The contribution of the Precambrian continental lithosphere to global H₂ production

Microbial ecosystems can be sustained by hydrogen gas (H₂)-producing water-rock interactions in the Earth's subsurface and at deep ocean vents^1-4. Current estimates of globalH₂ production from the marine lithosphere by water-rock reactions (hydration) are in the range of 1011 moles per year^5-9. Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H² as hydrothermal vents and seafloorspreading centres^1,2 and have suggested a link between dissolved H₂ and the radiolytic dissociation ofwater^10,11.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.