Radiolytic H₂ in continental crust: Nuclear power for deep subsurface microbial communities

H₂ is probably the most important substrate for terrestrial subsurface lithoautotrophic microbial communities. Abiotic H₂ generation is an essential component of subsurface ecosystems truly independent of surface photosynthesis. Here we report that H₂ concentrations in fracture water collected from deep siliclastic and volcanic rock units in the Witwatersrand Basin, South Africa, ranged up to two molar, a value far greater than observed in shallow aquifers or marine sediments. The high H2 concentrations are consistent with that predicted by radiolytic dissociation of H₂O during radioactive decay of U, Th, and K in the host rock and the observed He concentrations. None of the other known H_2-generating mechanisms can account for such high H₂ abundance either because of the positive free energy imposed by the high H₂ concentration or pH or because of the absence of required mineral phases. The radiolytic H ₂ is consumed by methanogens and abiotic hydrocarbon synthesis. Our calculations indicate that radiolytic H₂ production is a ubiquitous and virtually limitless source of energy for deep crustal chemolithoautotrophic ecosystems.