**3. Results and discussions**

142 International Perspectives on Global Environmental Change

Fig. 4. (a) Schematic sketch showing experimental setup for SR μ-XRF mapping at BL37XU of SPring-8 and XRF spectra in ranges of (b) 0–40 keV and (c) 0–20 keV for upper section of HDP-06 core 1-1 from Lake Hovsgol. Sample of red rectangle in (a) corresponds to

Fig. 3d.

### **3.1 Distribution of elements in Lake Hovsgol sediment**

Successive maps and profiles of the 11 elements in the upper section of HDP-06 core 1-1 are shown in Fig. 5. From a visual inspection of the distribution and features, the 11 elements were classified into three assemblages–group 1: Ti, Fe, Cu, Zn, As, Rb, Sr, Zr, and Nb; group 2: Br; and group 3: Mn, Fe, and As. Their distributions in the sediment are characterized as follows:


Based on the distributions and assemblages of the elements, group 1 is recognized to be elements composed of rock-forming minerals in the bedrock of the Hovsgol basin (Murakami et al., 2010). The group 1 is therefore terrigenous elements, which were supplied from the drainage basin by erosion and weathering processes. Bromine belonging to group 2 is a biophilic element whose abundance variation in the sediment reflects bioproduction in the lake (Phedorin et al., 2008). In Lake Hovsgol, the record of Br as alternative to diatom and bioSi, especially in the bottom sediment below 5.74 m (Fedotov et al., 2004b), is an important source of information to estimate biogenic production of the lake because the diatom frustules may be dissolved in the sediment high-pH pore water resulting from the presence of carbonates.

Manganese, Fe, and As of group 3 are elements sensitive to redox condition in the sediment. Existences of these three elements were indentified also in core X106 (Murakami et al. 2010). According to the study, the Mn in the core showed irregular distributions from the last glacial/Holocene transition to the Holocene section. The XRD analyses didn't indicate peaks of minerals containing Mn, thereby suggesting that the Mn in the sediment exists in an amorphous state. On the other hand, the coexistence of Fe and As identified in this study (red arrows in Figs. 5c and f) were identified in the last glacial/Holocene transition section of core X106. Because the section contains pyrite and dolomite as well as arsenic, Murakami et al. (2010) suggested the presence of sulfate-reducing bacteria.

These three elements of group 3 may have responded to redox changes and have subsequently migrated during the diagenetic process in the sediment. We therefore investigate paleoenvironmental implications for variations in the group 1 terrigenous elements and group 2 Br records of core HDP-06, as discussed below.

#### **3.2 Continental erosion/weathering changes in central Asia during the Holocene period**

Comparison of the terrigenous elements of group 1 (Ti, Fe, Cu, Zn, As, Rb, Sr, Zr, and Nb) with the Br of group 2 in the HDP-06 core 1-1 indicates a strong counterphase in the Holocene section (Fig. 5). At the present stage, the age of the section has not yet been determined. However, as shown in Figs. 6b and c, the content variation of Ti in the HDP-06 sediment is well correlative with that of dated X106 core (Murakami et al., 2010). Accordingly, by a correlation between these two profiles, we discuss hereafter nature of the temporal content variations of Ti and Br in HDP-06 core, together with paleoproxy records of X106 core.

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

Sediment from Lake Hovsgol, Northwest Mongolia, by Synchrotron μ-XRF Mapping Analyses 145

Figure 6 shows successive profiles for sediment chemistry of Lake Hovsgol, atmospheric CO2 (Indermühle et al. 1999), and general circulation model (GCM)-predicted climatic parameters in central Asia (Bush, 2005). The second principal component (PC-2) score (Fig. 6d) was obtained by principal component analysis of 21 chemical components in bulksediment of X106 core by the ICP-MS (inductively coupled plasma mass spectrometry) analyses (Murakami et al., 2010). The variability of the PC-2 score was controlled by chemical elements from detrital materials, thereby indicating erosion/weathering intensity in the Hovsgol drainage basin. Temporal variations of annual mean temperature and precipitation minus evaporation (PME) in central Asia (Figs. 6e and f) are GCM outputs of simulation accounting to the combined effect of orbital and CO2/H2O (Fig. 6g) forcing.

