我国表土中129I 的空间分布及129I 年代方法学初探

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	我国表土中129I 的空间分布及129I 年代方法学初探
	范煜坤
学位类型	博士
导师	周卫健 ; 侯小琳
	2013-11
学位授予单位	中国科学院研究生院
学位授予地点	北京
学位专业	第四纪地质学
关键词	129i 核活动黄土-古土壤红粘土初始值年代学
其他摘要	129I 作为核反应的主要产物之一，成为现今核环境安全监测的一个重要核素。对环境中129I 进行分析，不但可以直接探测到核事故污染的波及程度，结合其它的核素分析还可以了解不同源对研究地点的影响，另外129I 还能够用于当某次核事故向环境中释放的短寿命放射性核素（如131I）在衰变至无法探测时估算环境中所接受到的剂量。目前，我国环境中129I 研究还处于初期阶段，有必要分析我国环境中的129I 水平，可为将来核安全监测提供宝贵的本底数据（背景值）。129I 具有较长的半衰期（15.7 Ma），为较老的地质年代（如新生代）测定提供了可能。 129I 年代学方面的研究，国际上目前仅围绕海洋沉积物开展工作，而对于陆生129I 在天然水平的调查及其是否可用于测年工具等问题还尚无报道。本文针对上述两个方面，采集了胶州湾潮间带沉积物钻孔、中国部分地区表层土壤以及中国黄土和红粘土样品，分析了其中的129I，分别对我国环境129I 水平的时空变化、天然 129I 水平及其在年代学方面的应用获得了以下认识： 1. 我国表层土壤中 129I/127I 比值在 (0.24-380.29)×10-10 水平，平均值为 1.93×10-9，比核前时代海洋沉积物中天然129I 水平（1.5×10-12）高1-4 个数量级，是人类核活动向环境中释放了大量129I 的结果。绝大部分样品（>90%）在10-10- 10-9 水平，其中比值为 (1-10)×10-10 的样占53.8%，(1-10)×10-9 占40.9%。碘浓度在0.21-35.63 μg/g 之间，平均值为4.63 μg/g。129I 的空间分布整体上具有南低北高的趋势。北方较高的129I/127I 比值可能是欧洲、前苏联等同纬度带的几个核燃料后处理厂向环境中排放129I 的结果。新疆罗布泊附近、甘肃高台和四川广元等地的129I/127I 高值可能分别与当地的核武器试验以及核设施运行有关。 2. 我国胶州湾潮间带沉积物中 129I/127I 比值范围为 (1.21-189.07)×10-12，平均值为61.36×10-12。129I/127I 比值在时间分布上总体可分为五部分：(1.21-4.48) ×10- 12 (1931-1955)，(5.10-77.78) ×10-12 (1955-1969)，(13.64-104.05) ×10-12 (1969-1978)， (14.97-180.76) ×10-12 (1978-1990)，(17.68-189.07) ×10-12 (1990-2002)。各阶段较好的对应了海洋环境中核前天然129I 水平、1963 年全球核武器试验、70 年代初我国大气核试验、切尔诺贝利事故、以及英法核燃料后处理厂废料排放，显示出129I 是一个灵敏的环境示踪剂。另外，胶州湾沉积物中核前129I 水平与已报道的太平洋和大西洋沉积物中天然129I 水平一致，表明海洋系统中天然碘同位素混合均匀， 129I 水平无地域差异。 3. 西峰黄土-古土壤序列中129I/127I 比值为 (0.47-16.30)×10-11，洛川剖面为 (0.50-571.32)×10-11。两剖面中表层129I/127I 比值较高，向下呈指数衰减，为人工 129I 向下缓慢且有限迁移的表现。通过空白分析、铀元素分析、人工129I 污染可能性分析等，确定了深层恒定的129I/127I 比值2.0×10-11 是黄土-古土壤的天然水平，比海洋沉积物中天然水平高一个数量级。估算由粉尘传输携带的大气Xe 成因129I 对黄土中129I 贡献甚微，不超过1%；黄土中极低的碘含量（~2 ppm）可能是导致129I/127I 比值高于海洋沉积物的主要原因。对7 个已知古地磁年龄红粘土中的129I 分析，拟合了其中129I/127I“初始值”为5.34×10-12，约为黄土的四分之一。这可能是因为红粘土中含量较高的超细粒组分使碘得到了富集，从而稀释了129I/127I 比值。对于风成沉积物中碘同位素特征认识还十分有限，今后需进一步深入开展相关的研究工作。; 129I, one of the main fission products in nuclear reactor, is significant in nuclear safety monitoring. Analysis of 129I in the environmental samples can be used not only for detection of nuclear pollution in a nuclear accident, but also for distinguishing the sources while combined with other nuclides. Besides, long-lived 129I can be used to reconstruct the short-lived 131I level and its dispersion pattern after released from the accident. The previous investigation on environmental 129I is very limited in China, the analysis of 129I in the environmental samples is important for obtaining the valuable background data for nuclear safety monitoring in the future. Morever, the long half-life of 129I (15.7 Ma) has shown a potential in the geological dating. The reported works on 129I dating only focused on the marine system and none was reported on terrestrial environment. In this work, the 129I in the marine sediments in the intertidal zone in Jiaozhou Bay, surface soils in some regions of China, and Chinese loess-paleosol and red clay has been analyzed, and some point views were concluded as below. 1. 