Abiotic Controls on H2 Production from Basalt−Water Reactions and Implications for Aquifer Biogeochemistry

Abiotic production of H2 from basalt reactions in aqueous solutions is hypothesized to support microbial ecosystems in deep subsurface aquifers, such as those found in the Columbia River Basalt group (CRB). We investigated factors controlling this phenomenon, including rock composition, pH, temperature, sterilization method, reducing agents, and product removal. Ferrous silicate minerals were found to be the basalt components responsible for H2 production from anaerobic water−rock interactions. H2 evolution was faster at pH < 7 but occurred from pH 5 to pH 11, which covers the pH range (7−10) measured in CRB groundwaters. The onset of H2 evolution coincided with the appearance of dissolved Fe2+, and an apparent alkaline inhibition could be alleviated by the addition of excess FeCl2. This may reflect, in part, the low redox conditions needed for H2 evolution and suggests that H2 may be controlled by reaction rates of ferrous silicate minerals. Rates were higher at 60 °C than at 30 °C, suggesting that the geothermal gradient may lead to increased H2 production at depth. The results were consistent with the hypothesis that basalt−redox reactions support primary production by microorganisms in some terrestrial subsurface environments.