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

湖底沉积物核中铀、钍同位素变化的基础研究及其在古环境变化中的应用

基本信息

批准号：
16510005
负责人：
YAMAMOTO Masayoshi
金额：
$ 2.5万
依托单位：
Kanazawa University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2004
资助国家：
日本
起止时间：
2004 至 2005
项目状态：
已结题

项目摘要

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.

核心ver98。采集自西伯利亚贝加尔湖院士岭北部的6个样品，测量了U和Th同位素（U-238、U-234、Th-232和Th-230）。测定了中位粒径和Bio-Si含量等理化参数。采用基于中位粒径波动的轨道校正方法，沉积物岩心覆盖了最后2 ~ 4个间冰期，约为20 ky，沉积速率较低，约为4.7 cm/ky。铀-238的浓度波动幅度为4倍，范围为50至200 Bq/kg。U-238浓度在温暖期较高，并伴有高Bio-Si积累，在寒冷期较低。铀-234/铀-238比值的变化与整个堆芯铀-238浓度的变化相似。钍-232浓度变化的范围很窄，只有2倍。低Th-232含量是暖段富硅藻段的特征。Th-232通量剖面与矿物通量剖面具有较好的相关性，表明Th是陆地物质的代用品。对自生U-238和陆源U-238组分的区分表明，自生U-238组分在冰期非常低，特别是在MIS 6期几乎为零。沉积物中的自生U是气候敏感过程的候选者。根据沉积物中自生U-238、U-234和Th-230含量估算的U-234-U-238和Th-230定年方法，分别独立估算出平均沉积速率分别为3.84±0.43和4.71±0.21 cm/ky。这些速率接近于轨道调谐方法计算的平均速率4.7 cm/ky。