A modern framework for the interpretation of 238U/235U in studies of ancient ocean redox
A modern framework for the interpretation of 238U/235U in studies of ancient ocean redox
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DOI：
10.1016/j.epsl.2014.05.051
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发表时间：
2014-08-15
影响因子：
5.3
通讯作者：
Lyons, T. W.
中科院分区：
文献类型：
--
作者：
Andersen, M. B.;Romaniello, S.;Lyons, T. W.
古沉积物中氧化还原敏感元素的丰度和同位素组成越来越多地用于了解过去海洋的地球化学状态和地球的氧合历史。铀(U)从可溶的铀+6到相对不溶的铀+4的氧化还原转变及其随后进入还原沉积物的过程已被用来推断过去海洋的氧化还原状态。此外，最近的分析改进揭示了在氧化还原转变过程中显著的U-238/U-235分馏，这为U同位素作为氧化还原代理提供了潜力。然而，铀同位素作为地球化学示踪剂的发展需要与氧化还原变化相关的铀同位素系统得到很好的表征。本研究对黑海和卡里亚科盆地两个最大的现代缺氧海洋盆地近期沉积物中的U同位素进行了研究，旨在促进我们对还原海洋环境中U同位素系统的认识。与海水相比，这些缺氧沉积物具有较高的铀积累速率和较高的U-238/U-235比率，这与积累具有重同位素组成的还原铀的过程基本一致。利用Al和Ca浓度校正碎屑和生物碳酸盐结合的U，我们估计了沉积物中积累的自生铀及其U-238/U-235的还原量。这些结果强调了同位素质量平衡约束在U从海水和通过孔隙水扩散传输和反应过程中的重要性，影响了沉积物中观测到的U-238/U-235。利用这些约束条件，可以估算出从水柱顶部到底部U耗竭的平均百分比，假设U在受限盆地中分批去除到缺氧沉积物中。利用这一框架，黑海现代缺氧沉积物中的U-238/U-235意味着水柱中U的消耗接近30%，这与在这些深度观察到的现代黑海水柱中U的消耗接近40%。从卡里亚科盆地的缺氧沉积物样品中估计了水柱中类似的U损耗。这些新近缺氧沉积物为解释古沉积物中的自生U-238/U-235提供了基础。特别是，基于质量平衡关系，这种分析可能会提供一些见解，以了解特定的古代沉积物是沉积在开阔的海洋还是局限的盆地中。因此，这种方法可以提供对局部氧化还原与海洋氧化还原控制的关键见解，并且在这种情况下，限制其他代理捕获缺氧/缺氧全球信号的能力。(C) 2014 Elsevier B.V.版权所有
The abundance and isotope composition of redox sensitive elements in ancient sediments are increasingly used to understand the past ocean's geochemical state and the oxygenation history of the Earth. The redox transition of uranium (U) from soluble U+6 to relatively insoluble U+4 and its subsequent incorporation into reduced sediments has been used to deduce the redox state of the oceans in the past. Furthermore, recent analytical improvements have revealed significant U-238/U-235 fractionation during this redox transition, offering the potential for U isotopes to act as a redox proxy. However, the development of U isotopes as a geochemical tracer requires that U isotope systematics associated with redox changes, are well-characterized.This study focuses on U isotopes in recent sediments from the two largest modern anoxic ocean basins, the Black Sea and the Cariaco Basin, with the aim of advancing our understanding of the U isotope systematics in reducing marine environments. These anoxic sediments have high U accumulation rates and high U-238/U-235 ratios relative to seawater, in general agreement with a process that accumulates reduced U with a heavy isotopic composition. Using Al and Ca concentrations to correct for detrital and biogenic carbonate-bound U, we estimate the reduced authigenic U accumulated in the sediments and its U-238/U-235.These results highlight the importance of isotopic mass balance constraints during diffusive transport and reaction of U from seawater and through pore-water, affecting the observed U-238/U-235 in sediments. Using these constraints, the average percentages of U depletion from top to bottom of the water column can be estimated, assuming batch-removal of U into anoxic sediments in a restricted basin. Using this framework, U-238/U-235 in modern anoxic sediments from the Black Sea imply U depletions in the water column of similar to 30%, which is close to the observed similar to 40% U depletion in the modern Black Sea water column at these depths. Similar U depletion in the water column is estimated from anoxic sediment samples of the Cariaco Basin. These recent anoxic sediments provide a basis for interpreting authigenic U-238/U-235 in ancient sediments. In particular, such analyses may offer insights, based on mass balance relationships, into whether particular ancient sediments were deposited in an open ocean or restricted basin. As such, this approach may provide key insight into the controls on local versus ocean-scale redox and, in that light, constraints the capacity of other proxies to capture global signals for anoxia/euxinia. (C) 2014 Elsevier B.V. All rights reserved.