| 新疆137Cs定年时标解析 |
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| 引用本文： | 兰波,李佳秀,张东良,杨运鹏. 新疆137Cs定年时标解析[J]. 干旱区地理, 2017, 40(3): 504-511 |
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| 作者姓名： | 兰波 李佳秀 张东良 杨运鹏 |
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| 作者单位： | 1.中国科学院新疆生态与地理研究所, 环境演变实验室, 新疆 乌鲁木齐 830011;2.中国科学院大学, 北京 100049 |
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| 基金项目： | 国家自然科学基金（No.U1203281 L08）资助 |
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| 摘 要： | 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。
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| 关 键 词： | 137Cs 时标 湖泊沉积 新疆 |
| 收稿时间： | 2016-09-29 |
Interpretation of 137Cs time markers of Xinjiang |
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| LAN Bo,LI Jia-xiu,ZHANG Dong-liang,YANG Yun-peng. Interpretation of 137Cs time markers of Xinjiang[J]. Arid Land Geography, 2017, 40(3): 504-511 |
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| Authors: | LAN Bo LI Jia-xiu ZHANG Dong-liang YANG Yun-peng |
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| Affiliation: | 1.Laboratory of Environment Change in Arid Lands, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumuqi 830011, Xinjiang, China;2.University of Chinese Academy of Sciences, Beijing 100049, China |
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| 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. |
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| Keywords: | 137Cs time marker lake deposition Xinjiang |
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