新疆137Cs定年时标解析

¹³⁷Cs在年代测定和土壤流失等领域得到了广泛的应用。在认定¹³⁷Cs时标前，需要消除粘土和有机质（TOC）对¹³⁷Cs 的富集作用。通过对比新疆多个湖芯钻孔¹³⁷Cs 序列，发现它们异于北半球¹³⁷Cs 沉降模式，即出现了1954、1963、1975 和1986 这四种时标且1986 年的¹³⁷Cs 比活度可能异常的高。通过分析艾里克湖钻孔的¹³⁷Cs序列，并结合新疆其他地区多个钻孔的¹³⁷Cs序列，与北半球¹³⁷Cs沉降特征以及靠近北疆的Belukha冰芯²⁴⁰Pu²³⁹Pu沉降记录进行对比，分析了新疆¹³⁷Cs时标的可靠性以及1986年¹³⁷Cs比活度异常高的原因。在新疆¹³⁷Cs多个时标中，仅1963年的时标是高信度的（即北半球¹³⁷Cs最大沉积年）。1954年¹³⁷Cs蓄积峰代表其全球初始显著沉降，距今已有2个¹³⁷Cs半衰期，当年沉积通量低且可能发生¹³⁷Cs垂直迁移，仅能作为次要时标。1975年的时标反映了1960s- 1970s的中国核试验，但距离罗布泊～300 km的博斯腾湖对中国核试验有截然相反的记录，表明中国核试验不是稳定信号，其核沉降范围以及对湖芯的影响作用还需进一步评估，不能作为可信的时标。1986年蓄积峰对应切尔诺贝利核事故，因核尘埃未大量进入平流层且距离新疆较远（～4 000 km），但同时考虑到新疆位于核事故的西风环流下风区，所以该事故对新疆有影响但影响不大。1986年的¹³⁷Cs时标可作为次要时标。新疆湖芯1986年的异常高蓄积峰可能是由新疆最近几十年的人类活动（如过牧、耕种等）加上1987/1988年的高降水导致的地表侵蚀增强引起的。增强的地表侵蚀导致含有高¹³⁷Cs含量的表层土壤颗粒被冲刷进入湖泊，叠加在切尔诺贝利核事故¹³⁷Cs沉降之上，进而导致了湖芯沉积物记录到显著的1986年¹³⁷Cs蓄积峰。

Interpretation of 137Cs time markers of Xinjiang

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.