle's sedimentation in a fluid also depends on the size of the particle. Hence, by extracting sediment from the column after a predetermined time following agitation permits a desired grain size to be isolated. As indicated earlier, while fine grains can be analysed as polymineralic fractions, some studies extract pure quartz from the fine grains by digesting the feldspars using fluorosilicic acid [for example, 76]. Detailed information on separation procedures of fine grains for luminescence dating have been provided by [77]. Once separated, coarse grains (quartz or feldspar) or the fine grains (mixed polymineralic or pure quartz) are mounted on appropriate sample discs prior to analysis. Typically, the measurement discs are made of stainless steel or aluminum and measure around 10 mm in diameter and 0.5 mm thick [29]. To mount coarse grains, a monolayer of the sample is deposited on the disc in dry form. Silicone oil can be used to help the sample grains adhere to the disc. For fine grain particles, on the other hand, once the desired grain sizes have been isolated, these are mounted on the discs in a solution of acetone, ethanol or water which is then allowed to evaporate, leaving the sediment deposited on the disc. In either case, the disc will now be

would provide a good mechanism for comparison as a cross-checking method.

information on separation procedures of coarse grains can be obtained from [74, 75].

*6.2.2. Sample preparation prior to measurement*

ready for analysis.

where Dα, Dβ, Dγ and Dc refer to the alpha, beta, gamma and cosmic ray dose contributions respectively. Alternatively, in some studies, fine grained quartz is extracted from the poly‐ mineral mixture of fine grains using procedures outlined below and analyzed separately. There are some studies that have used intermediate size grains in the range 35-63 μm, which is coarse silt [for example, 72]. For such studies, alpha particle contribution also has to be taken into account when calculating the dose rate.

#### **6.2. Sample collection and laboratory procedures**

#### *6.2.1. Sample collection*

When collecting samples for dating using luminescence methods, a primary requirement that has to be fulfilled is that the mineral grains to be analyzed should not be exposed to light from the time they are initially buried up until the point they are exposed to the stimulating source during measurement. This restriction necessitates the adoption of special precautions during sample collection and a number of procedures have been devised over the years. For archaeological artifacts this may entail extracting material from the interior of the artifact using a drill under safe light conditions. For sediments, however, measures taken include sampling at night [29, 64]. Not only is this an inconvenient method be‐ cause of the need to work in the dark, but there is a greater risk of accidentally exposing the sample to light during the collection [73]. For sediments that are adequately firm, an alternative approach is to cut out a block of sample from the depositional unit being investigated and transfer it to the laboratory where a portion for analysis is extracted from the sample's interior [29, 64]. A sampling approach that has become a method of choice because of its ease and relative guarantee for retrieval of an unadulterated sample is to insert an opaque pipe made of metal, PVC or ABS plastic into a freshly prepared profile face [29]. Once retrieved, the pipe is immediately capped on both ends with an opaque and preferably airtight seal. At the laboratory, sediment at the ends of the pipe is removed and the sample for luminescence measurements is taken from the central portion of the pipe. In places where depositional units are not directly accessible, drilling has also been used to reach targeted units and methods that can be used for such sampling are reviewed in [73].

### *6.2.2. Sample preparation prior to measurement*

where Dβ, Dγ and Dc refer to the beta, gamma and cosmic ray dose contributions respectively [29, 31]. When the dose rates are expressed as annual rates, the age will be given in years. The beta contribution in Equation 2 is factored by 0.9 to account for the attenuation due to grain size as well as the etching that removes some parts of the grain that received the beta dose [29, 31]. Exceptions to Equation 2 are only those cases where the quartz itself has some uranium and thorium within it. Also for feldspar dating, where K-feldspars are usually isolated from Na-feldspars and dated separately, an additional parameter would also have to be included

In fine-grain dating procedures, dated materials are often not separated into mineral specific concentrations. Rather, polymineral grains with diameter in the range 4-11μm (fine silt) are extracted and analysed. Because of their size, alpha particles can penetrate these particles completely and alpha contribution has to be taken into account when calculating the dose rate.

where Dα, Dβ, Dγ and Dc refer to the alpha, beta, gamma and cosmic ray dose contributions respectively. Alternatively, in some studies, fine grained quartz is extracted from the poly‐ mineral mixture of fine grains using procedures outlined below and analyzed separately. There are some studies that have used intermediate size grains in the range 35-63 μm, which is coarse silt [for example, 72]. For such studies, alpha particle contribution also has to be taken

When collecting samples for dating using luminescence methods, a primary requirement that has to be fulfilled is that the mineral grains to be analyzed should not be exposed to light from the time they are initially buried up until the point they are exposed to the stimulating source during measurement. This restriction necessitates the adoption of special precautions during sample collection and a number of procedures have been devised over the years. For archaeological artifacts this may entail extracting material from the interior of the artifact using a drill under safe light conditions. For sediments, however, measures taken include sampling at night [29, 64]. Not only is this an inconvenient method be‐ cause of the need to work in the dark, but there is a greater risk of accidentally exposing the sample to light during the collection [73]. For sediments that are adequately firm, an alternative approach is to cut out a block of sample from the depositional unit being investigated and transfer it to the laboratory where a portion for analysis is extracted from the sample's interior [29, 64]. A sampling approach that has become a method of choice because of its ease and relative guarantee for retrieval of an unadulterated sample is to insert an opaque pipe made of metal, PVC or ABS plastic into a freshly prepared profile face [29]. Once retrieved, the pipe is immediately capped on both ends with an opaque and

k*D<sup>α</sup>* + *D<sup>β</sup>* + *D<sup>γ</sup>* + *DC*

(3)

in the denominator in Equation 2 to account for the internal dose from potassium.

*Luminescence Age* <sup>=</sup> *Paleodose*

As a result, general age equation is modified to:

52 Geochronology - Methods and Case Studies

into account when calculating the dose rate.

*6.2.1. Sample collection*

**6.2. Sample collection and laboratory procedures**

Once the collected sample has reached the laboratory, it has to be prepared for analysis and a number of procedures have been established depending on the material targeted for analysis. As outlined in Section 6.1, samples for luminescence analysis are either measured as fine or coarse grains. It was also indicated that for coarse grains, analysis is usually made on pure mineral separates (for example quartz, or K-feldspars) whereas for fine grains, either poly‐ mineral fractions or fine grained quartz extracts can be used. When dating fine grained sediments such as loess, for instance it would be preferable to use the fine grain procedure whereas for eolian dune sands, the coarse grain approach would be more appropriate. In sediments that have equal components of fine grains and coarse grains, dating both fractions would provide a good mechanism for comparison as a cross-checking method.

For coarse grained materials, quartz or feldspar are typically separated using a heavy liquid such as sodium polytungstate solution. Fig. 9 is a flowchart of a separating procedure for quartz and feldspar using heavy liquids with successively lighter or heavier specific densities. Prior to separating minerals using the heavy liquid, carbonates and organic materials that often occur in sediments, and are usually introduced during the postdepositional phase of the deposit, are removed using hydrochloric acid and hydrogen peroxide respectively. Additional information on separation procedures of coarse grains can be obtained from [74, 75].

For fine grains, carbonates and organic materials are also removed using dilute hydrochloric acid and hydrogen peroxide respectively. Dilute sodium oxalate solution is then added to prevent flocculation of the particles after which appropriate grain sizes are separated using a sedimentation column. Sedimentation columns employ Stoke's law which states that the velocity of a particle's sedimentation in a fluid also depends on the size of the particle. Hence, by extracting sediment from the column after a predetermined time following agitation permits a desired grain size to be isolated. As indicated earlier, while fine grains can be analysed as polymineralic fractions, some studies extract pure quartz from the fine grains by digesting the feldspars using fluorosilicic acid [for example, 76]. Detailed information on separation procedures of fine grains for luminescence dating have been provided by [77].

Once separated, coarse grains (quartz or feldspar) or the fine grains (mixed polymineralic or pure quartz) are mounted on appropriate sample discs prior to analysis. Typically, the measurement discs are made of stainless steel or aluminum and measure around 10 mm in diameter and 0.5 mm thick [29]. To mount coarse grains, a monolayer of the sample is deposited on the disc in dry form. Silicone oil can be used to help the sample grains adhere to the disc. For fine grain particles, on the other hand, once the desired grain sizes have been isolated, these are mounted on the discs in a solution of acetone, ethanol or water which is then allowed to evaporate, leaving the sediment deposited on the disc. In either case, the disc will now be ready for analysis.

**7.1. Heated materials**

should be datable using luminescence methods.

luminescence methods [for example, 79].

timing of the volcanic eruption [32].

As discussed in an earlier section, luminescence dating initially began as an archaeological technique that was used for dating materials that had been heated to temperatures adequately high (for example, > 500 °C) to expel all electrons from their traps. Hence, the heating process provides a starting point that can serve as year zero when dating such artifacts. Fired artifacts such as ancient pottery, tiles, bricks or terracotta figures are all examples of archeological materials that can be dated using luminescence methods, especially TL, because these materials usually contain dosimeters such as quartz and feldspar. By separating the quartz and dating quartz inclusions, or by using the fine grained components to date polymineralic fractions or quartz separates, ages can be obtained. Humans are generally believed to have discovered the art of making pottery during the Neolithic period, which dates back to about 30 000 yrs ago in some places [for example, 78], but the art only became widespread about 10 000 years ago [32]. Hence, unless dose rates are very high, most ancient pottery artifacts are not yet saturated and

Luminescence Chronology http://dx.doi.org/10.5772/58554 55

Apart from pottery, other heated materials of archaeological significance that can be dated using luminescence methods include burnt flint and burnt stones that were associated with human settlements and may have been heated to high temperatures. Flint is a sedimentary form of quartz and hence possesses dosimetric properties [32]. Paleolithic humans used flint extensively as a tool for scraping and for cutting as well as for projectile points. Chips or debitage left over from the manufacture of such implements can be found associated with ancient settlements. If any of these tools or chips were at some stage heated, either deliberately or accidentally during the occupation of the site by humans, dating the objects using lumi‐ nescence methods will provide a chronology commensurate with the timing of human habitation. Thus, heated flint can be a useful chronometer especially for timescales beyond those commonly covered by other methods such as radiocarbon [32]. Other heated geological materials include stones that were used as 'pot-boilers' by some societies prior to the discovery of pottery. In some settings stones were also used for constructing fireplaces. In both instances, where these previously heated stones contain appropriate minerals they can be dated using

A different class of heated materials that can be dated using luminescence methods are geological materials that have been heated to appropriate temperatures to have zeroed them during the last approximately 100,000 years. Such materials include contact-baked sediment that is heated to high temperatures following a volcanic eruption. In such cases, the soil can be collected and grains extracted for dating using either coarse grain or fine grain methods [80]. If the sediment contains larger clasts such as gravel, constituent quartz or feldspar grains can be extracted from the pebbles for analysis. The age obtained would be congruent with the

Also associated with volcanic eruptions are the products of the eruption itself such as lava and ash. The heat associated with the volcanic eruption is sufficient to zero these products of any previously acquired dose, if at all, since the lava would mostly be in liquid form rather than crystalline state. Fine grained glass (4-11μm) extracted from volcanic ash has been used in

**Figure 9.** Separation procedure for coarse grains using a heavy liquid. Starting off with a liquid of specific gravity (s.g.) 2.62 gcm-3, the sample is separated by centrifuge into a fraction that is lighter (floats) and one that is heavier (sinks). The fraction that sinks is passed through a liquid of s.g. 2.75 gcm-3 and the fraction that floats will comprise mostly quartz or plagioclase while the sinking fraction will comprise heavy minerals, for example, zircon. The fraction that floats in the liquid of density 2.62 gcm-3 is passed through another liquid of s.g. 2.58 gcm-3. The fraction that sinks will be Na-feldspar whereas the lighter fraction is further separated using a liquid of s.g. 2.53 gcm-3 into K-feldspar and clay.

