A seawater throttle on H production in Precambrian serpentinizing systems.

Since the initial discovery of low-temperature alkaline hydrothermal vents off the Mid-Atlantic Ridge axis nearly 20 y ago, the observation that serpentinizing systems produce abundant H has strongly influenced models of atmospheric evolution and geological scenarios for the origin of life. Nevertheless, the principal mechanisms that generate H in these systems, and how secular changes in seawater composition may have modified serpentinization-driven H fluxes, remain poorly constrained. Here, we demonstrate that the dominant mechanism for H production during low-temperature serpentinization is directly related to a Si deficiency in the serpentine structure, which itself is caused by low SiO(aq) concentrations in serpentinizing fluids derived from modern seawater. Geochemical calculations explicitly incorporating this mechanism illustrate that H production is directly proportional to both the SiO(aq) concentration and temperature of serpentinization. These results imply that, before the emergence of silica-secreting organisms, elevated SiO(aq) concentrations in Precambrian seawater would have generated serpentinites that produced up to two orders of magnitude less H than their modern counterparts, consistent with Fe-oxidation states measured on ancient igneous rocks. A mechanistic link between the marine Si cycle and off-axis H production requires a reevaluation of the processes that supplied H to prebiotic and early microbial systems, as well as those that balanced ocean-atmosphere redox through time.