中国湖泊沉积物137 Cs和210 Pbex断代的一些问题

137 Cs和 210 Pbex是湖泊沉积物断代常用的两种核素,沉积物中核素的深度分布,不但随核素的大气沉降量变化,也与流域环境变化和沉积后核素的再分布有关。我国环境复杂,人类活动强烈,137 Cs和 210 Pbex深度分布的非理想曲线常见于我国一些湖泊的沉积物。本文简要介绍了湖泊沉积物 137 Cs和 210 Pbex深度分布理想曲线和断代基本原理,在参阅大量有关中国湖泊沉积物 137 Cs和 210 Pbex断代文献的基础上,讨论了 137 Cs和 210 Pbex非理想深度分布曲线的解译和沉积物断代的几个常见问题。主要观点如下: 1)我国湖泊沉积物不存在所谓的1974年和1986年蓄积峰;2)一些湖泊沉积出现的非单峰型 137 Cs深度分布曲线,可能与人类活动或湖流扰动湖泊底泥、流域侵蚀产沙环境变化和 137 Cs主要来源于冰川融水补给等因素有关;3)沉积物中的核素垂向运移,不改变剖面中1963年 137 Cs蓄积峰的位置,不影响其断代标志意义;4)流域环境突发事件引起的沉积剖面中的 137 Cs和 210 Pbex明显异常,是断代的可靠标志等。最后介绍了根据沉积剖面 137 Cs和 210 Pbex总量与本底值对比,求算沉积速率的核素质量平衡法。

DISCUSSION ON APPLYING 137Cs AND 210Pbex FOR LAKE SEDIMENT DATING IN CHINA

137 Cs and 210 Pbex are two environmental fallout nuclides,which are commonly used for dating of lake sediments.Depth distributions of those nuclides in sediments are influenced not only by variations of their direct deposition on the lake water bodies but also by environmental changes in the lake catchment and their post-depositional redistribution in sediments.As the complex environments and strong human activities,nonideal 137 Cs and 210 Pbex depth distribution curves often exist in the sediments of Chinese lakes.In this paper,Ideal 137 Cs and 210 Pbex depth distribution curves in lake sediments and basis sediment dating principles of the two nuclides are firstly introduced.Then,by reviewing a number of relevant publications related to 137 Cs and 210 Pbex dating of sediments in Chinese lakes,several common questions on interpretations of nonideal 137 Cs and 210 Pbex depth distribution curves as well as on the relevant problematic sediment dating results are discussed.Important views in the paper are following:(1)So-called "1974 and 1986 137 Cs peaks" do not exist in the sediments of Chinese lakes.(2)137 Cs depth distribution profiles without a single peak existing in some lakes are probably related to disturbances of surface sediments in shallow lakes caused by human activities or lake currents,changes of erosion environments in the catchment and that the 137Cs nuclide in the lake sediments is mainly contributed from the melting glaciers.(3)Vertical movements of those nuclides in sediments have little influences on the 1963's 137 Cs peak location in a profile as well as on the dating significance,and have slight influences on the shape of 210 Pbex depth distribution profiles and very limited effects on calculation of sedimentation rates from the profiles.(4)Outlandish 137 Cs and 210 Pbex peaks,caused by environmental incidents in the catchment,are reliable marks for dating of sediments.Finally,mass balance models based on comparisons of the 137 Cs or 210 Pbex inventories in lake sediments with the local reference values of those nuclides for assessments of lake sedimentation are briefly introduced.