**Continental Erosion/Weathering Changes in Central Asia Recorded in the Holocene Sediment from Lake Hovsgol, Northwest Mongolia, by Synchrotron μ-XRF Mapping Analyses**

Nagayoshi Katsuta1, Takuma Murakami2,3, Yuko Wada2, Masao Takano2, Masayuki Kunugi4 and Takayoshi Kawai2,5 *1Faculty of Education, Gifu University 2Graduate School of Environmental Studies, Nagoya University 3Low Level Radioactivity Laboratory, Kanazawa University 4Environment Safety Center, Tokyo University of Science 5Association of International Research Initiatives for Environmental Studies Japan* 

#### **1. Introduction**

136 International Perspectives on Global Environmental Change

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Kuzmin, M. I. (1997). Lake Baikal record of continental climate response to orbital insolation during the past 5 million years. *Science*, Vol.278, No.5340, (November Lake Hovsgol (Fig. 1) is located in the southernmost part of the Baikal rift valley basins and occupies the second largest basin next to Lake Baikal. The lake lies 1645 m above sea level, and its surface area is 2,760 km2 (136 km long, 20~40 km wide). It has a water volume of 380.7 km3 and a maximum depth of 262.4 m (Goulden et al., 2006). The lake is surrounded by three types of vegetation regions: taiga-forest, steppe, and steppe-forest. The annual mean temperature is below zero (above zero during May to September), and the precipitation is 300~500 mm per year, most of which falls from April to October (Namkhaijantsan, 2006). The lake water contains Ca2+ at 797 μM. Its alkalinity is 2.60 (mEq/L), and it has a pH of 8.1 (Hayakawa et al., 2003). Geophysical observations reveal that Hovsgol's sediment is several kilometers thick (Fedotov et al., 2006), which suggests that the sedimentary sequences may document a long-term history of environmental changes in arid central Asia.

Recent studies on Lake Hovsgol cores indicate that the sediment chemistry records are important sources of information to understand environmental variations in the region and related climate changes. Oscillations in the climate proxy data acquired by stack of elements hosted in the carbonates and organic matter have been found to coincide with abrupt climate shifts in the Holocene and the last glacial/Holocene transition observed in the North Atlantic region (Fedotov et al., 2004a). Periodic variations in the 21 chemical elements in the bulk-sediment suggested that moisture change in central Asia occurred on glacialinterglacial scales, as well as with a period of ~8.7 kyr, through the last glacial/Holocene (Murakami et al., 2010). Phedorin et al. (2008) analyzed the past 1 Myr geochemical records

Continental Erosion/Weathering Changes in Central Asia Recorded in the Holocene

**2. Materials and analytical methods** 

section (Prokopenko et al., 2005).

**2.2 Sample preparations** 

glass slide using a solvent.

**2.3 SR μ-XRF mapping** 

(Fig. 4a). The measurement time was 25 s per step.

**2.1 Core HDP-06** 

Sediment from Lake Hovsgol, Northwest Mongolia, by Synchrotron μ-XRF Mapping Analyses 139

In the present study, we used core HDP-06 which was collected at the base of the southeast slope of Lake Hovsgol, in northwest Mongolia (Fig. 1). The HDP-06 core drilling took place in March 2006 at 50°54'25" N, 100°27'03" E at a water depth of 235.5 m using an improvised push-coring type technology (Hovsgol Drilling Project Members, 2009). Core HDP-06 was about 26 m long and the sediment recovery was on the order of 80%. In the present study, we analyzed the upper section (about 18 cm) of HDP-06 core 1-1 (about 140 cm), which was undisturbed and fully recovered. The lithologic observation revealed that the core 1-1 bears sedimentary layers distinctive of Hovsgol's sediment, which is composed of diatomaceous clayey ooze in the Holocene section and calcareous clayey silt to silty clay in the last glacial

