Microbially-mediated Mn redox cycling and Mn carbonate precipitation in the Marinoan glacial aftermath, South China

摘要

Carbonate-type Mn deposits developed at the top of the Ediacaran Doushantuo Formation (Marinoan glacial aftermath) along the northern margin of the Yangtze Block, South China, provide an exceptional opportunity to study Mn redox cycling in deep time, which remains a controversial topic of Mn biogeochemistry. To address this issue, here we conduct integrated petrological and geochemical work. Results show that Mn oxidizing micro-organisms may fix CO2 into organic matter by oxidizing Mn(II). This process provides energy for metabolism of the microorganisms under obligatory oxidation conditions in natural geological environments. At the same time, Mn(II) is oxidized to unstable todorokite, with accompanying Co and Ce oxidation and enrichment. Under suboxic conditions, heterotrophic Mn-reducing bacteria use organic matter as an electron donor and Mn bio-oxide as a terminal electron acceptor to generate Mn(II) and 12C-rich dissolved inorganic carbon (DIC; car-bonate and/or bicarbonate). Furthermore, microorganisms and macroalgae also provide nucleation sites for the precipitation of Mn carbonate minerals with structures consistent with microbially precipitated carbonates. Spherical and ellipsoidal Ca-rich rhodochrosite was formed in the syngenetic stage and cemented by Mg-rich kutnohorite in the early diagenetic stage. Microorganisms thus play an important role in Mn redox cycling and precipitation of Mn carbonate minerals, in a possibly universal phenomenon in geological evolution. The bloom of microorganisms and associated formation of mineral deposits may be a general phenomenon in the aftermath of a glaciation.