129I/127I ratios in Chinese surface soils varied within (0.24-380.29)×10-10 with average of 1.93􀵈10-9, approximately 1-4 orders of magnitude higher than the prenuclear level of 129I in marine sediments, 1.5×10-12. Most samples (>90%) were between (1-100)×10-10, among which 53.8% samples were between (1-10) ×10-10, 40.9% samples between (1-10) ×10 -9. The iodine content varied from 0.21 to 35.63 μg/g with an average of 4.63μg/g. The spatial distribution of 129I in surface soils showed the high values in northern China while 129I level in southern China was low. This feature might attribute to the release of 129I from the nuclear fuel reprocessing plants in Europe and Russia which located in the same latitude zone as that of the northern China. Some high 129I signals in Nop Nor Xinjiang, Gaotai Gansu Province, and Guanyuan Sichuan Province were probably the results of Chinese nuclear weapon tests and operations of local nuclear facilities in the past half century. 2. 129I/127I atomic ratios in the marine sediments in the intertidal zone in Jiaozhou Bay varied in (1.21-189.07)×10-12 with average of 61.36×10-12. Based on the deposition date and of measured 129I/127I ratios, the sediment can be regarded as five sections: years of 1931-1955, 1955-1969, 1969-1978, 1978-1990, and 1990-2002, corresponding well to the pre-nuclear level of 129I in marine system, the major global nuclear weapon tests, the Chinese atmospheric nuclear tests in the early 1970s, the Chernobyl accident, and the discharge of 129I by the nuclear fuel reprocessing plant (NFRP) in La Hague and Sellafield, respectively, which indicated that the 129I acted as a sensitive environmental tracer. In addition, the pre-nuclear level of 129I investigated in this work agreed very well with the previous value studied in Pacific and Atlantic Ocean, demonstrating that the natural iodine isotopes in marine system were well mixed and the pre-nuclear 129I/127I ratio did not varied with locations. 3. In the depth profile of loess, observed 129I/127I ratios were (0.47-16.30)×10-11 and (0.50-571.32)×10-11 in Xifeng and Luochuan sections, respectively. High signals of 129I were observed in surface layers and exponentially decreased with depth in the upper part, showing a slow and limited vertical migration of anthropogenic 129I. Based on the analysis of chemical blank, uranium content, and contamination by anthropogenic 129I, a constant 129I/127I ratio of 2.0×10-11 in deep layer in both sections was proved to represent the natural 129I level in loess-paleosol deposition. This value was about 1 order of magnitude higher than that in marine system which has been used as the initial value for 129I dating in marine system. The contribution of 129I by transport via the high-flux dust to the Chinese Loess Plateau (CLP) was calculated to be negligible, and the ultralow content of stable iodine in the loess and paleosol compared to that in marine system was probably the major reason for the higher pre-nuclear 129I/127I ratio in loess sediment. Based on the measurement of 129I in 7 red clay samples with measured ages by paleomagnetic dating, a fitting curve was obtained and the “initial value” was 5.34×10- 12, about a quarter of the pre-nuclear level of 129I in loess-paleosol sequence. This could be explained by that the relatively high iodine content resulted by greater amount of ultrafine component in red clay might dilute the 129I/127I ratio. This work is a preliminary study of 129I dating in terrestrial environment, more work needs to be carried out for a deep understanding of the geochemical behavior of iodine isotopes in the eolian dust deposition.
学科领域	地质学
语种	中文
文献类型	学位论文
条目标识符	http://ir.ieecas.cn/handle/361006/2597
专题	博士研究生论文
推荐引用方式 GB/T 7714	范煜坤. 我国表土中129I 的空间分布及129I 年代方法学初探[D]. 北京. 中国科学院研究生院,2013.