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Uranium reduction and isotopic fractionation in reducing sediments: Insights from reactive transport modeling
Geochimica et Cosmochimica Acta ( IF 4.5 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.gca.2020.01.021
Kimberly V. Lau , Timothy W. Lyons , Kate Maher

Abstract Uranium concentrations and isotopic ratios (238U/235U, denoted as δ238U) have been used to provide quantitative information about the degree of oxygenation and de-oxygenation of past oceans. The potential to constrain changes in global redox conditions, in contrast to many other proxies that reflect local conditions, is a particular strength of the uranium isotope approach. Because uranium reduction primarily occurs in sediments underlying anoxic water columns rather than in the anoxic water column itself, the removal of uranium in organic-rich shales is the largest lever on seawater δ238U. Accordingly, accumulation and isotopic fractionation are modulated by local variations in productivity, basin connectivity, sedimentation rate, and bottom-water redox conditions. To isolate the processes at the sediment-water interface that control δ238U and uranium accumulation in reducing sediments, we constructed a reactive transport model that couples biogeochemical reactions to diffusive transport and the burial of solutes and minerals. Using the model framework, we test the sensitivity of authigenic uranium isotopic fractionation and accumulation to oxygenation, permeability, sedimentation rate, organic carbon delivery, and basin restriction. Our results demonstrate that these external forcings produce diagnostic patterns in isotopic fractionation. Specifically, the model predicts that authigenic δ238U is sensitive to productivity because of the associated organic carbon burial rate. Moreover, our results suggest that the isotopic offset does not vary significantly due to changing bottom-water O2 concentrations, but the amount of accumulation does—a result that differs from previous estimates. Water column uranium reduction adds additional complexity to the ultimate δ238U value. The predictive patterns derived from model results can offer insight into local depositional conditions, such as sedimentation patterns. Collectively, these effects—including bottom-water redox conditions and related reducing sediments—alter the isotopic signature of the overlying water column according to the authigenic δ238U value and the diffusive fluxes arising from porewater concentration gradients. More broadly, this work provides important new constraints on the major controls on the δ238U of sediments while also supporting its use as a proxy for global marine redox conditions.

中文翻译:

还原沉积物中的铀还原和同位素分馏:反应输运模型的见解

摘要 铀浓度和同位素比 (238U/235U,表示为 δ238U) 已被用于提供有关过去海洋氧化和脱氧程度的定量信息。与反映当地条件的许多其他代理相比,限制全球氧化还原条件变化的潜力是铀同位素方法的一个特殊优势。由于铀还原主要发生在缺氧水柱下方的沉积物中,而不是缺氧水柱本身,因此富含有机质页岩中铀的去除是海水 δ238U 的最大杠杆。因此,积累和同位素分馏受生产力、盆地连通性、沉积速率和底水氧化还原条件的局部变化的调节。为了隔离沉积物-水界面上控制 δ238U 和还原沉积物中铀积累的过程,我们构建了一个反应性输运模型,将生物地球化学反应与扩散输运和溶质和矿物质的掩埋结合起来。使用模型框架,我们测试了自生铀同位素分馏和积累对氧化、渗透性、沉降速率、有机碳输送和盆地限制的敏感性。我们的结果表明,这些外强迫在同位素分馏中产生了诊断模式。具体而言,该模型预测自生 δ238U 由于相关的有机碳埋藏率而对生产力敏感。此外,我们的结果表明,由于底水 O2 浓度的变化,同位素偏移量不会发生显着变化,但积累的数量确实如此——这一结果与之前的估计不同。水柱铀还原增加了最终 δ238U 值的复杂性。从模型结果得出的预测模式可以深入了解当地的沉积条件,例如沉积模式。总的来说,这些影响——包括底水氧化还原条件和相关的还原沉积物——根据自生 δ238U 值和孔隙水浓度梯度产生的扩散通量改变了上覆水柱的同位素特征。更广泛地说,这项工作为沉积物 δ238U 的主要控制提供了重要的新限制,同时也支持将其用作全球海洋氧化还原条件的代理。水柱铀还原增加了最终 δ238U 值的复杂性。从模型结果得出的预测模式可以深入了解当地的沉积条件,例如沉积模式。总的来说,这些影响——包括底水氧化还原条件和相关的还原沉积物——根据自生 δ238U 值和孔隙水浓度梯度产生的扩散通量改变了上覆水柱的同位素特征。更广泛地说,这项工作为沉积物 δ238U 的主要控制提供了重要的新限制,同时也支持将其用作全球海洋氧化还原条件的代理。水柱铀还原增加了最终 δ238U 值的复杂性。从模型结果得出的预测模式可以深入了解当地的沉积条件,例如沉积模式。总的来说,这些影响——包括底水氧化还原条件和相关的还原沉积物——根据自生 δ238U 值和孔隙水浓度梯度产生的扩散通量改变了上覆水柱的同位素特征。更广泛地说,这项工作为沉积物 δ238U 的主要控制提供了重要的新限制,同时也支持将其用作全球海洋氧化还原条件的代理。如沉积模式。总的来说,这些影响——包括底水氧化还原条件和相关的还原沉积物——根据自生 δ238U 值和孔隙水浓度梯度产生的扩散通量改变了上覆水柱的同位素特征。更广泛地说,这项工作为沉积物 δ238U 的主要控制提供了重要的新限制,同时也支持将其用作全球海洋氧化还原条件的代理。如沉积模式。总的来说,这些影响——包括底水氧化还原条件和相关的还原沉积物——根据自生 δ238U 值和孔隙水浓度梯度产生的扩散通量改变了上覆水柱的同位素特征。更广泛地说,这项工作为沉积物 δ238U 的主要控制提供了重要的新限制,同时也支持将其用作全球海洋氧化还原条件的代理。
更新日期:2020-10-01
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