**Response of Biogenic Silica Production in Lake Baikal and Uranium Weathering Intensity in the Catchment Area to Global Climate Changes**

Takuma Murakami1,6, Nagayoshi Katsuta2, Takejiro Takamatsu3, Masao Takano1, Koshi Yamamoto1, Toshio Nakamura4 and Takayoshi Kawai5 *1Graduate School of Environmental Studies, Nagoya University, 2Faculty of Education, Gifu University, 3National Institute for Environmental Studies, 4Center for Chronological Research, Nagoya University, 5Association of International Research Initiative for Environmental Studies, 6Low Level Radioactivity Laboratory, Institute of Nature, Environmental Technology, Kanazawa University, Japan*

#### **1. Introduction**

120 International Perspectives on Global Environmental Change

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Lake Baikal, in southeast Siberia, is a structural basin in the Baikal rift valley (Fig. 1a) and is the largest lake on earth in terms of fresh water volume (23,000 km3). With a surface 454 m above sea level (asl), it covers an area of 31,500 km2 (length, 636 km; maximum width, 80 km) and has a maximum depth of 1,620 m. The vegetation around the lake is characteristic of the steppe and taiga. And the annual mean temperature and rainfall around the lake are -2.2 ºC and 400–500 mm per year, respectively. The catchment area of the lake is 540,000 km2, extending from northwest Mongolia to southeast Siberia (Fig. 1b), of which 83% constitutes the drainage basin of the Selenga River (Fig. 1b and c). This is the largest river flowing into the lake and its water inflow makes up 50% of the total riverine input. The climate in the Lake Baikal region is influenced by westerly wind weather systems (Mackay, 2007). Therefore, most of the atmospheric moisture in southeast Siberia is from the North Atlantic Ocean and the Arctic Ocean.

The bottom sediment of Lake Baikal documents the long-term history of environmental changes in the Asian continental interior (southeast Siberia), showing the shift in climate on various time-scales. The main proxy records obtained from the sediment are based on the concentration of diatom frustules and biogenic silica (bioSi) (Colman et al., 1995; Kashiwaya et al., 2001; Mackay, 2007; Prokopenko et al., 1999, 2001, 2002; Williams et al., 1997) and the amount of pollen fossils (Shich et al., 2007; Tarasov et al., 2005) in the sediment indicating the bioproductivity in the lake and its surrounding watersheds. The main source of the bioSi

Response of Biogenic Silica Production in Lake Baikal and

transported to from the watershed of the Selenga River.

labeled 8-0 from the past to the present day.

**2. Materials and methods** 

**2.1 Sediment cores** 

Members, 1997).

(Goldberg et al., 2005).

Uranium Weathering Intensity in the Catchment Area to Global Climate Changes 123

On the other hand, the uranium record in Lake Baikal sediments as well as the biological records provides information on the environmental changes in the region. The variation in the uranium concentration is thought to be due to the weathering intensity in the Selenga drainage basin associated with changes in rainfall/moistures levels (Goldberg et al., 2010; Murakami et al., under review). The U-Th isotope study of Edgington et al. (1996, 1997) revealed that the uranium in Lake Baikal sediment is composed mainly of authigenic components, which originated in uranium-bearing rocks distributed in Mongolia and southeast Siberia, and that the uranium is transported from the source rock into the lake via the Selenga River. Based on geochemical evidence, Edgington et al. also concluded that the variation of uranium concentration in the sediment resulted from changes in the input from the Selenga River and its tributary. The uranium variations are reported to have corresponded to the Pleistocene glacial-interglacial cycles (Chebykin et al., 2007; Edgington et al., 1996; Goldberg et al., 2010) and the Holocene Bond events

The purpose of the present study is to investigate the degree of similarity in the variations in the Lake Baikal records of bioSi and uranium and the paleoproxy records of global climate changes on a centennial-to-millennial-scale as well as a glacial-interglacial time scale. An examination of the correlation among these paleoclimate proxy datasets has been conducted for lake sediment from the underwater Academician ridge (Fig. 1) in Lake Baikal (Edgington et al., 1996), focusing on the variations on a glacial-interglacial scale. However, we analyzed the geochemical data of sediment from the Buguldeika saddle in Lake Baikal (Fig. 1). This site is located at the opposite side of the Selenga Delta, where uranium is directly

