Thermochemical oxidation of methane by manganese oxides in hydrothermal sediments

Rhodochrosite with very negative carbon isotope values is identified in association with hausmannite in manganese ores from the Late Triassic Heqing deposit, China, and could have formed from high-temperature oxidation of methane during early diagenesis. Microbial anaerobic oxidation of methane coupled to the reduction of Mn(IV)-oxides, typically MnO2, derived from continental weathering and riverine transport has been proposed as a globally important sink of methane. However, the potential role of hydrothermal Mn(IV) oxide-rich sediments as a methane sink, and the mechanisms of underpinning methane oxidation at high-temperature hydrothermal fields remain poorly understood. Here, we report the occurrence of almost pure rhodochrosite with extremely negative & delta;C-13(PDB) values (as low as -76.4 & PTSTHOUSND;) in direct association with hausmannite formed through hydrothermal activity in the Late Triassic Heqing Mn deposit in Southern China. Based on detailed petrography and geochemistry, this rhodochrosite is interpreted as the result of the thermochemical oxidation of methane by hausmannite during early diagenesis. Given high hydrothermal Mn2+ and CH4 release coupled to low sulfate concentration observed in the Archean oceans, we propose that hydrothermal Mn(IV) oxides may have been effective methane sinks in Earth's ancient oceans.