Dynamic biogeochemical cycling and mineralization of manganese of hydrothermal origin after the Marinoan glaciation

Abstract

The aftermath of the Marinoan glaciation is an important period in the evolutionary history of Earth. Here we report on the dynamically biogeochemical cycling and organic mineralization processes of Mn in the Doushantuo Formation, Yangtze Block, South China, which was sourced hydrothermally during this time. This process occurred in an extensional tectonic setting as the water depth became shallower. Mn(+2) ascended with deep reducing hydrothermal fluids and accumulated in the form of Mn(+4) near the redox interface of locally oxidized water, which may be associated with metabolism by autotrophic microorganisms. Mn(+4) was reduced to Mn(+2), with organic matter acting as an electron donor during its degradation, which may be mediated by heterotrophic microorganisms. Mn(+2) in the water combined with CO32− generated by the oxidation and degradation of organic matter to form rhodochrosite. Algae and bacteria provided nucleation sites for the crystallization of rhodochrosite. Finally, spherical and ellipsoidal rhodochrosite with a biological structure and negative carbon isotope values (average δ13CPDB = −7.04‰) under the influence of Mn bacteria was formed. The input of hydrothermal material, mineralization involving the participation of organic matter, and mixed enrichment during syngenesis and syngenetic–early diagenetic stage were unique factors in the formation of Mn deposits in the Doushantuo Formation along the northern margin of the Yangtze Block. This represents the first report of Mn mineralization in carbonate form in the Ediacaran globally (Doushantuo Formation), and provides new insights into the biogeochemical cycling of Mn in the Ediacaran.