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Microbial Mn redox cycling and mineralization of black shale-hosted Mn ore in Ediacaran Doushantuo Formation, South China

Manganese,Mn microbial oxidation,Mn microbial reduction,Black shale-hosted Mn carbonate,Doushantuo Formation

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4a: Control of Microbial and Biogeochemical Cycling on Hydrocarbon Formation and Critical Metal Mineralization

Manganese is an important redox-active transition metal and bio-element in the Earth system, with transformation between dissolved Mn(II) and insoluble Mn(IV) in geological history being a response to environmental redox conditions (Wittkop et al., 2020). The predominant mechanism of Mn cycling involves redox conversion between soluble Mn(II) and insoluble Mn(III/IV), controlled mainly by the type and availability of oxidants and reductants (Havig et al., 2015), and external environmental conditions (Barley and Groves, 1992). In addition, the recent research show that microorganisms may play a vital role in mediating Mn redox cycling in laboratory simulation (Myers and Nealson, 1988; Learman et al., 2011; Li et al., 2019; Yu and Leadbetter, 2020) and the formation of modern oceanic Mn nodules (Shiraishi et al., 2016; Jiang et al., 2020; Li et al., 2021). However, there is no solid evidence for microbially-mediated redox cycling of Mn under geological conditions in deep time. The black shale-hosted Mn deposit in the Ediacaran Doushantuo Formation along the northern margin of the Yangtze Block, South China, provide an excellent natural laboratory for the research of ancient Mn redox cycling and organic-inorganic interaction.

In this study, we performed a simulation experiment of oxidizing Mn by bacteria and applied high-resolution in situ mineralogical and geochemical analyses based on the morphological characteristics of different types of microorganisms and geochemical markers generated by their biological activities processes. The geochemical results indicate that Mn in carbonates at the top of the Doushantuo Formation along the northern margin of Yangtze Block was derived from deep submarine hydrothermal fluids as Mn(II), increasing the concentration of Mn(II) in the seawater reservoir. According to the mineralogical nature of products derived from microbial simulation experiment, we presume the process which microorganisms produced organic matter by oxidizing Mn(II), providing energy for their metabolic activities during the mineralization of Mn carbonate in Doushantuo Formation. In oxidation condition, Mn-oxidizing bacteria oxidize Mn(II) to generate unstable todorokite with low crystallization symbiotic with organic matter, with accompanying Co and Ce oxidation and enrichment. Under O₂-poor conditions, heterotrophic Mn-reducing bacteria use organic carbon as an electron donor and Mn bio-oxide as a terminal electron acceptor to generate Mn(II) and ¹²C-rich dissolved inorganic carbon (DIC), which are the necessary materials for the formation of Mn carbonate minerals. Meanwhile, the coupled bio-reduction of Mn(IV) and mineralization of organic C provide the ideal alkaline geochemical environment for Mn carbonate minerals precipitation. On this basis, microorganisms and organic matter also provide nucleation sites for the precipitation of Mn carbonate minerals with biological morphology.

Our findings provided robust evidence of the central role of microorganisms and organic matter in Mn redox cycling and Mn carbonate mineral precipitation. Microbial activity facilitates Mn redox cycling and promotes precipitation of Mn carbonate minerals by providing nucleation sites. At the same time, microbially mediated Mn redox cycling also stimulates the cycling of carbon between inorganic and organic environments, with C isotopic fractionation being a by-product of Mn biogeochemical cycling in deep time. Therefore, the process of microorganisms mediate Mn redox cycling and mineralization of black shale-hosted Mn ore provided a new example of organic-inorganic interaction in Earth Science System.

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