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

对采自西伯利亚贝加尔湖院士岭北部的VER98st.6岩心进行了U-238、U-234、Th-232和Th-230的U、Th同位素测定。还测量了中值晶粒度和生物硅含量等物理和化学参数。采用基于中值粒度起伏的轨道调整方法，沉积岩芯覆盖了最后两到四个间冰期，约为20Ky，具有低沉积速率的特点，约为4.7 cm/Ky。铀-238的浓度波动了4倍，从50到200Bq/kg不等。U-238浓度在暖期表现出较高的值，伴随着生物硅的高积累，在冷期表现出较低的值。U-234/U-238比值的变化与整个岩芯中U-238浓度的变化相似。Th-232浓度变化幅度很小，只有2倍。Th-232含量低是温暖时段富硅藻区的特征。Th-232通量剖面与矿物通量剖面具有很好的相关性，表明Th是陆源物质的替代物。U-238对自生和陆源U-238组分的区分表明，在冰期，自生U-238组分很低，特别是在MIS6期间几乎为零。沉积物中的自生U是气候敏感过程的候选者。根据U-234-U-238和Th-230-U-238测年方法，分别估算出暖期沉积物中自生U-238、U-234和Th-230的平均沉积速率分别为3.84±0.43和4.71±0.21 cm/Ky。这些速率接近用轨道调谐法计算的4.7厘米/Ky的平均速率。

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.