Abstract Fundamental questions persist regarding the redox structure and trace metal content of the Mesoproterozoic oceans. Multiple lines of evidence suggest more widespread anoxia in the deep oceans compared to today, and iron speciation indicates that anoxia was largely accompanied by dissolved ferrous iron (ferruginous conditions) rather than free sulfide (euxinia). Still, exceptions exist—euxinic conditions have been reported from some ocean margin and epeiric sea settings, and oxic conditions were reported in one deeper water environment and are also known from shallow waters. Constraining the temporal evolution of Mesoproterozoic marine redox structure is critical because it likely governed redox-sensitive trace metal availability, which in turn played a significant role in marine diazotrophy and the evolution of early eukaryotes. Here, we present a new, multi-proxy geochemical dataset from the ~1.2 Ga Bijaygarh Shale (Kaimur Group, Vindhyan Basin, India) emphasizing total organic carbon, iron speciation, and trace metal concentrations, as well as sulfur, nitrogen, and molybdenum isotopes. This unit was deposited in an open shelf setting near or just below storm wave base. Taken together, our data provide a unique snapshot of a biologically important shallow shelf setting during the Mesoproterozoic Era, which includes: 1) locally ferruginous waters below the zone of wave mixing, 2) muted enrichment of trace metals sensitive to general anoxia (e.g., chromium) and variable enrichment of trace metals sensitive to euxinia (e.g., molybdenum and, to a lesser extent, vanadium), 3) general sulfate limitation, and 4) nitrogen fixation by molybdenum-nitrogenase and a dominantly anaerobic nitrogen cycle in offshore settings. Differential patterns of trace metal enrichment are consistent with data from other basins and suggest a largely anoxic ocean with limited euxinia during the Mesoproterozoic Era. Our new molybdenum isotope data—the first such data from unambiguously marine shales deposited between 1.4 and 0.75 Ga—record values up to +1.18 ± 0.12‰ that are analogous to data from other Mesoproterozoic shale units. Ultimately, this study provides a broad, multi-proxy perspective on the redox conditions that accompanied early eukaryotic evolution.

中文翻译：

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富含铁的中元古代海洋中的钼同位素和痕量金属信号：来自印度温迪亚盆地的快照

摘要 关于中元古代海洋的氧化还原结构和痕量金属含量的基本问题仍然存在。多项证据表明，与今天相比，深海中存在更广泛的缺氧现象，而铁物种形成表明，缺氧主要伴随着溶解的亚铁（铁质条件）而不是游离硫化物（euxinia）。尽管如此，仍然存在例外情况——一些海洋边缘和上表海环境中报道了富氧条件，而在一个较深的水环境中报道了含氧条件，浅水区也有报道。限制中元古代海洋氧化还原结构的时间演化至关重要，因为它可能控制着对氧化还原敏感的痕量金属的可用性，这反过来又在海洋固氮和早期真核生物的演化中发挥了重要作用。这里，我们展示了来自 ~1.2 Ga Bijaygarh 页岩（Kaimur Group，Vindhyan 盆地，印度）的新的多代理地球化学数据集，强调总有机碳、铁形态和痕量金属浓度，以及硫、氮和钼同位素。该单元存放在风暴波基附近或正下方的开放式架子环境中。综上所述，我们的数据提供了中元古代时期具有生物学重要性的浅层陆架环境的独特快照，其中包括：1) 波浪混合区以下的局部含铁水，2) 对一般缺氧敏感的痕量金属的温和富集（例如，铬）和对微量元素敏感的微量金属的不同富集（例如，钼和在较小程度上，钒），3) 一般硫酸盐限制，和 4) 近海环境中钼-氮酶的固氮作用和主要的厌氧氮循环。痕量金属富集的差异模式与来自其他盆地的数据一致，表明中元古代时期海洋主要缺氧，微量元素有限。我们的新钼同位素数据——第一个来自沉积于 1.4 至 0.75 Ga 之间的明确海相页岩的此类数据——记录的值高达 +1.18 ± 0.12‰，类似于来自其他中元古代页岩单元的数据。最终，这项研究为伴随早期真核生物进化的氧化还原条件提供了一个广泛的、多代理的视角。痕量金属富集的差异模式与来自其他盆地的数据一致，表明中元古代时期海洋主要缺氧，微量元素有限。我们的新钼同位素数据——第一个来自沉积于 1.4 至 0.75 Ga 之间的明确海相页岩的此类数据——记录的值高达 +1.18 ± 0.12‰，类似于来自其他中元古代页岩单元的数据。最终，这项研究为伴随早期真核生物进化的氧化还原条件提供了一个广泛的、多代理的视角。痕量金属富集的差异模式与来自其他盆地的数据一致，表明中元古代时期海洋主要缺氧，微量元素有限。我们的新钼同位素数据——第一个来自沉积于 1.4 至 0.75 Ga 之间的明确海相页岩的此类数据——记录的值高达 +1.18 ± 0.12‰，类似于来自其他中元古代页岩单元的数据。最终，这项研究为伴随早期真核生物进化的氧化还原条件提供了一个广泛的、多代理的视角。