Thermochemical oxidation of methane induced by high-valence metal oxides in a sedimentary basin

Thermochemical oxidation of methane (TOM) by high-valence metal oxides in geological systems and its potential role as a methane sink remain poorly understood. Here we present evidence of TOM induced by high-valence metal oxides in the Junggar Basin, located in northwestern China. During diagenesis, methane from deeper source strata is abiotically oxidized by high-valence Mn(Fe) oxides at 90 to 135 °C, releasing (13)C-depleted CO(2), soluble Mn(2+) and Fe(2+). Mn generally plays the dominant role compared to Fe, due to its lower Gibbs free energy increment during oxidation. Both CO(2) and metal ions are then incorporated into authigenic calcites, which are characterized by extremely negative δ(13)C values (−70 to −22.5‰) and high Mn content (average MnO = 5 wt.%). We estimate that as much as 1224 Tg of methane could be oxidized in the study area. TOM is unfavorable for gas accumulation but may act as a major methane sink in the deep crustal carbon cycle.