The Phanerozoic surface ocean is characterized by its high dissolved oxygen content owing to mixing with the atmosphere. However, atmospheric oxygen levels varied in the early Paleozoic and it remains unclear whether the surface ocean was susceptible to significant redox fluctuations in response to extreme environmental events. In this study, we probed the redox structures of shallow middle Cambrian marine depositional environments across the North China Platform, ranging from open tidal flats to relatively deep subtidal environments. We utilized a combination of least diagenetically altered carbonate materials (such as ooid cortices, calcimicrobes, and their fringing cements), as well as in situ element measurement and imaging techniques. By analyzing a set of redox-related elements (e.g., Ce anomaly, Zn/Fe molar ratio, Mn and Cr) and mineralogical proxies (hydrogenetic Fe oxides), we revealed a stratified redox structure in the Drumian surface oceans. Compared to earlier Drumian conditions, late Drumian surface oceans experienced significant intrusions of ferruginous waters, probably reaching into shallow subtidal environments with water depths less than 10 m. Furthermore, we identified shallow subtidal microbial O-producing factories, characterized by dendritic thalli. These calcimicrobes exhibited more oxygenated signatures (negative Ce anomalies and enrichment of hydrogenetic Fe oxides) relative to contemporaneous less oxic shallower and deeper environments. This finding indicates that they produced oxygen oases or refuges during periods of both normal and poor dissolved O conditions. This study has the potential to broaden our understanding of redox conditions and microbial oxygen-producing mechanisms in the surface ocean, particularly during intervals characterized by low atmospheric oxygen levels or episodic anoxic events.

中文翻译：

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低氧水域强烈侵入寒武纪中表层海洋碳酸盐工厂


显生宙表层海洋的特点是由于与大气混合而具有高溶解氧含量。然而，古生代早期大气中的氧含量发生了变化，目前尚不清楚表层海洋是否容易因极端环境事件而受到显着的氧化还原波动的影响。在本研究中，我们探讨了华北地台浅层中寒武统海相沉积环境的氧化还原结构，范围从开阔的潮坪到相对较深的潮下环境。我们结合了成岩改变最小的碳酸盐材料（例如鲕粒皮质、钙微生物及其边缘胶结物）以及原位元素测量和成像技术。通过分析一组与氧化还原相关的元素（例如 Ce 异常、Zn/Fe 摩尔比、Mn 和 Cr）和矿物学指标（水成 Fe 氧化物），我们揭示了德鲁米亚表层海洋中的分层氧化还原结构。与早期德鲁姆期的情况相比，德鲁姆期晚期的表层海洋经历了铁质水域的显着入侵，可能到达水深小于10 m的浅潮下环境。此外，我们还发现了浅层潮下微生物产氧工厂，其特征是树枝状菌体。相对于同时期含氧量较低的浅层和深层环境，这些钙微生物表现出更多的含氧特征（负 Ce 异常和氢化铁氧化物富集）。这一发现表明，它们在正常和溶解氧条件较差的时期都产生了氧气绿洲或避难所。 这项研究有可能扩大我们对海洋表层氧化还原条件和微生物产氧机制的理解，特别是在以低大气氧水平或偶发性缺氧事件为特征的时期。