关键词: ¹³⁷Cs, 时标, 湖泊沉积, 新疆

Abstract: Caesium-137(137Cs)has been widely used in dating and soil erosion in the world. ¹³⁷Cs dating depends on the global nuclear fallout pattern. However,quite a few ¹³⁷Cs profiles of lacustrine cores in Xinjiang happen to be different from the characteristics of atmospheric ¹³⁷Cs deposition,showing four time markers(1954, 1963,1975 and 1986)and the strongest specific activity of ¹³⁷Cs in 1986. By analyzing the ¹³⁷Cs profile of Elike Lake,together with the other ¹³⁷Cs records in Xinjiang,this paper compared the ¹³⁷Cs features in Xinjiang with the ¹³⁷Cs fallout of Northern Hemisphere and the ²⁴⁰Pu/²³⁹Pu record of Belukha Glacier core which is situated to the north tip of Xinjiang,trying to clarify the reliability of those ¹³⁷Cs time markers in Xinjiang and interpret the strongest ¹³⁷Cs peak in 1986. Considering the ¹³⁷Cs enrichment effects of particle size and organic matter,it is important to diminish the effects by normalizing the ¹³⁷Cs activities with the aforementioned two factors. The normalization procedure will redefine ¹³⁷Cs activity peaks and help us to obtain the accurate ages. And based on the comparisons,several conclusions can be drawn as follows:(1)only the 1963 time mark is the most reliable,corresponding to the maximum ¹³⁷Cs fallout. While 1954 and 1986 could only be taken as secondary time references unless they coincide with another dating techniques(e.g.,²¹⁰Pb dating technique);(2)The first detection of ¹³⁷Cs in the sediments is assumed to correspond to a date of 1952 or 1954(initial testing of thermonuclear weapons). However,the low fallout flux,2 half-life periods to date and the possible mobility of ¹³⁷Cs will compromise the application of this time mark;(3)The ¹³⁷Cs signal of 1975,corresponding to Chinese nuclear tests,is questionable because this feature was not yet unified in Xinjiang(e.g.,the ¹³⁷Cs archives of Lake Bosten). More details about the effects of Chinese nuclear tests on the environment should be investigated in future．(4)The latest ¹³⁷Cs peak may correspond to the Chernobyl accident in 1986,verified by the ²⁴⁰Pu/²³⁹Pu record of Belukha Glacier core. This ¹³⁷Cs peak is assumed to be minor because of the great distance from Chernobyl(～4 000 km),the low accidental ¹³⁷Cs flux injecting into the stratosphere and the downwind location．(5)What's noteworthy is that the ¹³⁷Cs activities in 1986 are surprising high in some lacustrine cores．They may be associated with human activities,such as agricultural expansion and the resulting vegetation degradation. The effects of human activities,together with the higher precipitation in 1987/1988,led to an increased erosion. It is precisely the erosion that scoured the surface soil layer with high ¹³⁷Cs content into lake sediment,imposed on the ¹³⁷Cs deposition of Chernobyl accident,ultimately resulted in a high accumulation of ¹³⁷Cs around 1986.

Key words: ¹³⁷Cs, time marker, lake deposition, Xinjiang