Fundamental studies on variations of uranium and thorium isotopes in Lake bottom sediment core and their application to the paleoenvironmetal changes

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The core VER98st.6 taken from the northern part of the Academician Ridge of the Lake Baikal, Siberia, was measured for U and Th isotopes (U-238, U-234, Th-232 and Th-230). Physical and chemical parameters such median grain size and Bio-Si content were also measured. By using the orbital tuning method based on the median grain size fluctuation, the sediment core covers the last two to four interglacial stages, ca.20 ky and is characterized by low sedimentation rate, ca.4.7 cm/ky. The concentration of U-238 fluctuates by a factor of 4,ranging from 50 to 200 Bq/kg. The U-238 concentration displays the higher values during the warm intervals, accompanied by high accumulation of Bio-Si, and the lower values during the cold intervals. The variation of U-234/U-238 ratios is similar to the variation of U-238 concentrations throughout the core. The Th-232 concentration varies in a narrow range, only by a factor of 2. The low Th-232 contents characterize the diatom-rich intervals during the warm intervals. The profile of Th-232 flux correlates well with that of mineral flux, indicating that Th is a proxy for terrestrial materials. The discrimination of the U-238 into the authigenic and terrigenous U-238 fractions reveals that the authigenic U-238 fraction is very low during the periods of glacial intervals, especially being almost zero during MIS 6. The authigenic U in sediments is candidate for the climate-sensitive processes. Based on the U-234-U-238 and Th-230-U-238 dating methods using the estimated authigenic U-238, U-234 and Th-230 contents in sediments during the warm intervals, mean sedimentation rates are independently estimated as 3.84±0.43 and 4.71±0.21 cm/ky respectively. These rates are close to a mean rate of 4.7 cm/ky calculated by the orbital tuning method.