## **7. What materials can be dated using luminescence methods?**

For all practical purposes, an important aspect about luminescence dating is knowing what type of materials or sediments can be dated using luminescence methods. Discussions in the preceding sections have touched on this topic albeit indirectly. As detailed earlier, lumines‐ cence methods date materials by measuring energy that has accumulated in materials called dosimeters. Hence, one prerequisite for dating using the method is that the material to be dated must contain a dosimeter. The second prerequisite is that there has to be an event that reset the energy previously stored by the dosimeter to provide a starting point for counting the time. In essence, all energy stored in the dosimeter will be assumed to have accumulated since that point. A third requirement for material to be datable using luminescence methods is that the electron traps in the dosimeter should not be exhausted at the time of dating because once energy storage has reached a point of saturation, the relationship between time and dose rate breaks down. Hence, the sample for dating has to be younger than the upper age limit that can be attained for that particular dosimeter and, as indicated earlier, the exact limit ultimately depends on the dose rate too. With these considerations in mind, materials datable with luminescence techniques broadly fall into two main categories: materials that have been heated and clastic sediments of sand and silt size that have been reset (zeroed) through exposure to solar radiation.

## **7.1. Heated materials**

**7. What materials can be dated using luminescence methods?**

solar radiation.

clay.

54 Geochronology - Methods and Case Studies

For all practical purposes, an important aspect about luminescence dating is knowing what type of materials or sediments can be dated using luminescence methods. Discussions in the preceding sections have touched on this topic albeit indirectly. As detailed earlier, lumines‐ cence methods date materials by measuring energy that has accumulated in materials called dosimeters. Hence, one prerequisite for dating using the method is that the material to be dated must contain a dosimeter. The second prerequisite is that there has to be an event that reset the energy previously stored by the dosimeter to provide a starting point for counting the time. In essence, all energy stored in the dosimeter will be assumed to have accumulated since that point. A third requirement for material to be datable using luminescence methods is that the electron traps in the dosimeter should not be exhausted at the time of dating because once energy storage has reached a point of saturation, the relationship between time and dose rate breaks down. Hence, the sample for dating has to be younger than the upper age limit that can be attained for that particular dosimeter and, as indicated earlier, the exact limit ultimately depends on the dose rate too. With these considerations in mind, materials datable with luminescence techniques broadly fall into two main categories: materials that have been heated and clastic sediments of sand and silt size that have been reset (zeroed) through exposure to

**Figure 9.** Separation procedure for coarse grains using a heavy liquid. Starting off with a liquid of specific gravity (s.g.) 2.62 gcm-3, the sample is separated by centrifuge into a fraction that is lighter (floats) and one that is heavier (sinks). The fraction that sinks is passed through a liquid of s.g. 2.75 gcm-3 and the fraction that floats will comprise mostly quartz or plagioclase while the sinking fraction will comprise heavy minerals, for example, zircon. The fraction that floats in the liquid of density 2.62 gcm-3 is passed through another liquid of s.g. 2.58 gcm-3. The fraction that sinks will be Na-feldspar whereas the lighter fraction is further separated using a liquid of s.g. 2.53 gcm-3 into K-feldspar and

As discussed in an earlier section, luminescence dating initially began as an archaeological technique that was used for dating materials that had been heated to temperatures adequately high (for example, > 500 °C) to expel all electrons from their traps. Hence, the heating process provides a starting point that can serve as year zero when dating such artifacts. Fired artifacts such as ancient pottery, tiles, bricks or terracotta figures are all examples of archeological materials that can be dated using luminescence methods, especially TL, because these materials usually contain dosimeters such as quartz and feldspar. By separating the quartz and dating quartz inclusions, or by using the fine grained components to date polymineralic fractions or quartz separates, ages can be obtained. Humans are generally believed to have discovered the art of making pottery during the Neolithic period, which dates back to about 30 000 yrs ago in some places [for example, 78], but the art only became widespread about 10 000 years ago [32]. Hence, unless dose rates are very high, most ancient pottery artifacts are not yet saturated and should be datable using luminescence methods.

Apart from pottery, other heated materials of archaeological significance that can be dated using luminescence methods include burnt flint and burnt stones that were associated with human settlements and may have been heated to high temperatures. Flint is a sedimentary form of quartz and hence possesses dosimetric properties [32]. Paleolithic humans used flint extensively as a tool for scraping and for cutting as well as for projectile points. Chips or debitage left over from the manufacture of such implements can be found associated with ancient settlements. If any of these tools or chips were at some stage heated, either deliberately or accidentally during the occupation of the site by humans, dating the objects using lumi‐ nescence methods will provide a chronology commensurate with the timing of human habitation. Thus, heated flint can be a useful chronometer especially for timescales beyond those commonly covered by other methods such as radiocarbon [32]. Other heated geological materials include stones that were used as 'pot-boilers' by some societies prior to the discovery of pottery. In some settings stones were also used for constructing fireplaces. In both instances, where these previously heated stones contain appropriate minerals they can be dated using luminescence methods [for example, 79].

A different class of heated materials that can be dated using luminescence methods are geological materials that have been heated to appropriate temperatures to have zeroed them during the last approximately 100,000 years. Such materials include contact-baked sediment that is heated to high temperatures following a volcanic eruption. In such cases, the soil can be collected and grains extracted for dating using either coarse grain or fine grain methods [80]. If the sediment contains larger clasts such as gravel, constituent quartz or feldspar grains can be extracted from the pebbles for analysis. The age obtained would be congruent with the timing of the volcanic eruption [32].

Also associated with volcanic eruptions are the products of the eruption itself such as lava and ash. The heat associated with the volcanic eruption is sufficient to zero these products of any previously acquired dose, if at all, since the lava would mostly be in liquid form rather than crystalline state. Fine grained glass (4-11μm) extracted from volcanic ash has been used in some studies to date the eruption [81]. Other studies have attempted to use minerals from the lava instead of volcanic ash [for example, 82]

Quartz and feldspar grains extracted from eolian sediments have been dated in numerous studies which have compared the chronologies obtained to those from radiocarbon ages from associated sediments and have provided results that are congruent, validating luminescence dating of eolian sediments as reliable chronometers [for example, 86]. In many cases, because eolian deposits are proxy indicators of dry conditions from the past, luminescence ages from the eolian deposits have been used to provide a temporal framework for environmental changes from the past. Fossil dunes from inland deserts of Australia [for example 87, 88], southern Africa [for example 88, 89], Mongolia [for example, 90], United States [for example, 91, 92], Canadian Prairies [for example, 93, 94], the coversands of northern Europe [for example, 95], South America [for example, 96] and many other regions have all been dated using luminescence methods. Reported ages range from a few decades to over 100,000 years and, in many ways, luminescence dating has revolutionised the study of the geomorphology

Luminescence Chronology http://dx.doi.org/10.5772/58554 57

Apart from inland deserts, luminescence dating has also been used to date deposits from coastal dune deposits [for example, 97] where the ages obtained provide a chronological framework for processes in the coastal environment, including sea level change [for

Sequences of fine grained eolian sediments (silt size) give rise to loess deposits which can reach hundreds of meters in thickness [85]. Fine grain dating methods have been used to provide chronological frameworks for the deposition of such sediments from places such as the Loess Plateau of China [for example, 99, 100] to the North American Great Plains [101]. Ultimately the results in such studies are used for paleoenvironmental reconstruction too. As indicated earlier, methods that use fine grains can employ IRSL stimulation of polymineral aliquots, which targets the feldspars. Alternatively, quartz from the loess can be extracted and dated

Sediments deposited by water, either fine grain or coarse grain, have been dated using luminescence methods. The abundance of sand in fluvial systems makes luminescence dating an attractive dating method in such settings. However, it is the case that sediments transported by fluvial processes are not always completely zeroed, such that the grains are often partially bleached [29, 103, 104]. Statistical approaches for dealing with the partial bleaching have been proposed but there is no consensus on how these should be applied. This has led Cunningham and Wallinga [104] to propose a protocol for analysing OSL data from fluvial sediment using a Bayesian approach. Nonetheless, in studies that have investigated fluvial sediments, luminescence chronology has provided information on modern and ancient sedimentation rates [104]. It has also enabled investigators to assess response patterns of river systems to climatic and tectonic forcing [104]. Results from dating of fluvial sediments can similarly be applied to paleoseismic and archaeological studies [103]. Comprehensive reviews of lumines‐

and paleoclimates of arid regions over the last three decades [84].

using blue OSL stimulation [for example, 102].

cence dating of fluvial sediments can be found in [103, 104].

example, 98].

*7.2.2. Water-lain deposits*

Finally, also related to the effects of heating to reset the luminescence signal are materials that have been zeroed by heat emanating from the impact of a meteorite. The thermal shock associated with such impacts can reach temperatures high enough to zero constituent grains of the affected geological material [for example, 83].

## **7.2. Dating of sediments reset by sunlight**

A class of materials that has fostered the rapid development of luminescence dating methods over the last three decades are clastic sediments that have been zeroed by solar bleaching. Since the confirmation by Huntley et al. [1] that energy from sunlight was capable of adequately erasing previously accumulated energy from dosimeters, there have been many applications of the method to obtain chronologies from sedimentary materials, initially using TL and later using OSL. As outlined above, the prerequisites for dating using luminescence methods which include a presence of a dosimeter in the material, the occurrence of a bleaching episode that erases any previously accumulated energy, and the absence of saturation in the dosimeters (sediment grains) need to be satisfied if a material is to be dated. A number of sedimentary materials satisfy these criteria and have been dated using luminescence methods, with some presently constituting formidable chronological archives of environmental change. These include sediments deposited by wind (eolian sediments), water-laid sediment, glacial deposits, and earthquake related sediments. These sediment classes are discussed below under the respective headings. In all instances, reference to ages obtained from sediments bleached by solar resetting denotes burial ages or time that has elapsed since the last time the sediments were exposed to light from the sun.

### *7.2.1. Eolian deposits*

Wind deposited (eolian) sediments are the sediment of choice for dating using luminescence methods. This is because the subaerial transport that the sediment experiences during transportation is expected to provide adequate time to bleach the sediment grains of any previously accumulated energy [29]. While this may not be true in some cases, results generally show that in most settings, that assumption is valid [84]. As a result, this class of sediments has provided the majority of luminescence ages reported to date. Eolian sediments generally fall into one of two main classes. One class comprises sand grains in the fine to medium size range (63-250 μm) that are generally transported by wind through a series of low jumps along the surface of a sedimentary bed in a process referred to as saltation [85]. These grains are usually deposited as dunes. The other sediment category is silt size grains (2-63 μm) that are transported by wind in suspension. Fine silt grains can remain airborne for extended periods of time [85]. For both sand size and silt size grains, the particle transport at the surface is often adequate to zero the grains.

