for methanethiol synthesis (SI Appendix, Fig. ). Given the thermodynamic equilibrium between mackinawite and siderite (FeCO3(s) + H2S(aq) ⇌ FeS(s) + CO2(aq) + H2O(l)), mackinawite is more stable than siderite when the activity ratio of H2S/CO2 exceeds 0.0064 under our experimental condition (SI Appendix, Fig. ). Thus, unless the dissolved CO2 is extremely concentrated, several to dozens of mmol kg -1 of H2S, which are typical H2S concentrations in fluids from submarine hydrothermal systems , are sufficient for mackinawite precipitation and the subsequent H2 generation.