Whether Mn carbonates can be used as a proxy for the oxygenation event is debated. Here we examined the Early Cretaceous lacustrine Mn carbonates from North China, which contain abundant microbial fossils. The extremely positive δ¹³C (up to +15‰ relative to Vienna Peedee belemnite) and micro-area enrichment of Ni strongly indicate a methanogenic archaea origin of these microorganisms. Transmission electron microscope and electron energy loss spectroscopy show the nanoscale transformation of Mn-oxides (Mg-exchanged phyllomanganate) to Mn carbonates (kutnohorite), on extracellular polymeric substances. The reaction of the Mn oxides with organic matter resulted in increasing pH and alkalinity, together with the fluctuating pH, offering a suitable micro-environment for the transformation processes. These Mn carbonates are therefore indicative of an oxidized, sulfate-absent environment. The depicted scenario serves as a reference to ocean of the early Earth and provides a referable Mn oxide tracer for determining the emergence of the Great Oxidation Event.

中文翻译：

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以氧化锰为代价的产甲烷菌介导的碳酸锰沉淀

碳酸锰是否可以用作氧化事件的代表存在争议。在这里，我们研究了华北早白垩世湖相碳酸锰，其中含有丰富的微生物化石。极端正的 δ ¹³ C（相对于维也纳 Peedee 箭石高达 +15‰）和 Ni 的微区域富集强烈表明这些微生物的产甲烷古菌起源。透射电子显微镜和电子能量损失光谱显示细胞外聚合物上锰氧化物（镁交换叶锰酸盐）向锰碳酸盐（锰锰酸锂）的纳米级转变。 Mn氧化物与有机物的反应导致pH值和碱度增加，以及pH值波动，为转化过程提供了合适的微环境。因此，这些锰碳酸盐表明存在氧化的、缺乏硫酸盐的环境。所描绘的场景可以作为早期地球海洋的参考，并为确定大氧化事件的出现提供可参考的氧化锰示踪剂。