Fig. 6. Comparison of (a)-(d) paleoenvironmental proxy records from Lake Hovsgol, (e)-(f) GCM-simulated selected climatic parameters over central Asia (Bush, 2005), and (g)

atmospheric CO2 concentration record from Antarctica ice core at Talyor Dome (Indermühle et al., 1999) over the Holocene interval. (a) Br and (b) Ti profiles in Lake Hovsgol sediment of HDP-06 core 1-1 are captured by the SR μ-XRF analysis. (c) Ti and (d) PC-2 score in Lake Hovsgol core X106 are from Murakami et al. (2010). PME in (f) stands for precipitation minus evaporation. Arrows with curved or straight lines denoted in each plot emphasize significant environmental/climatic trend. The horizontal scale in (a)-(b) and that in (c)-(g) are shown by the core depth in the upper axis and by the age in lower axis, respectively. Vertical gray dashed lines in (a)-(d) indicates a visual correlation of Ti in cores between

HDP-06 and X106.

Fig. 5. Successive maps and profiles of major and trace elements in upper section of HDP-06 core 1-1: (a) Ti, (b) Mn, (c) Fe, (d) Cu, (e) Zn, (f) As, (g) Br, (h) Rb, (i) Sr, (j) Zr, and (k) Nb. The black arrows in (a) show the boundaries between the neighboring measurement areas, which were overlapped by approximately several millimeters. The vertical distance of each element profile hence represents the apparent core depth. The red arrows in (e) and (f) show the coexistence of Fe and As. The low XRF counts seen on the left side of each map were caused by the epoxy resin.

Fig. 5. Successive maps and profiles of major and trace elements in upper section of HDP-06 core 1-1: (a) Ti, (b) Mn, (c) Fe, (d) Cu, (e) Zn, (f) As, (g) Br, (h) Rb, (i) Sr, (j) Zr, and (k) Nb. The black arrows in (a) show the boundaries between the neighboring measurement areas, which were overlapped by approximately several millimeters. The vertical distance of each element profile hence represents the apparent core depth. The red arrows in (e) and (f) show the coexistence of Fe and As. The low XRF counts seen on the left side of each map were

caused by the epoxy resin.

Figure 6 shows successive profiles for sediment chemistry of Lake Hovsgol, atmospheric CO2 (Indermühle et al. 1999), and general circulation model (GCM)-predicted climatic parameters in central Asia (Bush, 2005). The second principal component (PC-2) score (Fig. 6d) was obtained by principal component analysis of 21 chemical components in bulksediment of X106 core by the ICP-MS (inductively coupled plasma mass spectrometry) analyses (Murakami et al., 2010). The variability of the PC-2 score was controlled by chemical elements from detrital materials, thereby indicating erosion/weathering intensity in the Hovsgol drainage basin. Temporal variations of annual mean temperature and precipitation minus evaporation (PME) in central Asia (Figs. 6e and f) are GCM outputs of simulation accounting to the combined effect of orbital and CO2/H2O (Fig. 6g) forcing.

Fig. 6. Comparison of (a)-(d) paleoenvironmental proxy records from Lake Hovsgol, (e)-(f) GCM-simulated selected climatic parameters over central Asia (Bush, 2005), and (g) atmospheric CO2 concentration record from Antarctica ice core at Talyor Dome (Indermühle et al., 1999) over the Holocene interval. (a) Br and (b) Ti profiles in Lake Hovsgol sediment of HDP-06 core 1-1 are captured by the SR μ-XRF analysis. (c) Ti and (d) PC-2 score in Lake Hovsgol core X106 are from Murakami et al. (2010). PME in (f) stands for precipitation minus evaporation. Arrows with curved or straight lines denoted in each plot emphasize significant environmental/climatic trend. The horizontal scale in (a)-(b) and that in (c)-(g) are shown by the core depth in the upper axis and by the age in lower axis, respectively. Vertical gray dashed lines in (a)-(d) indicates a visual correlation of Ti in cores between HDP-06 and X106.

