Abiotic controls on H-2 production from basalt-water reactions and implications for aquifer biogeochemistry

Abiotic production of H-2 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 H-2 production from anaerobic water-rock interactions. H-2 evolution was faster at pH < 7 hut occurred from pH 5 to pH 11, which covers the pH range(7-10)measured in CRB groundwaters. The onset of H-2 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 H-2 evolution and suggests that H-2 may be controlled by reaction rates of ferrous silicate minerals. Rates were higher at 60 degrees C than at 30 degrees C, suggesting that the geothermal gradient may lead to increased H-2 production at depth. The results were consistent with the hypothesis that basalt-redox reactions support primary production by microorganisms in some terrestrial subsurface environments.