Anoxic pyrite oxidation by water radiolysis products — A potential source of biosustaining energy

Radiolysis on rocky planetary bodies provides chemical species across redox gradients that can supply energy for microbial life in subsurface environments. We investigated the oxidation of pyrite to aqueous sulfate (SO42−) by water γ-radiolysis products with concomitant production of molecular hydrogen (H2). The production of H2, the only gaseous product recovered at the end of pyrite–water irradiation experiments, was found to be dependent on pyrite/water ratios. The yield of radiolytically-produced SO42− correlated with the total irradiation dose. The effectiveness of γ-radiation in oxidative dissolution of pyrite is determined by (1) redox reactions between radiolytically-produced oxidants and pyrite, and (2) the interaction between γ-radiation and pyrite's crystalline structure. Radiolytic oxidation of reduced sulfur occurs with the oxidants HO● (hydroxyl radical) and Fe3+ (ferric iron) involving two different pathways. The radiolytic production of these two chemical oxidants is self-sustaining in the presence of water and Fe2+ in the system. Radiolytic oxidation can produce significant sulfur isotope effects by preferentially bringing 34S into solution as sulfate and leaving a 32S-enriched elemental sulfur layer on the pyrite surface. Experimental abiotic fractionations of sulfur isotopes between original pyrite and its sulfur oxidation products are significant and indicate that isotopically distinct sulfate is being produced during oxidation. Based on measured radiolysis constants for pyrite and radiation dose estimates for continental crust, we show that radiolysis of water coupled to oxidation of metallic sulfides could be a significant source of sulfate in many geological environments. Implications of this work are broad, impacting our assessment of the potential for life to exist in subsurface environments on Earth as well as in extraterrestrial environments.