Supercritical CO2 as the carbon source and reaction medium for methanethiol synthesis in primordial subseafloor

Abiotic synthesis of methanethiol through CO2 reduction is a presumptive initiation reaction of protometabolism in primordial ocean hydrothermal systems. However, reported artificial means of methanethiol production indicate the necessity of gas-phase condition, rather than aqueous-phase setting, for this prebiotic reaction to occur. Here we demonstrate experimentally that this dilemma was mitigated by subseafloor supercritical CO2. Under supercritical CO2 with a geochemically feasible concentration of hydrogen, up to 7.9% conversion of hydrogen sulfide to methanethiol was achieved at 200 °C on molybdenum sulfide catalyst deposited in a carbonate-NaCl solution. Effective formations of carbon monoxide, methane, and formate were also observed. On the present ocean floor, discharges of liquid and supercritical CO2 occasionally occur in close vicinity to hydrothermal vents. Geological records of ancient seafloor suggest prevalence of such CO2 fluxes associated with hydrothermal activities. Our experimental results link these facts with the protometabolism scenario, leading to the possibility that the generation and transportation of methanethiol through the subseafloor CO2 fluxes constituted the beginning steps of protometabolism in primordial ocean hydrothermal systems. Thus, the supply of supercritical-CO2-derived materials likely directed the chemical evolution of life in combination with the known geochemical processes at the primordial ocean-vent interface.