Ratios of (un)reactive iron species, authigenic molybdenum contents (Moauth), and molybdenum isotope compositions (δ98Moauth) in sedimentary rocks are geochemical proxies that are widely used to reconstruct past marine redox states, which have been calibrated in modern marine settings covering oxic to euxinic conditions. However, syn- and postdepositional processes can result in alterations and ambiguities of proxy-derived redox signals that can challenge the validity of paleoreconstructions. We present new data from modern organic-rich sediments of two oxygen minimum zone settings in the Gulf of California and the Peruvian margin. The results show that Mo is fully immobilized shortly after deposition by reaction with hydrogen sulfide (H2S) produced during organoclastic sulfate reduction. Thus, any H2S produced deeper in the sediment (e.g., by sulfate reduction coupled to anaerobic methane oxidation) leaves the initially deposited Mo concentrations and δ98Mo signatures unaltered, which supports the robustness of Mo-based redox proxies. In contrast, the Fe speciation data reveal continued pyritization due to constant exposure of Fe minerals to H2S. Importantly, both Fe bound to oxides and carbonates (highly reactive Fe) and also poorly reactive Fe (e.g., sheet silicates) undergo pyritization during early diagenesis. This process generates Fe-based proxy signatures that falsely imply ferruginous or euxinic conditions.

中文翻译：

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沉积后改变对铁和钼基氧化还原代理的影响

沉积岩中（非）反应性铁种类、自生钼含量 (Moauth) 和钼同位素组成 (δ98Moauth) 的比率是广泛用于重建过去海洋氧化还原状态的地球化学指标，这些指标已在现代海洋环境中进行了校准，涵盖了有氧到 euxinic 条件。然而，同沉积和沉积后过程可能导致代理衍生氧化还原信号的改变和模糊，从而挑战古建筑的有效性。我们提供了来自加利福尼亚湾和秘鲁边缘两个最低氧区设置的现代富含有机物沉积物的新数据。结果表明，通过与有机碎屑硫酸盐还原过程中产生的硫化氢 (H2S) 反应，Mo 在沉积后不久就被完全固定。因此，任何在沉积物更深处产生的 H2S（例如，通过硫酸盐还原与厌氧甲烷氧化相结合）使最初沉积的 Mo 浓度和 δ98Mo 特征保持不变，这支持基于 Mo 的氧化还原代理的稳健性。相比之下，Fe 物种形成数据显示，由于 Fe 矿物持续暴露于 H2S，导致持续的黄铁矿化。重要的是，与氧化物和碳酸盐结合的铁（高活性铁）和低活性铁（如片状硅酸盐）在早期成岩过程中都会经历黄铁矿化。此过程会生成基于 Fe 的代理签名，这些签名错误地暗示了铁质或富氧条件。Fe 物种形成数据显示，由于 Fe 矿物不断暴露于 H2S 中而导致持续的黄铁矿化。重要的是，与氧化物和碳酸盐结合的铁（高活性铁）和低活性铁（如片状硅酸盐）在早期成岩过程中都会经历黄铁矿化。此过程会生成基于 Fe 的代理签名，这些签名错误地暗示了铁质或富氧条件。Fe 物种形成数据显示，由于 Fe 矿物不断暴露于 H2S 中而导致持续的黄铁矿化。重要的是，与氧化物和碳酸盐结合的铁（高活性铁）和低活性铁（如片状硅酸盐）在早期成岩过程中都会经历黄铁矿化。此过程会生成基于 Fe 的代理签名，这些签名错误地暗示了铁质或富氧条件。