In order to acquire two-dimensional XRF images of the sediment surface by the SR μ-XRF techniques, we prepared thin sections of the resin-embedded samples. Subsamplings from an open core surface were carried out using aluminum U-channels (Fig. 2). After carefully retrieving the sediment slab from the U-channel, the slabs were impregnated with epoxy resin following the procedure of Tiljander et al. (2002). The U-channels were produced from a 0.2 mm-thick aluminum sheet using a sheet bender (Fig. 2a), which we produced ourselves. A resin-embedded sediment block was shaped to a thickness of 0.3 mm for SR μ-XRF mapping. This was done to reduce the X-ray scattering from the inside of the sample. First, a pair of sediment slabs (Fig. 3a) was separated with a band saw because several sediment slabs were embedded together with the epoxy resin. Each sediment slab was cut into three pieces to produce approximately 5 cm × 5 cm samples at an angle of 45° to the bedding plane (Fig. 3b), in order to maintain the vertical continuity of the sediment. One face of each sample was polished and glued to a glass slide. After polishing the other face to achieve a thickness of about 0.3 mm, the thin section sample (Figs. 3c and d) was removed from the

The SR experiment was carried out at undulator beamline 37XU of SPring-8, Hyogo, Japan (Fig. 4a; Terada et al., 2004). A Si (111) double-crystal monochromator was used to acquire the incident X-ray beams at 37 keV. The X-ray beam size was adjusted using an XY slit and was 1 mm (V) × 0.5 mm (H). A thin-section sample was fixed on an acrylic holder (Fig. 3c). The sample holder was set on the X-Y computer-controlled step stage, which was rotated by 10º in the rectangular direction of the incident beam toward the detector (Fig. 3d). The stage had step sizes of 1 mm (V) and 0.5 mm (H). The XRF spectra from the sample were measured using a Ge solid-state detector coupled with a multichannel pulse-height analyzer

In Hovsgol's sediment, fluorescence X-rays of the 11 elements (Ti, Mn, Fe, Cu, Zn, As, Br, Rb, Zr, and Nb) were successfully detected (Figs. 4b and c). Based on the obtained XRF spectra, we determined the distribution map and profile of each element (Fig. 5). The XRF spectra were collected at each position on a sample. At each position, we computed the integrated numbers of X-ray photons with energy near each *Kα1* line (energy window within *Kα1* ± 0.10 keV), and consequently produced XRF maps of the 11 elements. Moreover, the XRF profiles were acquired by averaging three vertical lines of the XRF maps, which were appropriately selected.

of sediments from the two great Asian lakes (Hovsgol for Ti/Ca and Br; Baikal for diatoms and Br) with a synchrotron radiation (SR) induced μ-XRF line-scanning technique, and then found a 300-500 kyr long cycle, possibly associated with Earth's orbital eccentricity. Moreover, Phedorin et al. (2008) suggested that the variations with periods of 300-500 kyr influenced the evolution of terrestrial ecosystem.

Fig. 1. Maps of (a) Lake Hovsgol in (b) northern Mongolia. (a) Bathymetric map of the lake showing drill sites for cores X106 and HDP-06. HDP-06 was used for analyses in the present study. The gray shading and lines in (b) indicate the Lake Baikal catchment area and the borders of China, Mongolia, and Russia, respectively.

In the present study, we investigated the distributions and their origins of 11 major and trace elements in the Holocene section of Hovsgol's sediment using the SR μ-XRF mappings. With respect to the Hovsgol's sediment, the two-dimensional distribution of the elements has not been studied until now. A visualization of distribution of the elements enables us to recognize detailed sedimentary or geochemical structures such as laminations, local elementally enriched grains and layers, and so on (Katsuta et al., 2007). Moreover, it makes possible to distinguish between primary structures and secondary structures deposited in the sediment. As a result of this study, we discovered an alternation of terrigenous and biological elements in the Hovsgol Holocene sections. In this chapter, we first describe methods for capturing the SR μ-XRF data of the sediment surfaces. After investigating distributions and features of the detected 11 elements, we assess the sources and natures of each element. We discuss the paleoenvironmental implications for the observed alternated pattern in the sedimentation, together with a record of the detritus input into Lake Hovsgol (Murakami et al. 2010).
