Uranium isotopes (²³⁸U/²³⁵U) 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 δ²³⁸U values in natural ferruginous settings. We measured δ²³⁸U 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 δ²³⁸U data from core top sediments from anoxic but nonsulfidic settings in the Peru Margin oxygen minimum zone. We find that δ²³⁸U 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.

中文翻译：

---

非硫化物缺氧环境中的铀同位素分馏和全球铀同位素质量平衡

铀同位素（²³⁸ U / ²³⁵ U）在过去十年中被广泛用作海洋氧化还原条件的全球替代物。系统中最大的同位素分馏发生在铀还原，去除和埋葬期间。应用这一基本框架，全球铀同位素质量平衡模型已被用来预测整个地球历史上关键时间间隔内海床缺氧的程度。然而，尽管人们普遍认为，在整个地球的大部分历史中，普遍认为铁质条件已经广泛存在，但目前对缺氧和富铁（铁质）水生系统在还原和埋葬过程中发生的同位素分馏的限制很小。在这里，我们提供δ的第一口勘探²³⁸在天然铁质环境中的U值。我们测量δ ²³⁸ ü在沉积物中含铁条件下沉积两个现代铁质湖泊（布朗尼湖和帕文），布朗尼湖的水柱，和沉积岩，从志留纪，泥盆纪边界。此外，我们提供了来自秘鲁边缘含氧量最小区域中缺氧但非硫化环境的岩心顶部沉积物的新^δ238 U数据。我们发现，来自所有这些位置的沉积物中的^δ238 U值变化很大，但平均而言与相邻的含氧沉积物没有区别。这迫使人们重新评估全球U同位素质量平衡，以及如何使用U同位素值来重建海洋氧化还原景观的演变。