Paleoenvironmental implications of uranium concentrations in lacustrine calcareous clastic-evaporite deposits in the western Qaidam Basin
Paleoenvironmental implications of uranium concentrations in lacustrine calcareous clastic-evaporite deposits in the western Qaidam Basin
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DOI：
10.1016/j.palaeo.2014.10.002
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发表时间：
2015
期刊：
影响因子：
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通讯作者：
Yibo Yang;X. Fang;X. Fang;Minghui Li;A. Galy;A. Koutsodendris;Weilin Zhang
中科院分区：
文献类型：
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作者：
Yibo Yang;X. Fang;X. Fang;Minghui Li;A. Galy;A. Koutsodendris;Weilin Zhang
由于铀在表生环境中独特的地球化学性质和行为，它为重建古氧化还原环境提供了特别有用的信息。然而，在封闭的水文系统中，其在与地表水盐碱化相关的亚氧-好氧条件下的替代潜力尚未得到充分探讨。本文研究了柴达木盆地西部一个由钙质湖相碎屑岩和盐层组成的沉积序列中铀的丰度和分布。本研究探讨了U浓度的钙质碎屑沉积物（包括碳酸盐，酸不溶部分，以及铁和锰的氧化物/羟基氧化物）和盐层。在钙质碎屑沉积物中，U浓度的变化在酸不溶部分遵循重建的湖泊水位的波动，根据岩性调查，提供了一个工具，用于跟踪古湖泊底层水的氧化还原条件。碳酸盐含量的变化与碳酸盐组分中U和Sr浓度之间的相关性表明，碳酸盐矿物的类型和丰度控制了碳酸盐中U的分布。在盐层中，U和Al浓度密切相关，这意味着两者主要来自盐层中的碎屑部分。然而，富集的U和Mg系统较高的盐矿物中形成的浓度较高的盐水和较低的浓度形成的盐水。这些结果表明，在碳酸盐沉淀过程中，碳酸盐可以有效地去除湖水中的U。在盐矿物沉淀的后续阶段，U的行为是保守的，并且U在盐水中以溶解形式存在很长一段时间，从而在更浓的盐水和相关的沉淀盐矿物中引起增强的U富集。然而，我们的研究表明，在长期的古湖泊演化过程中，在亚氧-好氧环境中，与铀相关的氧化还原指标应谨慎解释。
Uranium (U) provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments. Its proxy potential under suboxic –oxic conditions associated with salinization of surface water in closed hydrologic systems, however, has not been adequately explored. This study examined the abundance and distribution of U in a sedimentary sequence of calcareous lacustrine clastics and salt layers from a high quality (938.5 m-long) core drilled in the western Qaidam Basin, NE Tibetan Plateau. This study examined U concentrations in calcareous clastic sediments (including carbonates, acid insoluble fractions, as well as the amounts of iron and manganese oxides/oxyhydroxides) and salt layers. In the calcareous clastic sediments, variations in U concentrations within the acid insoluble fraction follow the fluctuations of reconstructed lake levels based on lithologic investigations, providing a tool for tracing redox conditions in paleolake bottom water. Correlations between the variations in carbonate content with U and Sr concentrations in the carbonate fractions demonstrate that the types and abundance of carbonate minerals control the U distribution in carbonates. In the salt layers, U and Al concentrations correlate closely, implying that both derived mainly from the detrital fraction in the salt layers. However, enrichments of U and Mg are systematically higher in salt minerals formed in more concentrated brine and lower in those formed in less concentrated brine. These results indicate that during the process of carbonate precipitation, carbonate might effectively remove U from lake water. During the subsequent stage of salt mineral precipitation, U behavior is conservative, and U resides for a long period in dissolved forms within the saline water, giving rise to enhanced U enrichment in the more concentrated brines and the associated precipitated salt minerals. Our study suggests, however, that U-related redox proxies should be interpreted with caution in suboxic–oxic environments during the long-term paleolake evolution.