Quartz and feldspar grains extracted from eolian sediments have been dated in numerous studies which have compared the chronologies obtained to those from radiocarbon ages from associated sediments and have provided results that are congruent, validating luminescence dating of eolian sediments as reliable chronometers [for example, 86]. In many cases, because eolian deposits are proxy indicators of dry conditions from the past, luminescence ages from the eolian deposits have been used to provide a temporal framework for environmental changes from the past. Fossil dunes from inland deserts of Australia [for example 87, 88], southern Africa [for example 88, 89], Mongolia [for example, 90], United States [for example, 91, 92], Canadian Prairies [for example, 93, 94], the coversands of northern Europe [for example, 95], South America [for example, 96] and many other regions have all been dated using luminescence methods. Reported ages range from a few decades to over 100,000 years and, in many ways, luminescence dating has revolutionised the study of the geomorphology and paleoclimates of arid regions over the last three decades [84].

Apart from inland deserts, luminescence dating has also been used to date deposits from coastal dune deposits [for example, 97] where the ages obtained provide a chronological framework for processes in the coastal environment, including sea level change [for example, 98].

Sequences of fine grained eolian sediments (silt size) give rise to loess deposits which can reach hundreds of meters in thickness [85]. Fine grain dating methods have been used to provide chronological frameworks for the deposition of such sediments from places such as the Loess Plateau of China [for example, 99, 100] to the North American Great Plains [101]. Ultimately the results in such studies are used for paleoenvironmental reconstruction too. As indicated earlier, methods that use fine grains can employ IRSL stimulation of polymineral aliquots, which targets the feldspars. Alternatively, quartz from the loess can be extracted and dated using blue OSL stimulation [for example, 102].

#### *7.2.2. Water-lain deposits*

some studies to date the eruption [81]. Other studies have attempted to use minerals from the

Finally, also related to the effects of heating to reset the luminescence signal are materials that have been zeroed by heat emanating from the impact of a meteorite. The thermal shock associated with such impacts can reach temperatures high enough to zero constituent grains

A class of materials that has fostered the rapid development of luminescence dating methods over the last three decades are clastic sediments that have been zeroed by solar bleaching. Since the confirmation by Huntley et al. [1] that energy from sunlight was capable of adequately erasing previously accumulated energy from dosimeters, there have been many applications of the method to obtain chronologies from sedimentary materials, initially using TL and later using OSL. As outlined above, the prerequisites for dating using luminescence methods which include a presence of a dosimeter in the material, the occurrence of a bleaching episode that erases any previously accumulated energy, and the absence of saturation in the dosimeters (sediment grains) need to be satisfied if a material is to be dated. A number of sedimentary materials satisfy these criteria and have been dated using luminescence methods, with some presently constituting formidable chronological archives of environmental change. These include sediments deposited by wind (eolian sediments), water-laid sediment, glacial deposits, and earthquake related sediments. These sediment classes are discussed below under the respective headings. In all instances, reference to ages obtained from sediments bleached by solar resetting denotes burial ages or time that has elapsed since the last time the sediments

Wind deposited (eolian) sediments are the sediment of choice for dating using luminescence methods. This is because the subaerial transport that the sediment experiences during transportation is expected to provide adequate time to bleach the sediment grains of any previously accumulated energy [29]. While this may not be true in some cases, results generally show that in most settings, that assumption is valid [84]. As a result, this class of sediments has provided the majority of luminescence ages reported to date. Eolian sediments generally fall into one of two main classes. One class comprises sand grains in the fine to medium size range (63-250 μm) that are generally transported by wind through a series of low jumps along the surface of a sedimentary bed in a process referred to as saltation [85]. These grains are usually deposited as dunes. The other sediment category is silt size grains (2-63 μm) that are transported by wind in suspension. Fine silt grains can remain airborne for extended periods of time [85]. For both sand size and silt size grains, the particle transport at the surface is often

lava instead of volcanic ash [for example, 82]

56 Geochronology - Methods and Case Studies

**7.2. Dating of sediments reset by sunlight**

were exposed to light from the sun.

*7.2.1. Eolian deposits*

adequate to zero the grains.

of the affected geological material [for example, 83].

Sediments deposited by water, either fine grain or coarse grain, have been dated using luminescence methods. The abundance of sand in fluvial systems makes luminescence dating an attractive dating method in such settings. However, it is the case that sediments transported by fluvial processes are not always completely zeroed, such that the grains are often partially bleached [29, 103, 104]. Statistical approaches for dealing with the partial bleaching have been proposed but there is no consensus on how these should be applied. This has led Cunningham and Wallinga [104] to propose a protocol for analysing OSL data from fluvial sediment using a Bayesian approach. Nonetheless, in studies that have investigated fluvial sediments, luminescence chronology has provided information on modern and ancient sedimentation rates [104]. It has also enabled investigators to assess response patterns of river systems to climatic and tectonic forcing [104]. Results from dating of fluvial sediments can similarly be applied to paleoseismic and archaeological studies [103]. Comprehensive reviews of lumines‐ cence dating of fluvial sediments can be found in [103, 104].

#### *7.2.3. Sediments of glacial origin*

For luminescence dating purposes, sediments of glacial origin can generally be classified into three broad categories with regards to solar bleaching possibilities [67]. One class of sediments is transported below the glacier where no bleaching occurs at all. Hence this class would not be appropriate for luminescence dating. The second class is transported within the glacier itself where limited bleaching could occur under some circumstances but often does not. The third class of sediment is transported above the glacier and, for such sediment, some bleaching could occur. The greatest opportunity for bleaching of sedi‐ ment associated with glaciers, however, is noticed in sediment transported from the glacier as outwash materials by meltwaters [67, 105]. As a result, the likelihood of the lumines‐ cence signal being zeroed increases appreciably with distance of transportation away from the glacier [105, 106, 107]. In practice, however, investigators have noted that, even for outwash deposits, partial bleaching is a problem that is encountered frequently [106]. It is thought that this occurs because the transport of sand and fine grained sediment in the proglacial environment occurs in meltwaters that are both deep and turbid such that penetration by sunlight is impeded [105]). Methods that can be used to date such sedi‐ ments include single grain methods that try to identify individual grains that were adequately bleached. Current research looks at identifying mineral characteristics such as rapidly bleaching components of the luminescence signal that can be used for dating such sediments [105]. Varying dose rates during the burial history of glacigenic deposits can also be of concern and these have to be reconstructed carefully if accurate ages are to be determined [67]. Additional information on dating of glaciofluvial deposits can be found in [67, 105].

faulting associated with earthquakes [29]. Contemporaneous deposition across the displaced surfaces will result in units of similar age on surfaces that differ in height, allowing the units

Luminescence Chronology http://dx.doi.org/10.5772/58554 59

**8. Case study: Luminescence chronology of postglacial eolian dunes from Alberta, Canada: Constraining the timing of Late Pleistocene deglaciation**

To demonstrate the diverse aspects that can be encountered in a luminescence dating study, this section presents a brief outline of an investigation carried out in Alberta, western Canada [94] to constrain the timing of the retreat of the Laurentide Ice Sheet from the region. The landscape of Alberta features in excess of sixty discreet eolian dune fields that are believed to have been deposited after the retreat of the Laurentide Ice Sheet that once covered the most of Canada, east of the Rocky Mountains, about 20,000 years ago (Fig.10) [94]. Source sediments for the eolian deposits are thought to be sandy glaciolacustrine and outwash deposits associ‐ ated with the retreating ice sheet. The eolian dunes in central and northern Alberta are currently stable, with many of them supporting boreal vegetation. It is thought that the eolian deposition was initiated in the immediate aftermath of the retreat of the Laurentide Ice Sheet but before the climate conditions ameliorated enough to allow vegetation to flourish. Once the climate improved, the landscape was stabilized by vegetation and the dunes have remained largely intact such that the depositional sequences they contain can be used as indicators of past environmental conditions. The exact chronology of the retreat of the Laurentide Ice Sheet from the region, however, still has to be firmly established [94, 112]. Efforts to constrain the ice sheet's retreat using radiocarbon chronology has been hampered by the scarcity of contemporaneous radiocarbon bearing material from the region [94, 113, 114] which contrasts with areas further to the east [112, 113], to the west [115] or to the south [116; 117] where radiocarbon ages have provided a sound chronological framework for Late Pleistocene deglaciation. To help address this lack of age controls for the postglacial period, a study was conceived to collect eolian dune sands from central and northern Alberta and date them using luminescence methods [94]. Ages from the eolian dunes would have a number of contributions. Because the transport and deposition of eolian sediment can only take place in ice-free conditions, ages from the dune sands would make it possible to determine by when the Laurentide Ice Sheet had retreated from the landscape. It would also be possible to construct a chronological framework for the environmental evolution of the region by putting maximum age constrains on the colonisation of the region by vegetation. A third important aspect of establishing the chronology of deglaciation of the region was that it would allow investigators to evaluate whether western Canada served as an inland migration route used by the first humans to reach the Americas. This is because the path followed by the first Americans remains a subject of contention. Evidence from central USA shows that humans had settled there by about 15,000-16,000 years ago [118]. These humans are thought to have migrated south from Beringia, having arrived from Eurasia earlier during a low sea-level phase. Around 15,000-16,000 years ago, however, some researchers believe that the Laurentide Ice Sheet still covered large parts of western Canada, including Alberta, making the region unnavigable for

to be correlated.

#### *7.2.4. Earthquake related studies*

A class of sediments that is of relevance to the category of earthquake related luminescence dating studies are sandy deposits emplaced during a tsunami event. Tsunami events following an earthquake commonly transport sandy materials inland to deposit them in proximal tidal marshes as well as in bogs and lakes. The sand is subsequently overlain by other materials such as peat or mud to form part of the coastal depositional sequence. Dating such sands would allow one to reconstruct the recurrence rate of the tsunami events and, by inference the earthquakes themselves, which is pertinent information for evaluating environmental hazards within the coastal zone [29]. Tsunami events, however, occur abruptly and the turbidity associated with such an event would not normally provide adequate time for effective bleaching of the sands before they are deposited. This problem is avoided by targeting sands that were previously part of the tidal flat and tidal channel environment where they lay exposed at the surface prior to the tsunami [29]. Working on the west coast of North America, close to Washington and Vancouver Island, Huntley and Clague [108] were able to date two tsunami events using this approach. Other more recent studies that have attempted to date tsunami events include [109, 110]. It should be noted, however, that the identification of tsunami deposits within the coastal zone remains a subject of debate [for example, 111].

Another class of earthquake related sediments that have been dated using luminescence methods are deposits emplaced on horizontal surfaces that have been vertically displaced by faulting associated with earthquakes [29]. Contemporaneous deposition across the displaced surfaces will result in units of similar age on surfaces that differ in height, allowing the units to be correlated.

*7.2.3. Sediments of glacial origin*

58 Geochronology - Methods and Case Studies

in [67, 105].

