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 H2 has strongly influenced models of atmospheric evolution and geological scenarios for the origin of life. Nevertheless, the principal mechanisms that generate H2 in these systems, and how secular changes in seawater composition may have modified serpentinization-driven H2 fluxes, remain poorly constrained. Here, we demonstrate that the dominant mechanism for H2 production during low-temperature serpentinization is directly related to a Si deficiency in the serpentine structure, which itself is caused by low SiO2(aq) concentrations in serpentinizing fluids derived from modern seawater. Geochemical calculations explicitly incorporating this mechanism illustrate that H2 production is directly proportional to both the SiO2(aq) concentration and temperature of serpentinization. These results imply that, before the emergence of silica-secreting organisms, elevated SiO2(aq) concentrations in Precambrian seawater would have generated serpentinites that produced up to two orders of magnitude less H2 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 H2 production requires a reevaluation of the processes that supplied H2 to prebiotic and early microbial systems, as well as those that balanced ocean–atmosphere redox through time.
|Original language||English (US)|
|Number of pages||8|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - Jun 30 2020|
Bibliographical noteFunding Information:
ACKNOWLEDGMENTS. B.M.T. acknowledges support for this paper and related research from the Natural Sciences and Engineering Research Council of Canada through Discovery Grant RGPIN-2018-03800. W.E.S. acknowledges support from the NSF under Grant OCE-1736679. Dr. Phillipe Blanc and Dr. Philippe Vieillard are thanked for their assistance in the production of thermodynamic data for the serpentine minerals. This manuscript benefited greatly from the insightful comments of two anonymous reviewers.
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- Atmosphere redox state
- Origins of life