Abstract The sedimentary Mo and U isotope systems have been commonly used as novel global ocean redox tracers due to their long oceanic residence times and redox-sensitive behavior. However, local sedimentary environments and global ocean redox conditions both influence the Mo and U isotope compositions of euxinic organic-rich mudrocks (ORM). Here, we further develop the coupled use of Mo and U isotope data from euxinic ORM to more robustly infer coeval global ocean redox conditions. We measured δ238U from eight late Neoproterozoic to middle Paleozoic ORM units that have previously reported Mo isotope and Fe speciation data. Integration of our new data with previously published Proterozoic and Phanerozoic Mo and U isotope data reveals that there is no overall correlation between the Mo and U isotope compositions of euxinic ORM. This observation confirms that the extent to which local versus global environments influenced the preserved Mo and U isotope compositions in ORM was variable. Individual ORM units can have negative, positive, or no correlation between δ98Mo and δ238U. A negative correlation between δ98Mo and δ238U in the Upper Devonian Kettle Point Formation is similar to the observations from modern euxinic basins, reflecting a major control on the Mo-U isotope systematics by changes in the local depositional environment, such as bottom-water sulfide concentrations. A positive correlation between δ98Mo and δ238U observed in the Upper Ordovician Fjacka Shale is best explained by changes in global ocean redox conditions that simultaneously shifted the Mo and U isotope compositions of the global seawater and the Fjacka Shale ORMs in the same direction. No correlations between δ98Mo and δ238U for euxinic ORM may be caused by specific local depositional changes, a lack of or a combination of local and global environmental changes, and/or is an artifact of limited data. For example, a vertical trend (variable δ98Mo but similar δ238U) is shown by most samples from Member IV of the Ediacaran Doushantuo Formation, implying a strong influence on the Mo isotope data by an Fe-Mn particulate shuttle. A horizontal trend (similar δ98Mo but variable δ238U) is observed from the Paleoproterozoic Zaonega Formation, implying that relatively constant bottom water sulfide concentrations caused similar magnitudes of Mo isotope fractionations whereas other factors (e.g., U reduction pathways, aqueous U species, productivity) were responsible for variable U isotope fractionations. Relatively constant elemental concentrations and isotope compositions from the Tanezzuft Formation are indicative of stable conditions at local and global scales. We further propose a method to estimate the coeval seawater Mo and U isotope compositions based on a coupled Mo-U isotope mass balance model and the observations from modern euxinic basins. The coupled Mo-U isotope data from euxinic ORMs provide more insights on the local and global environmental controls on the preservation of both isotope systems than previously realized. Our study highlights the importance of examining the local depositional environment and using large datasets of coupled Mo-U isotope compositions from euxinic ORM intervals to reconstruct paleocean redox conditions.

中文翻译：

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通过耦合富有机质泥岩的钼和铀同位素系统估计古代海水同位素组成和全球海洋氧化还原条件

摘要 沉积Mo和U同位素系统由于其在海洋中的停留时间长和氧化还原敏感行为而被广泛用作新型的全球海洋氧化还原示踪剂。然而，局部沉积环境和全球海洋氧化还原条件都会影响富有机质泥岩 (ORM) 的 Mo 和 U 同位素组成。在这里，我们进一步开发了来自 euxinic ORM 的 Mo 和 U 同位素数据的耦合使用，以更可靠地推断同时代的全球海洋氧化还原条件。我们测量了 8 个晚新元古代到中古生代 ORM 单位的 δ238U，这些单位以前曾报道过 Mo 同位素和 Fe 物种形成数据。我们的新数据与先前公布的元古代和显生宙 Mo 和 U 同位素数据的整合表明，优生 ORM 的 Mo 和 U 同位素组成之间没有整体相关性。这一观察证实，局部环境与全球环境对 ORM 中保存的 Mo 和 U 同位素组成的影响程度是可变的。单个 ORM 单元在 δ98Mo 和 δ238U 之间可能具有负相关、正相关或无相关。上泥盆统Kettle Point组δ98Mo和δ238U呈负相关，与现代富氧盆地的观测结果相似，反映了底水硫化物浓度等局部沉积环境变化对Mo-U同位素系统的主要控制. 在上奥陶统 Fjacka 页岩中观察到的 δ98Mo 和 δ238U 之间的正相关性最好用全球海洋氧化还原条件的变化来解释，这些变化同时使全球海水和 Fjacka 页岩 ORM 的 Mo 和 U 同位素组成向同一方向移动。δ98Mo 和 δ238U 之间对于 euxinic ORM 没有相关性可能是由特定的局部沉积变化、局部和全球环境变化的缺乏或组合引起的，和/或是有限数据的产物。例如，埃迪卡拉纪陡山沱组 IV 段的大多数样品显示出垂直趋势（变量 δ98Mo，但类似 ​​δ238U），这意味着 Fe-Mn 颗粒穿梭对 Mo 同位素数据的强烈影响。从古元古代 Zaonega 组观察到一个水平趋势（类似的 δ98Mo 但可变的 δ238U），这意味着相对恒定的底部水硫化物浓度导致类似的 Mo 同位素分馏幅度，而其他因素（例如，U 还原途径、含水 U 物种、生产力）负责可变 U 同位素分馏。来自 Tanezzuft 地层的相对恒定的元素浓度和同位素组成表明在局部和全球范围内条件稳定。我们进一步提出了一种基于耦合 Mo-U 同位素质量平衡模型和来自现代 euxinic 盆地的观测结果来估计同时期海水 Mo 和 U 同位素组成的方法。来自 euxinic ORM 的耦合 Mo-U 同位素数据提供了比以前意识到的更多关于本地和全球环境控制对两种同位素系统保存的见解。我们的研究强调了检查当地沉积环境和使用来自 euxinic ORM 间隔的耦合 Mo-U 同位素组成的大型数据集来重建古海洋氧化还原条件的重要性。