Uranium Isotope Fractionation in Non-sulfidic Anoxic Settings and the Global Uranium Isotope Mass Balance
Uranium Isotope Fractionation in Non-sulfidic Anoxic Settings and the Global Uranium Isotope Mass Balance
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DOI：
10.1029/2020gb006649
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发表时间：
2020-08-01
影响因子：
5.2
通讯作者：
Sperling, Erik A.
中科院分区：
文献类型：
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作者：
Cole, Devon B.;Planavsky, Noah J.;Sperling, Erik A.
在过去十年中，铀同位素（U-238/U-235）已被广泛用作海洋氧化还原条件的全球替代物。在系统中最大的同位素分馏发生在U还原，去除和埋葬。应用这一基本框架，全球铀同位素质量平衡模型已被用于预测在整个地球历史的关键间隔洋底缺氧的程度。然而，目前有最低限度的限制同位素分馏过程中发生的还原和埋葬在缺氧和富铁（含铁）的水生系统，尽管共识，含铁的条件被认为是广泛的，通过我们这个星球的大部分历史。在这里，我们提供了第一次探索三角洲U-238值在天然铁质设置。我们测量了三角洲U-238的沉积物从两个现代含铁湖泊（布朗尼湖和帕文湖），水柱的布朗尼湖，和沉积岩从志留纪-泥盆纪的边界下沉积的含铁条件。此外，我们提供了新的三角洲U-238数据从核心顶部沉积物缺氧，但nonsulfidic设置在秘鲁保证金氧气最低区。我们发现，三角洲的U-238值沉积在所有这些地方的沉积物是高度可变的，但平均而言是无法区分的邻近的氧化沉积物。这迫使重新评估全球铀同位素质量平衡以及如何使用铀同位素值来重建海洋氧化还原景观的演变。
Uranium isotopes (U-238/U-235) have been used widely over the last decade as a global proxy for marine redox conditions. The largest isotopic fractionations in the system occur during U reduction, removal, and burial. Applying this basic framework, global U isotope mass balance models have been used to predict the extent of ocean floor anoxia during key intervals throughout Earth's history. However, there are currently minimal constraints on the isotopic fractionation that occurs during reduction and burial in anoxic and iron-rich (ferruginous) aquatic systems, despite the consensus that ferruginous conditions are thought to have been widespread through the majority of our planet's history. Here we provide the first exploration of delta U-238 values in natural ferruginous settings. We measured delta U-238 in sediments from two modern ferruginous lakes (Brownie Lake and Lake Pavin), the water column of Brownie Lake, and sedimentary rocks from the Silurian-Devonian boundary that were deposited under ferruginous conditions. Additionally, we provide new delta U-238 data from core top sediments from anoxic but nonsulfidic settings in the Peru Margin oxygen minimum zone. We find that delta U-238 values from sediments deposited in all of these localities are highly variable but on average are indistinguishable from adjacent oxic sediments. This forces a reevaluation of the global U isotope mass balance and how U isotope values are used to reconstruct the evolution of the marine redox landscape.