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

**5. Acknowledgement** 

Science and Technology, Japan.

1040-6182

3941

**6. References** 

Sediment from Lake Hovsgol, Northwest Mongolia, by Synchrotron μ-XRF Mapping Analyses 147

early Holocene to the present day. These two-type variations suggest that the continental erosion/weathering in central Asia occurred on two different processes and time-scales.

The authors are very grateful to the staff of Technical Center of Nagoya University. K. Suzuki provided helpful support in designing a sheet bender. S. Yogo made many helpful suggestions on sample preparation. A part of μ-XRF analysis was performed with the approval of SPring-8 (Proposal No. 2006B1091 and 2007A1563). We also thank to Y. Terada for μ-XRF mappings using synchrotron radiation. We are grateful to the HDP members for subsampling from the HDP-06 core. This study was supported by the Sumitomo Foundation grant No.103359; the Saijiro Endo Memorial Foundation; awards from Dynamics of the Sun-Earth-Life Interactive System, No. G-4 at Nagoya Univ., and Environmental Monitoring and Prediction of Long- and Short-Term Dynamics of Pan-Japan Sea Area, No. E-07 at Kanazawa Univ., of the 21st Century COE Program of the Ministry of Education, Culture, Sports,

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organic carbon and fluorescence in Lake Hovsgol: factors reducing humic content

The variations of Ti and Br contents in core HDP-06 are similar to that of PME over the Holocene period (Figs. 6a, b, and f). The relationship between Ti and PME shows an inverse correlation, whereas the relationship between Br and PME shows a positive correlation. In the mid-Holocene, the Ti intensity peaks at low PME, and the Br content increases in the early- and late-Holocene when the PME rises. On the other hand, the PC-2 score in core X106 (Fig. 6d), together with annual mean temperature in central Asia (Fig. 6e) and atmospheric CO2 concentration (Fig. 6f), shows a gradual increase from about 8.0 ka to the present day.

These two-type variations observed in cores HDP-06 and X106 are considered to have resulted from changes in erosion/weathering intensity of central Asia (Asian continental interior) with moisture changes.

Evidences and suggestions supporting our hypothesis are provided by studies on the sediment from Lake Hovsgol. Prokopenko et al. (2007) showed that the early Holocene diatom/biogenic silica (bioSi) peaks correlated with the humidity maximum (Fig. 6f) reconstructed by the pollen fossil analyses and from predictions of GCMs (Bush, 2005). Based on these observations, Prokopenko et al. (2007) regarded that the early Holocene increase of diatom abundance in Lake Hovsgol was caused by increased nutrient supply with high precipitation and surface runoff. The early Holocene diatom/bioSi peaks correspond to the increased Br contents in core HDP-06 (Fig. 6a). Murakami et al. (2010) observed that low PC-2 detrital input occurred during the early Holocene (Fig. 4d). To explain the early Holocene decease of erosion/weathering intensity in the drainage basin with high humidity, Murakami et al. (2010) proposed that the detritus supply to Lake Hovsgol may have been controlled by the amount of vegetation cover: (1) vegetation cover in the catchment increased with high precipitation; (2) as a result, the nutrient supply to the lake enhanced, which in turn resulted in high productivity in the lake; (3) simultaneously, declined erosion through the drainage basin resulted in a reduced sediment supply into the lake.

The interpretation for the early Holocene detritus input by Murakami et al. (2010) can apply to the gradual increase from about 8.0 ka observed in the PC-2 score (Fig. 6d), the regional annual mean temperature (Fig. 6e), and atmospheric CO2 concentration (Fig. 6g): (1) the annual mean temperature increases with the rise in atmospheric CO2; (2) because of the exponential increase of the saturation vapor pressure with air temperature, the moisture decreases; (3) the resultant aridity of the continental interior has intensified the erosion/weathering processes.
