^(137)Cs和^(241)Am在滇池、剑湖沉积孔柱中的蓄积分布及时标意义

^(137)CS AND^(241) AM PROFILES AND DATING OF SEDIMENTS FROM TWO LAKES IN YUNNAN PROVINCE, CHINA

自70年代以来,对湖泊沉积物中核试验散落核素的研究已成为研究现代湖泊沉积过程和沉积年代及沉积速率的一个重要方面,本文通过对分别采自云南省滇池、剑湖的两个沉积孔柱中^(137)Cs和^(241)Am蓄积分布的研究,测定了它们60、70年代以来的沉积速率分别是42.1和28.7mg/(cm^2·a)。研究发现,虽然^(241)Am的含量远低于^(137)Cs,但^(241)Am具有沉积后迁移能力较小,半衰期长等特点,因而随着时间的推移,在未来湖泊现代沉积作用的研究中将会显示出越来越重要的作用。

Atmospheric nuclear weapons testing produced global radioactive fallout that has been widely studied for dating recent lacustrine sediments over the past 20 years. This paper describes the use of 137Cs and 241 Am radioactivity measurements to determine the accumulation rates of sediments from two lake cores, one is Core DC9301 from Dianchi Lake, the other is Core JL9301 from Jianhu Lake, in Yunnan Province, China. The vertical distributions of 137Cs and 241 Am in the sediment cores are markedly similar to the expectations based on estimated and measured temporal variation of the two radioisotopes fallout in this area. The variation of the 137Cs activity with depth in cores provides four distinct chronological markers with the onset of fallout in 1954, the peak fallouts from the atmospheric testing of nuclear weapons in 1963 and two lower peaks occurred in 1974 by nuclear tests and in 1986 from Chernobyl accident. The variation of 241 Am proves two chronological markers from the atmospheric testing of nuclear weapons in 1963 and 1974. According to the 137Cs and 241Am results, the dry mass accumulation rates of Core DC9301 and JL9301 are 42. 2 and 28.7mg/ (cm2 ?a), respectively. ?Because of the post-depositional mobility of 137 Cs in lake sediments, the 137 Cs chronological marker results of 1954 for these two cores were of no chronological value. Although the 241Am activities are extremely lower than those of 137Cs in these two cores, the former is significantly less mobile in lake sediments and longer half-life than the latter. So 241 Am is likely to play an increasingly important role in dating recent lacustrine sediments.