Carbonate associated uranium isotopes as a novel local redox indicator in oxidatively disturbed reducing sediments
Carbonate associated uranium isotopes as a novel local redox indicator in oxidatively disturbed reducing sediments
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DOI：
10.1016/j.gca.2021.07.025
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发表时间：
2021-10
影响因子：
5
通讯作者：
M. Clarkson;R. Hennekam;T. Sweere;M. B. Andersen;G. Reichart;D. Vance
中科院分区：
文献类型：
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作者：
M. Clarkson;R. Hennekam;T. Sweere;M. B. Andersen;G. Reichart;D. Vance
海洋沉积物中局部氧化还原指标的解释依赖于它们的保存。就其性质而言，这种生物地球化学示踪剂易受成岩蚀变的影响，特别是通过沉积后的再氧化作用。这可能导致独特的氧化还原依赖性信号的动员和损失，导致经典问题，即氧合条件的存在不能通过缺乏缺氧的证据来有力地推断。自生铀富集及其同位素特征（δ238U）被广泛用于推断底水和孔隙水氧化还原条件，但易受后期氧化成岩干扰。本文研究了东地中海64PE406-E1遗址（~1760 m水深）S1 Sapropel S1沉积物样品中自生δ238U特征的保存，该沉积物最初是在还原性条件下沉积的，但受到后期氧化成岩作用和上部Sapropel U（IV）的变化损失的严重影响。为此，我们将大块测量的U同位素特征（δ238Ubulk）与碎屑校正的自生U （δ238Uauth）和碳酸盐伴生U （δ238UCAU）进行了比较。与完全氧化条件下沉积的开阔海洋碳酸盐岩相比，腐泥型碳酸盐岩渗滤液的δ 238ucauth与计算的δ238Uauth相似，因此（主要）记录了依赖于氧化还原的同位素分馏。没有证据表明沉积后氧化成岩作用导致了显著的同位素分馏，但相关的自生U（IV）去除导致相对碎屑对δ238Ubulk的贡献较大，因此δ 238uauth_估算存在很大的不确定性。相比之下，δ 238ucau成功地避免了碎屑相，以及与碎屑校正相关的不确定性，提供了局部氧化还原条件变化的初步记录。因此，我们建议δ 238ucauca可以用于低U富集的沉积物中底栖生物脱氧的精确重建，包括沉积后U（IV）损失和还原环境较少的沉积物。
The interpretation of local redox indicators in marine sediments relies on their preservation. By their nature, such biogeochemical tracers are susceptible to diagenetic alteration, particularly through post-depositional re-oxidation of the sediment. This can result in the mobilization and loss of distinctive redox dependant signatures, leading to the classical problem that the presence of oxygenated conditions cannot be robustly inferred by the absence of evidence for anoxia. Authigenic uranium enrichments, and their isotope signatures (δ238U), are widely used to infer bottom-water and pore-water redox conditions but are susceptible to later oxidative diagenetic disturbance. Here we explore the preservation of authigenic δ238U signatures in sediment samples from Sapropel S1 at Site 64PE406-E1 in the Eastern Mediterranean Sea (~1760 m water depth), which was originally deposited under reducing conditions but severely affected by later oxidative diagenesis and the variable loss of authigenic U(IV) from the upper sapropel. To this end, we compare U isotope signatures from bulk measurements (δ238Ubulk) with detrital corrected authigenic U (δ238Uauth) and carbonate associated U (δ238UCAU). In contrast to open ocean carbonates deposited under fully oxic conditions, sapropelic carbonate leachates yield δ238UCAUsimilar to calculated δ238Uauth, thus recording (predominantly) redox dependant isotope fractionation. There is no evidence for significant isotope fractionation resulting from post-depositional oxidative diagenesis, but associated authigenic U(IV) removal results in larger relative detrital contributions to δ238Ubulk, and hence large uncertainties on δ238Uauthestimates. By contrast, δ238UCAUsuccessfully avoids detrital phases, and the uncertainty associated with detrital corrections, providing a primary record of changes in local redox conditions. Thus, we propose that δ238UCAUcan be used for accurate reconstructions of benthic de-oxygenation in sediments with low U enrichments, including sediments with post-depositional U(IV) loss and less reducing environments.