Microbial ecosystems can be sustained by hydrogen gas (H2)-producing water-rock interactions in the Earth's subsurface and at deep ocean vents1-4. Current estimates of globalH2 production from the marine lithosphere by water-rock reactions (hydration) are in the range of 1011 moles per year5-9. Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H2 as hydrothermal vents and seafloorspreading centres1,2 and have suggested a link between dissolved H2 and the radiolytic dissociation ofwater10,11.However, extrapolation of a regionalH2 flux based on the deep gold mines of the Witwatersrand basin in South Africa11 yields a contribution of the Precambrian lithosphere to global H2 production that was thought to be negligible (0.0093×1011moles per year)6. Here we present a global compilation of published and new H2 concentration data obtained fromPrecambrian rocks and find that theH2 production potential of the Precambrian continental lithosphere has been underestimated. We suggest that this can be explained by a lack of consideration of additional H2-producing reactions, such as serpentinization, and the absence of appropriate scaling ofH2measurements fromthese environments to account for the fact that Precambrian crust represents over 70 per cent of global continental crust surface area12. If H2 production via both radiolysis and hydration reactions is taken into account, ourestimate of H2 production rates from the Precambrian continental lithosphere of 0.36-2.273×1011 moles per year is comparable to estimates from marine systems.
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