When a subseafloor upwelling region at 300 °C is considered, CO2 is soluble up to 3.0 mol kg -1 under the hydrostatic pressure of 50 MPa, whereas the solubility reduces to 0.6 mol kg -1 as the pressure ) the (electro)catalytic reaction of methanethiol with CO2 to form methyl thioacetate, a crucial intermediate for prebiotic carbon fixation. Thus, our experimental results, coupled with the relevant geological evidences, suggest that protometabolism was not a mere aqueous-phase CO2 reduction on a mineral catalyst; it was a consequence of the active hydrothermal circulation beneath the primordial ocean floor (Fig. ) causing multiple geochemical processes including the generation of electric current and the flow of scCO2. It is noteworthy that the density (kg m -3 ) of liquid CO2 exceeds that of seawater below ~3,000 m (68). Thus, the delivery of methanethiol is feasible on seafloor environments shallower than the threshold. Given that the mean ocean depth in the Hadean era exceeded the present level , hydrothermal systems on oceanic plateaus may be more accessible than the ocean ridge systems in this regard.