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

对中纬度地区极地冰盖和湖泊沉积物的调查，使得研究大陆地区过去的气候环境条件对获得更多关于第四纪环境变化机制的信息具有重要意义。在西伯利亚东南部的贝加尔湖，沉积物中硅藻球茎数量、水分和有机碳含量的深度分布是古气候变化的重要信号。此外，微量元素，特别是铀，也被认为是最重要的化学指标之一。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代表，而且可以作为气候变化的指示物。蒙古最大的淡水湖杰克霍夫斯戈尔对周围流域的环境变化做出了敏感的反应。本研究试图获得霍夫斯戈尔湖沉积物芯中U、Th同位素分布的详细特征。重点讨论了238U-234U-230Th-232Th系统的测年结果，并与以往贝加尔湖沉积物岩心测年结果进行了比较。本研究的最终目的是阐明U、Th同位素在沉积物中的深度分布及其比值的变化，以深入了解它们对古气候变化的意义。对霍夫斯戈尔湖81m长的沉积岩芯(HDP-04)的U、Th同位素深度剖面进行了详细的测量，并成功地将U系列等时线技术应用于部分地层的测年。自生铀的深度分布与初级产品生物标志物的深度分布有很好的相关性，表明自生铀的波动至少与冰期-间冰期旋回有很好的相关性。在不同水质的霍夫斯戈尔湖和贝加尔湖，从24m到表层的沉积物中也观察到了相似的铀深度剖面。

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.