*7.2.4. Earthquake related studies*

For luminescence dating purposes, sediments of glacial origin can generally be classified into three broad categories with regards to solar bleaching possibilities [67]. One class of sediments is transported below the glacier where no bleaching occurs at all. Hence this class would not be appropriate for luminescence dating. The second class is transported within the glacier itself where limited bleaching could occur under some circumstances but often does not. The third class of sediment is transported above the glacier and, for such sediment, some bleaching could occur. The greatest opportunity for bleaching of sedi‐ ment associated with glaciers, however, is noticed in sediment transported from the glacier as outwash materials by meltwaters [67, 105]. As a result, the likelihood of the lumines‐ cence signal being zeroed increases appreciably with distance of transportation away from the glacier [105, 106, 107]. In practice, however, investigators have noted that, even for outwash deposits, partial bleaching is a problem that is encountered frequently [106]. It is thought that this occurs because the transport of sand and fine grained sediment in the proglacial environment occurs in meltwaters that are both deep and turbid such that penetration by sunlight is impeded [105]). Methods that can be used to date such sedi‐ ments include single grain methods that try to identify individual grains that were adequately bleached. Current research looks at identifying mineral characteristics such as rapidly bleaching components of the luminescence signal that can be used for dating such sediments [105]. Varying dose rates during the burial history of glacigenic deposits can also be of concern and these have to be reconstructed carefully if accurate ages are to be determined [67]. Additional information on dating of glaciofluvial deposits can be found

A class of sediments that is of relevance to the category of earthquake related luminescence dating studies are sandy deposits emplaced during a tsunami event. Tsunami events following an earthquake commonly transport sandy materials inland to deposit them in proximal tidal marshes as well as in bogs and lakes. The sand is subsequently overlain by other materials such as peat or mud to form part of the coastal depositional sequence. Dating such sands would allow one to reconstruct the recurrence rate of the tsunami events and, by inference the earthquakes themselves, which is pertinent information for evaluating environmental hazards within the coastal zone [29]. Tsunami events, however, occur abruptly and the turbidity associated with such an event would not normally provide adequate time for effective bleaching of the sands before they are deposited. This problem is avoided by targeting sands that were previously part of the tidal flat and tidal channel environment where they lay exposed at the surface prior to the tsunami [29]. Working on the west coast of North America, close to Washington and Vancouver Island, Huntley and Clague [108] were able to date two tsunami events using this approach. Other more recent studies that have attempted to date tsunami events include [109, 110]. It should be noted, however, that the identification of tsunami deposits within the coastal zone remains a subject of debate [for example, 111].

Another class of earthquake related sediments that have been dated using luminescence methods are deposits emplaced on horizontal surfaces that have been vertically displaced by

## **8. Case study: Luminescence chronology of postglacial eolian dunes from Alberta, Canada: Constraining the timing of Late Pleistocene deglaciation**

To demonstrate the diverse aspects that can be encountered in a luminescence dating study, this section presents a brief outline of an investigation carried out in Alberta, western Canada [94] to constrain the timing of the retreat of the Laurentide Ice Sheet from the region. The landscape of Alberta features in excess of sixty discreet eolian dune fields that are believed to have been deposited after the retreat of the Laurentide Ice Sheet that once covered the most of Canada, east of the Rocky Mountains, about 20,000 years ago (Fig.10) [94]. Source sediments for the eolian deposits are thought to be sandy glaciolacustrine and outwash deposits associ‐ ated with the retreating ice sheet. The eolian dunes in central and northern Alberta are currently stable, with many of them supporting boreal vegetation. It is thought that the eolian deposition was initiated in the immediate aftermath of the retreat of the Laurentide Ice Sheet but before the climate conditions ameliorated enough to allow vegetation to flourish. Once the climate improved, the landscape was stabilized by vegetation and the dunes have remained largely intact such that the depositional sequences they contain can be used as indicators of past environmental conditions. The exact chronology of the retreat of the Laurentide Ice Sheet from the region, however, still has to be firmly established [94, 112]. Efforts to constrain the ice sheet's retreat using radiocarbon chronology has been hampered by the scarcity of contemporaneous radiocarbon bearing material from the region [94, 113, 114] which contrasts with areas further to the east [112, 113], to the west [115] or to the south [116; 117] where radiocarbon ages have provided a sound chronological framework for Late Pleistocene deglaciation. To help address this lack of age controls for the postglacial period, a study was conceived to collect eolian dune sands from central and northern Alberta and date them using luminescence methods [94]. Ages from the eolian dunes would have a number of contributions. Because the transport and deposition of eolian sediment can only take place in ice-free conditions, ages from the dune sands would make it possible to determine by when the Laurentide Ice Sheet had retreated from the landscape. It would also be possible to construct a chronological framework for the environmental evolution of the region by putting maximum age constrains on the colonisation of the region by vegetation. A third important aspect of establishing the chronology of deglaciation of the region was that it would allow investigators to evaluate whether western Canada served as an inland migration route used by the first humans to reach the Americas. This is because the path followed by the first Americans remains a subject of contention. Evidence from central USA shows that humans had settled there by about 15,000-16,000 years ago [118]. These humans are thought to have migrated south from Beringia, having arrived from Eurasia earlier during a low sea-level phase. Around 15,000-16,000 years ago, however, some researchers believe that the Laurentide Ice Sheet still covered large parts of western Canada, including Alberta, making the region unnavigable for humans [119]. Hence, an alternative route must have been used, possibly a coastal route [120]. By accurately constraining the timing of the retreat of the ice sheet using the chronology of dune deposition, however, it would be possible to ascertain if the ice sheets retreated early enough to allow humans to trek from Beringia through western Canada to reach central USA by around 15,000-16,000 years ago using an inland route.

Results of the study showed that eolian deposition in central Alberta started at least 15,000-16,000 years ago and that by around 11,000-12,000 years ago, many of the dunes had become stable [94]. A separate investigation [93] that had dated feldspar separates from dune sands in the area using the additive dose protocol obtained similar ages. The age of 15,000-16,000 years ago for eolian deposition in the study area is important because it shows that, by that time, the Laurentide Ice Sheet had retreated from the region, allowing eolian processes to operate. However, it is not yet possible to establish when exactly the eolian conditions began but, with the arrival of humans in central USA at around 15,000-16,000 ka, it is possible that the ice sheet retreated from western Canada early enough to avail the first Americans an inland route to trek southwards [94]. With regards to the subsequent colonisa‐ tion of the region by boreal vegetation, the termination of eolian deposition around 11,00-12,000 years ago is consistent with other records which point to the proliferation of vegetal communities in the region associated with the postglacial climate amelioration of the Early Holocene [94]. Continuing work in the study area focuses on establishing a higher resolution chronology of eolian deposition, with emphasis on obtaining a more accurate

Luminescence Chronology http://dx.doi.org/10.5772/58554 61

Over the last forty years, luminescence dating has matured into a full-fledged and robust technique with many practising laboratories established across the world. Every three years, international practitioners gather for the Luminescence and Electron Spin Resonance Dating (LED) Conference where research in luminescence dating is presented. In the intervening years between the LED conferences, national and regional meetings such as the UK Luminescence Dating Conference, the German Luminescence and ESR Conference, and the New World Luminescence Dating Workshop in North America are also held. Such meetings feature fundamental research into luminescence dating methods as well as their applications in the environmental, geological and archaeological sciences. Advances in instrumentation are also

Prominent topics in current fundamental research include efforts to better understand the luminescence signal characteristics of both quartz and feldspar, which should allow for more accurate ages to be produced as well as for the dating ranges to be extended. For instance, ongoing studies are trying to characterise the behaviour of quartz at high doses. As indicated earlier, a number of studies have shown that at high doses (>150Gy), quartz OSL SAR protocols produce ages that underestimate the real ages of dated sedimentary units [for example, 70, 102, 121]. As a result, ongoing studies aim to identify the causes of these underestimations, particularly by looking at the characteristics of the individual components of the luminescence signal (for example, fast, medium and slow). Success in this quest could lead to the develop‐ ment of appropriate protocols that would make it possible to extend the dating range of quartz

For feldspar, researchers have been looking at identifying IRSL signals that are less susceptible to anomalous fading which would also allow much older ages to be determined [for example,

framework for the initiation of eolian processes in the region [94].

**9. Current and future trends in luminescence dating**

always an important component at the meetings.

beyond what is currently attainable.

Accordingly, samples for luminescence dating were collected from eolian dunes in central and northern Alberta from excavation pits as well from vertical profiles and these were sent to luminescence dating labs at the University of Washington and Utah State University for analysis [94]. At both labs the sample preparation methods followed standard procedures that included sieving, heavy liquid separation and etching with HF to separate quartz. The SAR protocol [26, 27] was used determine the paleodose and luminescence measurements were conducted on a Risø DA-20 instrument. For the dose rate, concentrations of U, Th and K were determined using ICP-MS and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Alternatively TSAC in conjunction with flame photometry and beta counting were also used. More experimental details can be found in [94].

**Figure 10.** Eolian sands in Alberta, Canada. Samples were collected from dunes in central and northern Alberta from which quartz was extracted for dating using the SAR protocol [94].

Results of the study showed that eolian deposition in central Alberta started at least 15,000-16,000 years ago and that by around 11,000-12,000 years ago, many of the dunes had become stable [94]. A separate investigation [93] that had dated feldspar separates from dune sands in the area using the additive dose protocol obtained similar ages. The age of 15,000-16,000 years ago for eolian deposition in the study area is important because it shows that, by that time, the Laurentide Ice Sheet had retreated from the region, allowing eolian processes to operate. However, it is not yet possible to establish when exactly the eolian conditions began but, with the arrival of humans in central USA at around 15,000-16,000 ka, it is possible that the ice sheet retreated from western Canada early enough to avail the first Americans an inland route to trek southwards [94]. With regards to the subsequent colonisa‐ tion of the region by boreal vegetation, the termination of eolian deposition around 11,00-12,000 years ago is consistent with other records which point to the proliferation of vegetal communities in the region associated with the postglacial climate amelioration of the Early Holocene [94]. Continuing work in the study area focuses on establishing a higher resolution chronology of eolian deposition, with emphasis on obtaining a more accurate framework for the initiation of eolian processes in the region [94].

## **9. Current and future trends in luminescence dating**

humans [119]. Hence, an alternative route must have been used, possibly a coastal route [120]. By accurately constraining the timing of the retreat of the ice sheet using the chronology of dune deposition, however, it would be possible to ascertain if the ice sheets retreated early enough to allow humans to trek from Beringia through western Canada to reach central USA

Accordingly, samples for luminescence dating were collected from eolian dunes in central and northern Alberta from excavation pits as well from vertical profiles and these were sent to luminescence dating labs at the University of Washington and Utah State University for analysis [94]. At both labs the sample preparation methods followed standard procedures that included sieving, heavy liquid separation and etching with HF to separate quartz. The SAR protocol [26, 27] was used determine the paleodose and luminescence measurements were conducted on a Risø DA-20 instrument. For the dose rate, concentrations of U, Th and K were determined using ICP-MS and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Alternatively TSAC in conjunction with flame photometry and beta counting were

**Figure 10.** Eolian sands in Alberta, Canada. Samples were collected from dunes in central and northern Alberta from

by around 15,000-16,000 years ago using an inland route.

60 Geochronology - Methods and Case Studies

also used. More experimental details can be found in [94].

which quartz was extracted for dating using the SAR protocol [94].

Over the last forty years, luminescence dating has matured into a full-fledged and robust technique with many practising laboratories established across the world. Every three years, international practitioners gather for the Luminescence and Electron Spin Resonance Dating (LED) Conference where research in luminescence dating is presented. In the intervening years between the LED conferences, national and regional meetings such as the UK Luminescence Dating Conference, the German Luminescence and ESR Conference, and the New World Luminescence Dating Workshop in North America are also held. Such meetings feature fundamental research into luminescence dating methods as well as their applications in the environmental, geological and archaeological sciences. Advances in instrumentation are also always an important component at the meetings.

