Fig. 4.

Geoelectrochemical mechanism of ammonia accumulation in the early ocean hydrothermal system. The schematic was depicted based on our experimental results together with the field and laboratory findings on the geoelectrochemical processes in submarine hydrothermal systems reported in the literature (10, 11, 23, 24, 29, 30). Thermodynamic calculations for the ${\mathrm{H}}^{+}$/${\mathrm{H}}_2$ redox potential as a function of pH and temperature (31) (Right) indicate that the electric potential necessary for the $\mathrm{FeS}-\mathrm{to}-{\mathrm{Fe}}^0$ reduction and the subsequent ${\mathrm{NH}}_3$ accumulation ($\le$$-0.6{\mathrm{V}}_{\mathrm{SHE}}$) is readily realizable in ${\mathrm{H}}_2$-rich, alkaline hydrothermal systems that were ubiquitous on the early ocean floor (51).