The concentration of redox sensitive trace metals (RSTEs) and their isotopic composition preserved in Precambrian marine sediments, are critical for the reconstruction of ocean–atmosphere oxygenation history. Particularly, the concentration of Fe, its redox speciation, and isotopic distribution, have gained widespread use for inferring the biogeochemical processes that controlled Fe cycling in Precambrian oceans linked to the reconstruction of Earth surface redox budget. However, questions remain about the biotic and abiotic processes involved in Fe cycling in these ancient oceans, including the impact of post-depositional alterative processes on the reliability of the Fe redox proxy. Here we present a multi-proxy mineralogical and geochemical study of the ∼1.1 Ga Atar and El Mreiti strata of the Taoudeni Basin in Mauritania, to better constrain pathways involved in Fe cycling, linked to Fe mineralogy, redox speciation, isotopic ratios during this time and metamorphism. We compare unmetamorphosed sedimentary deposits with facies metamorphosed by dolerite sill intrusion. The results reveal the occurrence of diagenetic Fe minerals in the basal unmetamorphosed samples associated with light δ56Fe signatures, reflecting dominant anoxic conditions that promoted microbial dissimilatory Fe reduction. Notably, δ56Fe composition of these rocks reveal several fluctuations in evolving seawater redox state from oxic to anoxic/sulfidic conditions associated with changes in sea level stand and periods of full bottom water oxygenation and redox stratification. Overall, Ce anomalies suggest a general up sequence increase in seawater oxygen content. Metamorphosed rocks display heterogeneous δ56Fe distribution, consisting of light and heavy signatures associated with secondary Fe-bearing minerals produced by metamorphic and metasomatic overprinting of carbonated rocks by hot circulating fluids. The results thus indicate metamorphic overprinting of primary seawater δ56Fe promoted by increased mobility of reactive Fe during post-depositional metamorphic transformation. They show that post-depositional metamorphic/metasomatic overprinting complicates direct reconstruction of seawater biogeochemical Fe cycling and redox state using δ56Fe systematics.

中文翻译：

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海相铁循环与毛里塔尼亚 1.1 Ga 中元古代 Taoudeni 盆地的动态氧化还原变化、生物活性和沉积后矿化有关


前寒武纪海洋沉积物中保存的氧化还原敏感痕量金属 （RSTE） 的浓度及其同位素组成对于重建海洋-大气氧合历史至关重要。特别是，Fe 的浓度、其氧化还原形态和同位素分布已被广泛用于推断控制前寒武纪海洋中 Fe 循环的生物地球化学过程，这些过程与地球表面氧化还原预算的重建有关。然而，关于这些古老海洋中 Fe 循环所涉及的生物和非生物过程仍然存在疑问，包括沉积后交替过程对 Fe 氧化还原代理可靠性的影响。在这里，我们介绍了毛里塔尼亚陶德尼盆地 ∼1.1 Ga Atar 和 El Mreiti 地层的多代理矿物学和地球化学研究，以更好地限制与铁矿物学、氧化还原形态、在此期间的同位素比值和变质作用有关的铁循环途径。我们将未变质的沉积矿床与由玄武岩基台侵入变质的相进行了比较。结果揭示了与光 δ56Fe 特征相关的基底未变质样品中成岩 Fe 矿物的出现，反映了促进微生物异化 Fe 还原的主要缺氧条件。值得注意的是，这些岩石的 δ56Fe 成分揭示了海水氧化还原状态从含氧到缺氧/硫化条件的演变中的几次波动，这与海平面支架的变化以及完全底层水氧化和氧化还原分层的时期有关。总体而言，Ce 异常表明海水氧含量普遍上升。 变质岩表现出非均质的 δ56Fe 分布，由与热循环流体对碳酸盐岩进行变质和交代叠印产生的次生含铁矿物相关的轻和重特征组成。因此，结果表明，在沉积后变质转变过程中，活性 Fe 的迁移率增加促进了原生海水 δ56Fe 的变质叠印。他们表明，沉积后变质/交代叠印使使用 δ56Fe 系统学直接重建海水生物地球化学 Fe 循环和氧化还原态变得复杂。