Prominent topics in current fundamental research include efforts to better understand the luminescence signal characteristics of both quartz and feldspar, which should allow for more accurate ages to be produced as well as for the dating ranges to be extended. For instance, ongoing studies are trying to characterise the behaviour of quartz at high doses. As indicated earlier, a number of studies have shown that at high doses (>150Gy), quartz OSL SAR protocols produce ages that underestimate the real ages of dated sedimentary units [for example, 70, 102, 121]. As a result, ongoing studies aim to identify the causes of these underestimations, particularly by looking at the characteristics of the individual components of the luminescence signal (for example, fast, medium and slow). Success in this quest could lead to the develop‐ ment of appropriate protocols that would make it possible to extend the dating range of quartz beyond what is currently attainable.

For feldspar, researchers have been looking at identifying IRSL signals that are less susceptible to anomalous fading which would also allow much older ages to be determined [for example, 6, 122, 123, 124]. As indicated earlier, though quartz is usually favoured for dating because it does not exhibit fading problems like those observed in feldspar, its drawback is that it saturates at much lower doses (<150 Gy). The advantage that feldspar has over quartz, however, is encumbered by the lack of fading correction methods that can be used on older ages (>100 Gy) because fading correction methods proposed to date are normally only effective for low doses [43, 44]. Hence, any methodology that would allow feldspar grains with high doses to yield accurate ages would be desirable. Accordingly, recent studies have shown that IRSL signals obtained by stimulating feldspars at a low temperature (for example, 50 °C) immediately followed by another IRSL measurement at an elevated temperature (for example, 290 °C) yields a signal that has a lower fading rate. This measurement protocol is referred to as post-IR IRSL [122] and results provided to date suggest this is a promising approach that has the potential to extend the dating range of feldspars significantly [6, 125].

[3] Wintle AG. Luminescence dating: where it has been and where it is going. Boreas

Luminescence Chronology http://dx.doi.org/10.5772/58554 63

[4] Murray A, Buylaert J-P, Henriksen M, Svendsen J-I, Mangerud J. Testing the reliabili‐ ty of quartz OSL ages beyond the Eemian. Radiation Measurements 2008; 43:

[5] Pawley SM, Bailey RM, Rose J, Moorlock BSP, Hamblin RJO, Booth SJ, Lee JR. Age limits on Middle Pleistocene glacial sediments from OSL dating, north Norfolk, UK.

[6] Buylaert JP, Murray AS, Thomsen KJ, Jain M. Testing the potential of an elevated temperature IRSL signal from K-feldspar. Radiation Measurements 2009; 44: 560-565.

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Developments in instrumentation are also keeping pace, with new luminescence measurement systems being developed [for example, 126]. Also worth mentioning is the recent development of portable OSL devices capable of conducting rapid measurements in the field [for example, 127]. Though limited in capability compared to regular OSL readers, such portable devices could, with additions such as internal X-ray radiation sources, introduce more options in the sphere of luminescence signal collection.

Overall, the field of luminescence dating is a vibrant research area and, if the recent past is any indication of what the future holds, it is a discipline guaranteed to witness innovative devel‐ opments in the coming years. With the continual refinement of both the laboratory procedures as well as the equipment, we should see chronologies being reported with greater precision and accuracy.

## **Author details**

Ken Munyikwa\*

Address all correspondence to: kenm@athabascau.ca

Centre For Science, Athabasca University, University Drive, Athabasca, Alberta, Canada

## **References**


[3] Wintle AG. Luminescence dating: where it has been and where it is going. Boreas 2008; 37: 471-482.

6, 122, 123, 124]. As indicated earlier, though quartz is usually favoured for dating because it does not exhibit fading problems like those observed in feldspar, its drawback is that it saturates at much lower doses (<150 Gy). The advantage that feldspar has over quartz, however, is encumbered by the lack of fading correction methods that can be used on older ages (>100 Gy) because fading correction methods proposed to date are normally only effective for low doses [43, 44]. Hence, any methodology that would allow feldspar grains with high doses to yield accurate ages would be desirable. Accordingly, recent studies have shown that IRSL signals obtained by stimulating feldspars at a low temperature (for example, 50 °C) immediately followed by another IRSL measurement at an elevated temperature (for example, 290 °C) yields a signal that has a lower fading rate. This measurement protocol is referred to as post-IR IRSL [122] and results provided to date suggest this is a promising approach that

Developments in instrumentation are also keeping pace, with new luminescence measurement systems being developed [for example, 126]. Also worth mentioning is the recent development of portable OSL devices capable of conducting rapid measurements in the field [for example, 127]. Though limited in capability compared to regular OSL readers, such portable devices could, with additions such as internal X-ray radiation sources, introduce more options in the

Overall, the field of luminescence dating is a vibrant research area and, if the recent past is any indication of what the future holds, it is a discipline guaranteed to witness innovative devel‐ opments in the coming years. With the continual refinement of both the laboratory procedures as well as the equipment, we should see chronologies being reported with greater precision

Centre For Science, Athabasca University, University Drive, Athabasca, Alberta, Canada

[1] Huntley DJ, Godfrey-Smith DI, Thewalt MLW. Optical dating of sediments. Nature

[2] Aitken MJ. Luminescence dating: a guide for non-specialists. Archaeometry 1989; 31:

has the potential to extend the dating range of feldspars significantly [6, 125].

sphere of luminescence signal collection.

62 Geochronology - Methods and Case Studies

Address all correspondence to: kenm@athabascau.ca

and accuracy.

**Author details**

Ken Munyikwa\*

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**Chapter 3**

**Varve Chronology**

http://dx.doi.org/10.5772/58630

growth rings in a tree known as tree-rings.

Additional information is available at the end of the chapter

Chronology indicates a sequence of time and refers back to *Chronos*, the Greek God of time. The word "varve" needs an explanation, however (*varv* is a Swedish word denoting a full circle). It refers to a rhythmic sequence representing the deposition of sediments or growth of a precipitate over a time of 1 single year (as defined by De Geer, 1884, Högbom, 1889 and Johnston, 1922). Consequently, a varve is a sedimentological equivalent to the biological

Like tree-rings, the varves are measured as to thickness. The variations in thick-ness over a varve sequence are then used to establish correlations with another, nearby sequences (Fig. 1). By extending these sequences piece by piece over time, we establish a varve

This method was invented in the late 19th century by Gerard De Geer in Sweden (Fig. 1; further described in De Geer, 1940; Mörner, 1978; Francus et al., 2013). Therefore, it was often termed "the Swedish Varve Chronology" or "the Swedish Time Scale". Today, this chronology spans about 14,000 years from the present back in time. The method has been successfully applied in Finland, and also applied in many other areas of the globe (e.g. North America, the Alps,

Some sedimentary basins contain varved sediments where the individual varves may be counted separately (or at least approximated) so that site-specific long-term chronologies are

The present paper will be devoted to the Swedish Time Scale and the application of varve

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

chronologies in general for precise dating of events, and calculations of rates.

Nils-Axel Mörner

**1. Introduction**

chronology.

and Argentina).

established.

**Chapter 3**

## **Varve Chronology**

Nils-Axel Mörner

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/58630

## **1. Introduction**

Chronology indicates a sequence of time and refers back to *Chronos*, the Greek God of time. The word "varve" needs an explanation, however (*varv* is a Swedish word denoting a full circle). It refers to a rhythmic sequence representing the deposition of sediments or growth of a precipitate over a time of 1 single year (as defined by De Geer, 1884, Högbom, 1889 and Johnston, 1922). Consequently, a varve is a sedimentological equivalent to the biological growth rings in a tree known as tree-rings.

Like tree-rings, the varves are measured as to thickness. The variations in thick-ness over a varve sequence are then used to establish correlations with another, nearby sequences (Fig. 1). By extending these sequences piece by piece over time, we establish a varve chronology.

This method was invented in the late 19th century by Gerard De Geer in Sweden (Fig. 1; further described in De Geer, 1940; Mörner, 1978; Francus et al., 2013). Therefore, it was often termed "the Swedish Varve Chronology" or "the Swedish Time Scale". Today, this chronology spans about 14,000 years from the present back in time. The method has been successfully applied in Finland, and also applied in many other areas of the globe (e.g. North America, the Alps, and Argentina).

Some sedimentary basins contain varved sediments where the individual varves may be counted separately (or at least approximated) so that site-specific long-term chronologies are established.

The present paper will be devoted to the Swedish Time Scale and the application of varve chronologies in general for precise dating of events, and calculations of rates.

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

With intensive work by De Geer himself and his students, a chronology was built up from Stockholm to central Sweden. By this he could demonstrate that it took 1073 years for the land-ice to retreat from Stockholm to Jämtland in central Sweden, a distance of 500 km. Already at the 1910 International Geological Meeting in Norden, he was able to give a detailed picture of the mode of ice recession after the last glaciation maximum around

Varve Chronology

75

http://dx.doi.org/10.5772/58630

Because De Geer's primary aim was to date the recession of land-ice over Sweden, his and his students' work was concentrated on the oldest varve deposited in front of the receding ice margin. During the time of deglaciation, the crust was isostatically depressed by the load of the ice, causing relative sea level to be significantly higher than today. From the subglacial drainage system glacifluvial material was deposited in the form of eskers and varved clay with

The first varves to be deposited in front of the receding ice margin are strongly influenced by the glacial melting giving rise to varves composed of a coarse-grained summer unit (sandysilty, sometimes even gravelly) and a fine-grained winter unit (clay to fine silt). In an esker environment, the first summer unit may include many meters of gravelly sediments (e.g. Bergström, 1968). De Geer (1940) interpreted the sand units of the proximal varves as being "flocculated" through the water column in front of the ice. Kuenen (1951) showed, however, that these units must have been deposited as turbidites (bed-load transport). The thick sandy summer units of proximal varves often exhibit rhythmic laminations. Ringberg (1984) counted some 50 laminae and proposed that they represented the number of summer days with open

The clayey winter units represent the slow setting of suspended matter during the winters. These beds are often dark to black, exhibiting a reducing environment. During the winters, the lake and sea levels froze over, turbulence ceased and calm water conditions were established

The annual rhythmicity behind De Geer's glacial varves was the annual changes between melting in the summers and freezing in the winters. When the ice was gone some 9000 years ago, climatic conditions like to day were established. Even then, however, "postglacial" varved sedimentary sequences were formed in some lakes (Renberg, 1983) and especially in the deposits of the main rivers in the north due to the annual rhythm of a strong spring melting

Besides the strict building up of "the Swedish Time Scale" via multiple short-distance varve correlations from Stockholm to north central Sweden, De Geer (1940) also attempted so-called "telecorrelations" over inter-continental and even inter-hemispheral distances. It seems to be

and low water discharge in late summers and winters (Lidén, 1913, 1938).

an unfortunate mistake, however (not further discussed in this paper).

20,000 years ago (De Geer, 1912).

water conditions in the Baltic.

allowing suspended matter to settle.

**2.1. Identifying and measuring varves**

one varve for each year (as illustrated in Fig. 2).

**Figure 1.** Gerard De Geer demonstrating the varve chronological method in Essex Junction, US, in 1920 (where he made the first measurements already in 1891).