In the present study, we used the SPECMAP δ18O record (Imbrie et al., 1984) and the North Atlantic IRD index (Bond et al., 1997, 2001) as a proxy of global climate change. The SPECMAP was acquired by stacking δ18O data of planktonic foraminifera collected from five deep-sea cores in low- and mid-latitudes, generally reflecting the continental ice sheet volume. The IRD index represents cooling events in the North Atlantic, which occurred nine times during the Holocene period. The IRD cooling events are referred to as Bond events

In the present study, we used two cores, BDP93-2 and BSS06-G2 taken at Buguldeika saddle in Lake Baikal, southeast Siberia (Fig. 1). Buguldeika saddle is a local elevation that developed at the opposite side of the Selenga Delta, separated from the mouth of the river by a deep trough. Because of these topographic features, the sedimentation of the saddle is controlled by the supply of a fine suspended load from the Selenga River. Seismic surveys and the lithologic features of the drilled cores indicate that the upper 50 m of sediment at the drill sites consists of continuous and sub-parallel layers (BDP-93 Baikal Drilling Project

Core BDP93-2 was collected in March 1993 at a water depth of 354 m (52º 31' 3.0"N and 106º 09' 6.01''E) using a piston corer (BDP-93 Baikal Drilling Project Members, 1997). This core was 102 m long. We measured the chemical components of 228 sediment samples from the core. The samples were collected at intervals of 30 to 80 cm corresponding to the

in the sediment is diatom frustules (Karabanov et al., 1998). Studies to date using the biological records revealed that the diatom and vegetation changes were correlated with the Milankovitich periods (Kashiwaya et al., 2001; Williams et al., 1997) and were in phase with the glacial-interglacial cycles (Colman et al., 1995; Prokopenko et al., 2001a, 2002; Shichi et al., 2007). Moreover, the variations are found to follow the centennial-to-millennial-scale climate changes that occurred in the North Atlantic region: Bond cooling cycles during the last glacial periods (Prokopenko et al., 2001b); a Younger Dryas cold period for the last glacial/Holocene transition (Prokopenko et al., 1999); and IRD (ice-rafted debris) cooling (Bond) events in the Holocene (Mackay, 2007; Tarasov et al., 2005).

Fig. 1. Maps of (a) northeastern part of continental interior Asia and (b) catchment basin of (c) Lake Baikal. Bathymetric map of the lake showing the collection sites of cores BSS06-G2 and BDP93-2 at Buguldeika saddle.

On the other hand, the uranium record in Lake Baikal sediments as well as the biological records provides information on the environmental changes in the region. The variation in the uranium concentration is thought to be due to the weathering intensity in the Selenga drainage basin associated with changes in rainfall/moistures levels (Goldberg et al., 2010; Murakami et al., under review). The U-Th isotope study of Edgington et al. (1996, 1997) revealed that the uranium in Lake Baikal sediment is composed mainly of authigenic components, which originated in uranium-bearing rocks distributed in Mongolia and southeast Siberia, and that the uranium is transported from the source rock into the lake via the Selenga River. Based on geochemical evidence, Edgington et al. also concluded that the variation of uranium concentration in the sediment resulted from changes in the input from the Selenga River and its tributary. The uranium variations are reported to have corresponded to the Pleistocene glacial-interglacial cycles (Chebykin et al., 2007; Edgington et al., 1996; Goldberg et al., 2010) and the Holocene Bond events (Goldberg et al., 2005).

The purpose of the present study is to investigate the degree of similarity in the variations in the Lake Baikal records of bioSi and uranium and the paleoproxy records of global climate changes on a centennial-to-millennial-scale as well as a glacial-interglacial time scale. An examination of the correlation among these paleoclimate proxy datasets has been conducted for lake sediment from the underwater Academician ridge (Fig. 1) in Lake Baikal (Edgington et al., 1996), focusing on the variations on a glacial-interglacial scale. However, we analyzed the geochemical data of sediment from the Buguldeika saddle in Lake Baikal (Fig. 1). This site is located at the opposite side of the Selenga Delta, where uranium is directly transported to from the watershed of the Selenga River.

In the present study, we used the SPECMAP δ18O record (Imbrie et al., 1984) and the North Atlantic IRD index (Bond et al., 1997, 2001) as a proxy of global climate change. The SPECMAP was acquired by stacking δ18O data of planktonic foraminifera collected from five deep-sea cores in low- and mid-latitudes, generally reflecting the continental ice sheet volume. The IRD index represents cooling events in the North Atlantic, which occurred nine times during the Holocene period. The IRD cooling events are referred to as Bond events labeled 8-0 from the past to the present day.
