Interlinked marine cycles of methane, manganese, and sulfate in the post-Marinoan Doushantuo cap dolostone

The strongly 13C-depleted calcite in the Doushantuo cap dolostones (-635 Ma) of South China is consid-ered to form via thermochemical or biological oxidation of methane by sulfate in an environment with sulfate concentrations close to that of modern seawater (-28 mM). Here we demonstrate that the coeval seawater sulfate levels were indeed low (-1 to 3 mM) because all bulk 834S values of pyrites are close to and higher than the coeval seawater, with only a few showing a wide in situ 834S range from 5 to 62%x. Such low sulfate concentrations were initially consumed by sulfate reducing microorganisms to extre-mely low levels (<-0.2 mM), and thus Mn-driven anaerobic oxidation of methane (AOM) by sulfate occurred in pore water to produce H2S, which was in turn oxidized by Mn oxides back to sulfate. Consequently, with increasing Mn reduction and decreasing sulfate concentrations in pore waters, the produced pyrite shows an increase of in situ 834S values, and is in association with calcite with more neg-ative 813C values, and an inherited positive Ce anomaly from Mn oxides. Finally, the pyrite with the high-est 834S value may have co-precipitated with the 13C-depelted calcite (with a nadir of -58%x), and thus show similarly high Mn concentrations (-20000 mu g/g). This is the first report of sulfate-dependent AOM driven by Mn-or Fe-oxides from the entire Precambrian record. Such a previously unrecognized biogeo-chemical cycling during Marinoan global deglaciation may serve as the mechanism for major methane sink during the Proterozoic and Archean with low sulfate concentrations in the oceans.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

The strongly 13C-depleted calcite in the Doushantuo cap dolostones (-635 Ma) of South China is formed via thermochemical or biological oxidation of methane by sulfate. The coeval seawater sulfate levels were low (-1 to 3 mM), initially consumed by sulfate reducing microorganisms. Mn-driven anaerobic oxidation of methane (AOM) by sulfate occurred in pore water to produce H2S, which was then oxidized by Mn oxides back to sulfate. This previously unrecognized biogeochemical cycling serves as the mechanism for major methane sink during the Proterozoic and Archean with low sulfate concentrations in the oceans.