## **2. Building up the Swedish varve chronology**

In 1884, the Swedish geologist Gerard De Geer observed in a channel excavation in Stockholm that the basal clay was laminated in a fashion, which made him think in terms of the annual growth rings in trees. He noted that the lamina consisted of a lower unit that was lighter in colour and courser in grain size and an upper clay unit that was quite dark. He named those couplets "varves" and claimed that they represented annual deposition (De Geer, 1884, 1940). In 1889, Högbom showed that the ratio of magnesium carbonate and calcium carbonate differed in the two units, and he interpreted this in terms the annual geochemical changes in the Baltic.

In 1904, a period of intensive construction of new houses stated in Stockholm. This gave rise to excellent exposures of the sediment beds, including the basal "varved clay" (as it was now called). De Geer measured new exposures, and to his surprise he noted that the new diagrams correlated well not only in between themselves, but also with the diagram he had measured 20 years before, located some 3 km to the east. This convinced him partly that the varves really were true annual varves, and partly that he would now be able to build up a continual chronology.

With intensive work by De Geer himself and his students, a chronology was built up from Stockholm to central Sweden. By this he could demonstrate that it took 1073 years for the land-ice to retreat from Stockholm to Jämtland in central Sweden, a distance of 500 km. Already at the 1910 International Geological Meeting in Norden, he was able to give a detailed picture of the mode of ice recession after the last glaciation maximum around 20,000 years ago (De Geer, 1912).

## **2.1. Identifying and measuring varves**

**Figure 1.** Gerard De Geer demonstrating the varve chronological method in Essex Junction, US, in 1920 (where he

In 1884, the Swedish geologist Gerard De Geer observed in a channel excavation in Stockholm that the basal clay was laminated in a fashion, which made him think in terms of the annual growth rings in trees. He noted that the lamina consisted of a lower unit that was lighter in colour and courser in grain size and an upper clay unit that was quite dark. He named those couplets "varves" and claimed that they represented annual deposition (De Geer, 1884, 1940). In 1889, Högbom showed that the ratio of magnesium carbonate and calcium carbonate differed in the two units, and he interpreted this in terms the annual geochemical changes in

In 1904, a period of intensive construction of new houses stated in Stockholm. This gave rise to excellent exposures of the sediment beds, including the basal "varved clay" (as it was now called). De Geer measured new exposures, and to his surprise he noted that the new diagrams correlated well not only in between themselves, but also with the diagram he had measured 20 years before, located some 3 km to the east. This convinced him partly that the varves really were true annual varves, and partly that he would now be able to build up a continual

made the first measurements already in 1891).

74 Geochronology - Methods and Case Studies

the Baltic.

chronology.

**2. Building up the Swedish varve chronology**

Because De Geer's primary aim was to date the recession of land-ice over Sweden, his and his students' work was concentrated on the oldest varve deposited in front of the receding ice margin. During the time of deglaciation, the crust was isostatically depressed by the load of the ice, causing relative sea level to be significantly higher than today. From the subglacial drainage system glacifluvial material was deposited in the form of eskers and varved clay with one varve for each year (as illustrated in Fig. 2).

The first varves to be deposited in front of the receding ice margin are strongly influenced by the glacial melting giving rise to varves composed of a coarse-grained summer unit (sandysilty, sometimes even gravelly) and a fine-grained winter unit (clay to fine silt). In an esker environment, the first summer unit may include many meters of gravelly sediments (e.g. Bergström, 1968). De Geer (1940) interpreted the sand units of the proximal varves as being "flocculated" through the water column in front of the ice. Kuenen (1951) showed, however, that these units must have been deposited as turbidites (bed-load transport). The thick sandy summer units of proximal varves often exhibit rhythmic laminations. Ringberg (1984) counted some 50 laminae and proposed that they represented the number of summer days with open water conditions in the Baltic.

The clayey winter units represent the slow setting of suspended matter during the winters. These beds are often dark to black, exhibiting a reducing environment. During the winters, the lake and sea levels froze over, turbulence ceased and calm water conditions were established allowing suspended matter to settle.

The annual rhythmicity behind De Geer's glacial varves was the annual changes between melting in the summers and freezing in the winters. When the ice was gone some 9000 years ago, climatic conditions like to day were established. Even then, however, "postglacial" varved sedimentary sequences were formed in some lakes (Renberg, 1983) and especially in the deposits of the main rivers in the north due to the annual rhythm of a strong spring melting and low water discharge in late summers and winters (Lidén, 1913, 1938).

Besides the strict building up of "the Swedish Time Scale" via multiple short-distance varve correlations from Stockholm to north central Sweden, De Geer (1940) also attempted so-called "telecorrelations" over inter-continental and even inter-hemispheral distances. It seems to be an unfortunate mistake, however (not further discussed in this paper).

*2.1.1. From field observations to chronological tools*

then used for inter-site correlations.

very wide lateral distribution.

Scale and dated in absolute varve ages BP.

(Mörner, 2003, 2013a).

The varves are observed and recorded in open pits or in cores. An open pit is always better because it allow us to view the lateral variations. In cores, very long and continual sequences can be obtained, however. The "Swedish Foil Piston Corer" was designed just for this purpose

Varve Chronology

77

http://dx.doi.org/10.5772/58630

De Geer introduced the simple method of rolling out a paper stripe over the section or core of varves and marking each individual varve on the stripe. Then the individual varve thicknesses were measured and plotted on a diagram. The saw-tooth patterns of the varve diagrams were

Varves of special characteristics, "marker-varves", were sometimes used for correlations (e.g. De Geer, 1940; Bergström, 1968; Strömberg, 1989). Mörner (e.g. 2013) did the opposite, used the varve chronology to prove that a "marker varve" represented one single event and had a

Fig. 3 shows two cores taken close to each other in two successive years. Even visually, it is easy to see the nearly identical variations in varve thickness. The varve diagram shows variations that allow the correlation with the main Swedish Time Scale, so that absolute ages are obtained. In this case, traces of two separate earthquakes were identified and dated

**Figure 3.** Varved clay cores take two years apart at approximately the same site. The inter-core correlations are very clear. The entire section includes 110 varves. Via the marker varves, the section can be correlated to the Swedish Time

allowing the retrieval of undisturbed cores of 11 m length (e.g. Järnefors, 1963).

**Figure 2.** *Above:* The mode of ice melting, subglacial drainage and esker formation with 3 esker centra (yellow) and 3 successive annual varves (pink). *Below:* De Geer's (1940) map of the ice marginal position in year 10,385 BP, variations in thickness of this year's varve (blue), esker accumulations (orange) and varve sites (red dots). From Mörner (2008).

## *2.1.1. From field observations to chronological tools*

The varves are observed and recorded in open pits or in cores. An open pit is always better because it allow us to view the lateral variations. In cores, very long and continual sequences can be obtained, however. The "Swedish Foil Piston Corer" was designed just for this purpose allowing the retrieval of undisturbed cores of 11 m length (e.g. Järnefors, 1963).

De Geer introduced the simple method of rolling out a paper stripe over the section or core of varves and marking each individual varve on the stripe. Then the individual varve thicknesses were measured and plotted on a diagram. The saw-tooth patterns of the varve diagrams were then used for inter-site correlations.

Varves of special characteristics, "marker-varves", were sometimes used for correlations (e.g. De Geer, 1940; Bergström, 1968; Strömberg, 1989). Mörner (e.g. 2013) did the opposite, used the varve chronology to prove that a "marker varve" represented one single event and had a very wide lateral distribution.

Fig. 3 shows two cores taken close to each other in two successive years. Even visually, it is easy to see the nearly identical variations in varve thickness. The varve diagram shows variations that allow the correlation with the main Swedish Time Scale, so that absolute ages are obtained. In this case, traces of two separate earthquakes were identified and dated (Mörner, 2003, 2013a).

**Figure 3.** Varved clay cores take two years apart at approximately the same site. The inter-core correlations are very clear. The entire section includes 110 varves. Via the marker varves, the section can be correlated to the Swedish Time Scale and dated in absolute varve ages BP.

**Figure 2.** *Above:* The mode of ice melting, subglacial drainage and esker formation with 3 esker centra (yellow) and 3 successive annual varves (pink). *Below:* De Geer's (1940) map of the ice marginal position in year 10,385 BP, variations in thickness of this year's varve (blue), esker accumulations (orange) and varve sites (red dots). From Mörner (2008).

76 Geochronology - Methods and Case Studies

## **2.2. Additional applications**

Lidén (1938) measured 7522 postglacial varves in the fluvial deposits occurring along the River Ångermanälven (plus a gap of 980 years to year 1900). By this, the varve chronology was fixed to the present and we were able to talk in terms of absolute years. De Geer's (1940) varve-1073 referring to the onset of the marine *Yoldia Sea* stage in the Baltic was now dated in absolute years at 9625 BP (later to be revised to 10,430 varves BP as discussed below).

Fromm (1938) measured pollen and diatoms in the same varves, implying that we from that time on were able to know the absolute ages of the immigration of different tree species, and the changes between fresh-water and marine stages of the Baltic.

Because Lidén's work referred to the succession of river deltas, he achieved a curve of the relative land uplift dated in absolute years BP (Fig. 4). It became a fundamental tool for the understanding of the concept of glacial isostsy (Gutenberg, 1941; Mörner, 1979).

bad errors in the varve ages were documented; especially in eastern US and Canada (e.g. Ridge

**Figure 5.** The Swedish Varve Chronology (or Swedish Time Scale) covers about 14,000 varve years back in time: (1) the drainage of the Central Jämtland Ice Lake at varve 9239 BP, (2) a major >8 earthquake at Hudiksvall at varve 9663 BP (and ~9150 C14-years BP), (3) a major >8 earthquake with ingression of salt water into the Baltic basin and the onset of the Yoldia Sea stage (*sensu strictu*) at 10,430 BP, (4) the Drainage of the Baltic Ice Lake, roughly corresponding to the end of the Younger Dryas Stadial, at 10,740 BP, (5) the onset of the Younger Dryas (YD) cold period, (6) the onset

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Internationally, the application of varve dating, rather switched from the ice recessional records in Sweden (De Geer, 1940), Finland (Sauramo, 1923) and North America (Antevs,

Annually varved sediments were discovered in a large number of non-glacial lakebeds from other parts of the world. This opened for local absolute dating of lake deposits. Many excellent papers were published (e.g. Anthony, 1977; Kelts & Hsü, 1978; Sturm, 1979; O'Sullivan, 1983;

In Sweden and Finland, we entered into a period of revision. The postglacial varves along River Ånermanälven and the connection to the present were revised by Cato (1987), and an error of +350 varves was established. The varves from Central Sweden to Stockholm were revised by Järnefors (1963) and later Strömberg (1989), who found a minor error of +19 varves. The number of varves between the "drainage of the Baltic Ice Lake" and the immigration of saltwater at Stockholm at De Geer's varve -1073 (De Geer, 1940; Mörner, 1995; Johnson et al., 2013) was set at 292 varves by Sauramo (1923), later at 299 varves by Mörner (1977) and finally 310 varves by Brunnberg (1995), who dated the two events at, respectively, 10,740 and 10,430 BP. Kristiansson (1986) extended the chronology through the Younger Dryas and Alleröd periods, with an additional sequence by Ringberg (1991). So, today, the Swedish Varve Chronology spans some 14,000 varves (Fig. 5) with, as it seems, quite a small margin of error in the varve dating. It must be noted, however, that there still remains a significant discrepancy with respect to calibrated C14-ages, which seems to be as much as in the order of 700 years (Mörner, 2003, p. 179) to 800 years (Wohlfart & Possnert, 2000); the varve ages being too young. The missing varves must be searched for at a time younger than 9663 varves BP, and maybe

and Larsen, 1990).

of the Alleröd warm period.

