Reconstruction of Paleoenvironmental Changes using Radionuclides:Uranium and Thorium Isotopes in the Lake Sediments

通过对中纬度地区极地冰盖和湖泊沉积物的考察，研究了大陆地区过去的气候环境条件，对了解第四纪环境变化机制具有重要意义。在西伯利亚东南部的贝加尔湖，硅藻硅藻壳数量、沉积物含水量和有机碳含量的深度分布都能反映沉积物中古气候变化的明显信号。此外，微量元素，特别是铀元素作为重要的化学指标之一也受到了人们的关注。因此，湖相沉积物中的U（和Th）同位素不仅可以作为年代学的良好替代物，而且可以作为气候变化的指示物。蒙古国最大的淡水湖-雅克湖对周围集水区的环境变化有着敏感的响应。通过对呼苏哥尔湖沉积物柱样中U、Th同位素分布特征的研究，揭示了该湖沉积物柱样中U、Th同位素的分布特征。重点讨论了238 U-234 U-230 Th-232 Th系统测年数据，并与贝加尔湖沉积物岩心测年数据进行了对比，旨在阐明贝加尔湖沉积物中U、Th同位素深度分布及其比值的变化，揭示其古气候变化的意义.详细测量了呼苏哥尔湖81 m长沉积柱样（HDP-04）的U、Th同位素深度剖面，并成功应用U系等时线技术对部分地层进行了定年.自生铀的深度分布与初级产物生物标志物的深度分布有很好的相关性，说明自生铀的波动至少与冰期-间冰期旋回有一定的相关性.在24 m至地表的深度上，水质差异较大的呼苏古尔湖和贝加尔湖沉积物中u深度剖面具有相似的模式。

Investigation on polar ice-sheet and lake sediments of mide-latitude area have made studies on past climatic and environmental conditions in continental areas very important to obtain a more information cu mechanisms of Quaternary environmental changes. In Lake Baikal in southeastern Siberia, the dear signal of changing paleoclimate in the sediments has been found in depth distributions of the diatom frustule numbers, water content and organic carbon content in sediments. Furthermore, microelements, especially, uranium have been also noticed as one of the most important chemical index. Thus, the U (and Th) isotopes in lacustrine sediments serve as good proxies for not only chronology but also indicator of climatic changes. Jake Hovsgol, the largest fresh water lake in Mongolia, promiePa the sensitive response to environmental changes in surrounding catchments. In this study, an attempt was made to get a detail feature of U and Th isotopes distribution in the sediment core of Lake Hovsgol. The obtained data were discussed laying stress on dating using 238U-234U-230Th-232Th system, and compared with those obtained previously in sediments core fit Lake Baikal The final purpose of this study was to elucidate the changes in the depth distribution of U and Th isotopes and their ratios in the sediments ad to obtain insight into their implication for paleoclimatic variation.1. Depth profiles of U and Th isotopes in a 81 m length sediment core (HDP-04) of Lake Hovsgol were measured in details, and U-series isochron techniques were successfully applied to the dating for some layers.2. The depth distribution of authigenic U well correlated with that of bio-marker of primary products, suggesting that the fluctuation of authigenic U well correlates with glacial-interglacial cycle at least about IMa.3. A similar pattern of u depth profiles in sediments was observed in the dated depth from 24 m to surface in Lake Hovsgol and lake Baikal with quite different water qualities.