H-2 is probably the most important substrate for terrestrial subsurface lithoautotrophic microbial communities. Abiotic H-2 generation is an essential component of subsurface ecosystems truly independent of surface photosynthesis. Here we report that H-2 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 H-2 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 H-2 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 H-2 production is a ubiquitous and virtually limitless source of energy for deep crustal chemolithoautotrophic ecosystems.
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