1932) to chronologies of continual lake records.

**4. A period of revision and extension**

between 5000 and 2000 BP (Wohlfarth et al., 1997).

Anderson et al., 1985; Saarnisto, 1985).

All this was, of course, quite remarkable at a time period were we generally lacked other means of establishing absolute time.

The varve chronology flourished also in Finland (Sauramo, 1923), and was also applied to eastern North America by Antevs (e.g. 1932) and Patagonia (Caldenius, 1932).

**Figure 4.** Varve-dated shorelevel displacement curve from Ångermanland by Lidén (1938; as redrawn in Mörner, 1979).

## **3. A period of hesitation and change of focus**

With the introduction of the radiocarbon dating method (Arnold and Libby, 1949) things changed, and there suddenly was an alternative method of obtaining absolute ages. Also, quite

**Figure 5.** The Swedish Varve Chronology (or Swedish Time Scale) covers about 14,000 varve years back in time: (1) the drainage of the Central Jämtland Ice Lake at varve 9239 BP, (2) a major >8 earthquake at Hudiksvall at varve 9663 BP (and ~9150 C14-years BP), (3) a major >8 earthquake with ingression of salt water into the Baltic basin and the onset of the Yoldia Sea stage (*sensu strictu*) at 10,430 BP, (4) the Drainage of the Baltic Ice Lake, roughly corresponding to the end of the Younger Dryas Stadial, at 10,740 BP, (5) the onset of the Younger Dryas (YD) cold period, (6) the onset of the Alleröd warm period.

bad errors in the varve ages were documented; especially in eastern US and Canada (e.g. Ridge and Larsen, 1990).

Internationally, the application of varve dating, rather switched from the ice recessional records in Sweden (De Geer, 1940), Finland (Sauramo, 1923) and North America (Antevs, 1932) to chronologies of continual lake records.

Annually varved sediments were discovered in a large number of non-glacial lakebeds from other parts of the world. This opened for local absolute dating of lake deposits. Many excellent papers were published (e.g. Anthony, 1977; Kelts & Hsü, 1978; Sturm, 1979; O'Sullivan, 1983; Anderson et al., 1985; Saarnisto, 1985).

## **4. A period of revision and extension**

**2.2. Additional applications**

78 Geochronology - Methods and Case Studies

of establishing absolute time.

1979).

Lidén (1938) measured 7522 postglacial varves in the fluvial deposits occurring along the River Ångermanälven (plus a gap of 980 years to year 1900). By this, the varve chronology was fixed to the present and we were able to talk in terms of absolute years. De Geer's (1940) varve-1073 referring to the onset of the marine *Yoldia Sea* stage in the Baltic was now dated in absolute

Fromm (1938) measured pollen and diatoms in the same varves, implying that we from that time on were able to know the absolute ages of the immigration of different tree species, and

Because Lidén's work referred to the succession of river deltas, he achieved a curve of the relative land uplift dated in absolute years BP (Fig. 4). It became a fundamental tool for the

All this was, of course, quite remarkable at a time period were we generally lacked other means

The varve chronology flourished also in Finland (Sauramo, 1923), and was also applied to

**Figure 4.** Varve-dated shorelevel displacement curve from Ångermanland by Lidén (1938; as redrawn in Mörner,

With the introduction of the radiocarbon dating method (Arnold and Libby, 1949) things changed, and there suddenly was an alternative method of obtaining absolute ages. Also, quite

**3. A period of hesitation and change of focus**

years at 9625 BP (later to be revised to 10,430 varves BP as discussed below).

understanding of the concept of glacial isostsy (Gutenberg, 1941; Mörner, 1979).

eastern North America by Antevs (e.g. 1932) and Patagonia (Caldenius, 1932).

the changes between fresh-water and marine stages of the Baltic.

In Sweden and Finland, we entered into a period of revision. The postglacial varves along River Ånermanälven and the connection to the present were revised by Cato (1987), and an error of +350 varves was established. The varves from Central Sweden to Stockholm were revised by Järnefors (1963) and later Strömberg (1989), who found a minor error of +19 varves. The number of varves between the "drainage of the Baltic Ice Lake" and the immigration of saltwater at Stockholm at De Geer's varve -1073 (De Geer, 1940; Mörner, 1995; Johnson et al., 2013) was set at 292 varves by Sauramo (1923), later at 299 varves by Mörner (1977) and finally 310 varves by Brunnberg (1995), who dated the two events at, respectively, 10,740 and 10,430 BP. Kristiansson (1986) extended the chronology through the Younger Dryas and Alleröd periods, with an additional sequence by Ringberg (1991). So, today, the Swedish Varve Chronology spans some 14,000 varves (Fig. 5) with, as it seems, quite a small margin of error in the varve dating. It must be noted, however, that there still remains a significant discrepancy with respect to calibrated C14-ages, which seems to be as much as in the order of 700 years (Mörner, 2003, p. 179) to 800 years (Wohlfart & Possnert, 2000); the varve ages being too young. The missing varves must be searched for at a time younger than 9663 varves BP, and maybe between 5000 and 2000 BP (Wohlfarth et al., 1997).

Niemelä (1971) revised the Finnish varve chronology. In Estonia and the St. Petersburg area, there are local varve sequences ("floating" varve chronologies) not yet connected to the Finnish and Swedish time scales (Hang & Kohv, 2013).

The deglaciation of the Ontario region in Canada is characterized by a number of end moraines representing halts or minor re-advances. By applying a relative varve chronology, it was possible to date the duration of the building up of the Tillsonburg Moraine at 22 years (Fig. 6). A varve sequence right in front of the Tillsonburg-Sparta I end-moraine includes 98 varves; 57 recessional varves, 19 readvance varves and 22 ice-marginal varves. This indicates a

In a few cases it has been possible also to pinpoint the season of an event. This is the case for a major earthquake in Sweden, which was shown to have occurred in the autumn of varve

Because different events in Sweden could be tied to one and the same varve, it was possible to document the spatial distribution of those events. This has been especially useful in paleoseismology (Mörner, 2003, 2011, 2013a; Mörner & Sun, 2008). Turbidites were recorded at single varves and their spatial distribution recorded. There is a relation between seismic magnitude and the spatial distribution of liquefaction. Thanks to the varve chronology in Sweden, a paleoseismic event occurring in varve 10,430 BP was shown to have generated liquefaction over an area of 320x100 km, indicating that this event must have had a magnitude

In the glacial varves in the Stockholm region, it was possible to document and date seven separate paleoseismic events within the period 10,490 to 10,388 varves BP (Mörner, 2011); i.e. 7 events in 102 years. This is a very high seismic frequency (or recurrence time). This record

At Hudiksvall at the coast of central Sweden, the difference in elevation (7,8 m) and difference in time (~25 varves) between the Baltic level at the deglaciation and at a tsunami event in varve 9663 BP was known (Mörner, 2003, p. 183). Consequently the relative land uplift must have been in the order of 31 cm per year (with eustatic calibration corresponding to a rate of absolute uplift in the order of 40 cm/yr). This is a unique value, which provides a very accurate

Micro-varved postglacial lake sequences occur both in Sweden and Finland. They provide excellent chronological tools for the recording and dating of environmental changes (e.g. Renberg, 1983; Renberg et al., 1984; Ojala & Tiljander, 2003; Ojala et al., 2008; Ojala et al., 2013). This also includes the recording and dating of secular paleomagnetic changes in the Holocene (Ojala & Tiljander, 2003). Maier et al. (2013) were able to assess the rate of sediment

Lake Kassjön at Umeå in northern Sweden has lake sediments that are annually varved for the last 6300 years. We applied paleomagnetic studies of these deposits (Mörner and Sylwan, 1989). A major swing in declination was recorded at around 2600 varves BP, which is about where the production of 14C records a major spike. Ten samples were C14-dated over the swing in declination (the same sample as paleomagnetically analysed). Declination swings to the

a "trans-polar VGP shift" (Mörner, 1991). It coincides with the main spike in 14C-production. Therefore, this event is likely to represent an internal perturbation of the Earth's own geo‐

per year. This change constitutes

Varve Chronology

81

http://dx.doi.org/10.5772/58630

could only have been achieved thanks to the firm varve dating.

measurement of the rate of uplift right after the free-melting.

compaction; after 3-5 years the varve thickness had decreased by 60%.

in 348 years, which implies implies a rate of 0.36o

readvance in the order of 8 km and time of ice-marginal halt of only 22 years.

10,430 BP (Mörner, 2003, 2011, 2013a).

of >8 on the Richter scale.

west by 117 <sup>o</sup>

## **5. The application of events, spatial distribution and rates**

Varve dating is very useful when it concerns the dating of the duration of a geological event. De Geer (1940) was able to show the mode of ice recession and date esker centra and moraine ridges as to single years (Fig. 2). Varve chronology also gives the back‐ ground for rate calculations. The classical example is the rate of ice retreat and its changes over time (De Geer, 1940). The rate of ice marginal recession over the Stockholm area was in the order of 300 m per year, despite the fact that the ice flow to the front was in the order of 500 m per year, implying a total annual melting of about 700-800 m. This is an enormous rate of ice melting (Fig. 3a). Still, the rate of global sea level rise was in the order of 10 mm/year. This value is of great significance, because all present-day sea level changes must be well below this value (Mörner, 2011).

**Figure 6.** Stratigraphy of the Kettle Creek at the northern shore of Lake Erie including three separate till beds, the last one of which is underlain by varves indicating a readvance of about 8 km and halt of 22 years for the building up the Tillsonburg–Sparta I (SI) Moraine (Mörner, *Ice recession and varve chronology in southern Ontario*, unpublished).

The deglaciation of the Ontario region in Canada is characterized by a number of end moraines representing halts or minor re-advances. By applying a relative varve chronology, it was possible to date the duration of the building up of the Tillsonburg Moraine at 22 years (Fig. 6). A varve sequence right in front of the Tillsonburg-Sparta I end-moraine includes 98 varves; 57 recessional varves, 19 readvance varves and 22 ice-marginal varves. This indicates a readvance in the order of 8 km and time of ice-marginal halt of only 22 years.

Niemelä (1971) revised the Finnish varve chronology. In Estonia and the St. Petersburg area, there are local varve sequences ("floating" varve chronologies) not yet connected to the Finnish

Varve dating is very useful when it concerns the dating of the duration of a geological event. De Geer (1940) was able to show the mode of ice recession and date esker centra and moraine ridges as to single years (Fig. 2). Varve chronology also gives the back‐ ground for rate calculations. The classical example is the rate of ice retreat and its changes over time (De Geer, 1940). The rate of ice marginal recession over the Stockholm area was in the order of 300 m per year, despite the fact that the ice flow to the front was in the order of 500 m per year, implying a total annual melting of about 700-800 m. This is an enormous rate of ice melting (Fig. 3a). Still, the rate of global sea level rise was in the order of 10 mm/year. This value is of great significance, because all present-day sea level changes

**Figure 6.** Stratigraphy of the Kettle Creek at the northern shore of Lake Erie including three separate till beds, the last one of which is underlain by varves indicating a readvance of about 8 km and halt of 22 years for the building up the Tillsonburg–Sparta I (SI) Moraine (Mörner, *Ice recession and varve chronology in southern Ontario*, unpublished).

**5. The application of events, spatial distribution and rates**

and Swedish time scales (Hang & Kohv, 2013).

80 Geochronology - Methods and Case Studies

must be well below this value (Mörner, 2011).

In a few cases it has been possible also to pinpoint the season of an event. This is the case for a major earthquake in Sweden, which was shown to have occurred in the autumn of varve 10,430 BP (Mörner, 2003, 2011, 2013a).

Because different events in Sweden could be tied to one and the same varve, it was possible to document the spatial distribution of those events. This has been especially useful in paleoseismology (Mörner, 2003, 2011, 2013a; Mörner & Sun, 2008). Turbidites were recorded at single varves and their spatial distribution recorded. There is a relation between seismic magnitude and the spatial distribution of liquefaction. Thanks to the varve chronology in Sweden, a paleoseismic event occurring in varve 10,430 BP was shown to have generated liquefaction over an area of 320x100 km, indicating that this event must have had a magnitude of >8 on the Richter scale.

In the glacial varves in the Stockholm region, it was possible to document and date seven separate paleoseismic events within the period 10,490 to 10,388 varves BP (Mörner, 2011); i.e. 7 events in 102 years. This is a very high seismic frequency (or recurrence time). This record could only have been achieved thanks to the firm varve dating.

At Hudiksvall at the coast of central Sweden, the difference in elevation (7,8 m) and difference in time (~25 varves) between the Baltic level at the deglaciation and at a tsunami event in varve 9663 BP was known (Mörner, 2003, p. 183). Consequently the relative land uplift must have been in the order of 31 cm per year (with eustatic calibration corresponding to a rate of absolute uplift in the order of 40 cm/yr). This is a unique value, which provides a very accurate measurement of the rate of uplift right after the free-melting.

Micro-varved postglacial lake sequences occur both in Sweden and Finland. They provide excellent chronological tools for the recording and dating of environmental changes (e.g. Renberg, 1983; Renberg et al., 1984; Ojala & Tiljander, 2003; Ojala et al., 2008; Ojala et al., 2013). This also includes the recording and dating of secular paleomagnetic changes in the Holocene (Ojala & Tiljander, 2003). Maier et al. (2013) were able to assess the rate of sediment compaction; after 3-5 years the varve thickness had decreased by 60%.

Lake Kassjön at Umeå in northern Sweden has lake sediments that are annually varved for the last 6300 years. We applied paleomagnetic studies of these deposits (Mörner and Sylwan, 1989). A major swing in declination was recorded at around 2600 varves BP, which is about where the production of 14C records a major spike. Ten samples were C14-dated over the swing in declination (the same sample as paleomagnetically analysed). Declination swings to the west by 117 <sup>o</sup> in 348 years, which implies implies a rate of 0.36o per year. This change constitutes a "trans-polar VGP shift" (Mörner, 1991). It coincides with the main spike in 14C-production. Therefore, this event is likely to represent an internal perturbation of the Earth's own geo‐ magnetic field and not Solar Wind driven change of the geomagnetic shielding and 14C production (Mörner, 2013b).

9500 varved BP has an error of about 700 missing varves with respect to the radiocarbon

Varve Chronology

83

http://dx.doi.org/10.5772/58630

Varve records have a great potential when it comes the determinations of durations and rates of a large variety of events recorded by the varves. In this case, the chronology needs not to be fixed to the present, but may also be a "floating chronology" just providing a short sequence of precise annual determination. This applies for all varve records from deposits older than

Continual varve sequences from lakes basins offer local chronologies of very high precision (e.g. Kitagawa & van der Plicht, 2000; Ojala & Alenius, 2005), and can be used to date a large number of local environmental changes. Today, this application of varve records seems to be more important (VWG, 2014) than the building up of local chronologies like the famous

[1] Anderson, R. Y., Dean, W. E., Bradbury, J. P. & Love, D. (1985). Meromictic lakes and varved lake sediments in North America, *U. S. Geological Survey Bulletin,* 1607, 1-19.

[2] Anthony, R. S. (1977). Iron-rich rhythmically laminated sediments in Lake of the

[3] Arnold, J. R. & Libby, W. F. (1949). Age determination by radiocarbon content: Check

[4] Antevs, E. (1932). Late-glacial clay chronology of North America, *Ann. Rep. Smithso‐*

[5] Bergström, R. (1968). Stratigrafi och isrecesion i södra Västerbotten. *Sveriges Geologis‐*

[6] Brunnberg, L. (1995). Clay-varve chronology and deglaciation during the Younger Dryas and Preboreal in the easternmost part of the Middle Swedish ice marginal

zone, Ph.D. thesis, Stockholm University, *Quaternaria,* Series A 2, 1–94.

Clouds, northeastern Minnesota, *Limnology and Oceanography*, 22, 45-54.

calibrated chronology.

**Author details**

Nils-Axel Mörner

**References**

with

*nian Inst*., 1931, 313-324.

*ka Undersökning*, C 634, 1–76.

the Last Ice Age (e.g. Williams, 1985).

Paleogeophysics & Geodynamics, Sweden

"Swedish Time Scale" or "Swedish Varve Chronology".

samples of known age, *Science,* 110, 678-680.

The very long varved core sequence from Lake Suigetsu in Japan extends the C14-calibation back to 45,000 BP (Kitagawa & van der Plicht, 2000).

Finally, it may be of historical interest to note that Wilson already in 1943 reported on a varvesequence of 12,223 varves from the southwestern part of Lake Erie (Wilson, 1943).

## **6. Pre-Quaternary varves**

Varved sediments are, of course, not restricted to the Quaternary period. Glacial varves are recorded for all previous glaciations, too. The Permian varves in Brazil provide fine examples of glacial varves, and have led to the establishment of a special exhibition park known as *Parque do Varvito* where the varves are excellently preserved (Fig. 7). The Late Precambrian (~650 Ma) varves of the Elatina Formation in Australia (Williams, 1985) are important because they provide records of a "~12-laminae cycle" interpreted as the 11-yr solar cycle.

**Figure 7.** Parque do Varvito exhibiting Permian varves. *Right:* view of the main sequence. *Left:* close-up of proximal varves in the centre including an ice-rafted block.

## **7. Conclusions**

"The Swedish Varve Chronology" was invented and built up by De Geer (1940). With much revision and addition, the chronology now covers a period of about 14,000 years. It is based on the successive correlation of varve segments representing the deposition of varved clay in front of the receding ice margin (on-lapping varves) plus the postglacial varves of deltaic river varves down the River Ångermanälven (off-lapping varves). The sequence older than about 9500 varved BP has an error of about 700 missing varves with respect to the radiocarbon calibrated chronology.

Varve records have a great potential when it comes the determinations of durations and rates of a large variety of events recorded by the varves. In this case, the chronology needs not to be fixed to the present, but may also be a "floating chronology" just providing a short sequence of precise annual determination. This applies for all varve records from deposits older than the Last Ice Age (e.g. Williams, 1985).

Continual varve sequences from lakes basins offer local chronologies of very high precision (e.g. Kitagawa & van der Plicht, 2000; Ojala & Alenius, 2005), and can be used to date a large number of local environmental changes. Today, this application of varve records seems to be more important (VWG, 2014) than the building up of local chronologies like the famous "Swedish Time Scale" or "Swedish Varve Chronology".

## **Author details**

magnetic field and not Solar Wind driven change of the geomagnetic shielding and 14C

The very long varved core sequence from Lake Suigetsu in Japan extends the C14-calibation

Finally, it may be of historical interest to note that Wilson already in 1943 reported on a varve-

Varved sediments are, of course, not restricted to the Quaternary period. Glacial varves are recorded for all previous glaciations, too. The Permian varves in Brazil provide fine examples of glacial varves, and have led to the establishment of a special exhibition park known as *Parque do Varvito* where the varves are excellently preserved (Fig. 7). The Late Precambrian (~650 Ma) varves of the Elatina Formation in Australia (Williams, 1985) are important because they

**Figure 7.** Parque do Varvito exhibiting Permian varves. *Right:* view of the main sequence. *Left:* close-up of proximal

"The Swedish Varve Chronology" was invented and built up by De Geer (1940). With much revision and addition, the chronology now covers a period of about 14,000 years. It is based on the successive correlation of varve segments representing the deposition of varved clay in front of the receding ice margin (on-lapping varves) plus the postglacial varves of deltaic river varves down the River Ångermanälven (off-lapping varves). The sequence older than about

sequence of 12,223 varves from the southwestern part of Lake Erie (Wilson, 1943).

provide records of a "~12-laminae cycle" interpreted as the 11-yr solar cycle.

production (Mörner, 2013b).

82 Geochronology - Methods and Case Studies

**6. Pre-Quaternary varves**

varves in the centre including an ice-rafted block.

**7. Conclusions**

back to 45,000 BP (Kitagawa & van der Plicht, 2000).

## Nils-Axel Mörner

Paleogeophysics & Geodynamics, Sweden

## **References**


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[33] Mörner, N.-A. (2011). Setting the frames of expected future sea level changes, In: *Evi‐*

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**Section 2**

**The Pre-Quaternary**

**The Pre-Quaternary**

**Chapter 4**

**Geochronology From The Castelo Branco Pluton**

The geochronology of granitic rocks is a key-issue in the crustal evolution and orogenic processes. Modern high precision techniques have been used to identify relevant geological

U-Th-Pb chemical dating by electron-microprobe (EPMA) is a potentially valuable method in monazite-bearing rocks. Monazite presents the fundamental conditions required to apply this procedures: 1) monazite is a U-Th enriched phase, 2) all Pb monazite is radiogenic; 3) its closure temperature has proved to be fairly high, up to 900º C [1] and 4) the system remains close [2]. Monazite presents a higher resistance than zircon to radiation damage effects [3] and low

Different studies have demonstrated that U-Th-Pb dating by EPMA is an accurate method of geochronology (e.g., [5-8]). U-Th-Pb monazite age determination can be obtained in small crystals (5 μm), allowing the study of mineral heterogeneities, without destruction and preserving textural relationships. Microanalytical techniques are an adequate way to study magmatic and polymetamorphic events registered in monazites with zoning textures (e.g., [2, 9-11]). The advantages of this technique are the high spatial resolution and the possibility to obtain rapidly a large number of ages. The main disadvantage is the low accuracy, conditioned by Pb content and statistical treatment of data. The analytical error frequently ranges from ± 40 to ± 120Ma (2σ) for ages of 300 to 3000 Ma, respectively [8]. However, a statistical treatment

Attemps to constraint the timing of high-temperature orogenic processes including crustal melting, metamorphism and deformation are typically based upon U-Pb age analysis of accessory minerals such as zircon and monazite. Although growth and recrystallization of

> © 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

of homogeneous ages promotes a decrease of uncertainty to ± 20 – 30 Ma [2].

**(Portugal) — Isotopic Methodologies**

Additional information is available at the end of the chapter

Antunes Imhr

**1. Introduction**

diffusion rates [4].

episodes.

http://dx.doi.org/10.5772/58686
