Abstract
The discovery of hydrogen-rich waters preserved below the Earth's surface in Precambrian rocks worldwide expands our understanding of the habitability of the terrestrial subsurface. Many deep microbial ecosystems in these waters survive by coupling hydrogen oxidation to sulfate reduction. Hydrogen originates from water–rock reactions including serpentinization and radiolytic decomposition of water induced by decay of radioactive elements in the host rocks. The origin of dissolved sulfate, however, remains unknown. Here we report, from anoxic saline fracture waters ∼2.4 km below surface in the Canadian Shield, a sulfur mass-independent fractionation signal in dissolved sulfate. We demonstrate that this sulfate most likely originates from oxidation of sulfide minerals in the Archaean host rocks through the action of dissolved oxidants (for example, HO· and H2O2) themselves derived from radiolysis of water, thereby providing a coherent long-term mechanism capable of supplying both an essential electron donor (H2) and a complementary acceptor (sulfate) for the deep biosphere.
Highlights
The discovery of hydrogen-rich waters preserved below the Earth’s surface in Precambrian rocks worldwide expands our understanding of the habitability of the terrestrial subsurface
Because the water radiolysis is driven by decay of radioactive elements in the host rocks, our results indicate that the fracture waters themselves, by interacting with their host rocks in the Precambrian cratons, can provide both an electron acceptor and a complementary electron donor (H2) that in turn could support microbial ecosystems in these isolated subsurface fracture systems
The D33S and D36S values of dissolved sulfate varied from borehole to borehole but remained nearly constant for a given borehole over the course of this study
Summary
The discovery of hydrogen-rich waters preserved below the Earth’s surface in Precambrian rocks worldwide expands our understanding of the habitability of the terrestrial subsurface. From anoxic saline fracture waters B2.4 km below surface in the Canadian Shield, a sulfur mass-independent fractionation signal in dissolved sulfate. We demonstrate that this sulfate most likely originates from oxidation of sulfide minerals in the Archaean host rocks through the action of dissolved oxidants (for example, HO and H2O2) themselves derived from radiolysis of water, thereby providing a coherent long-term mechanism capable of supplying both an essential electron donor (H2) and a complementary acceptor (sulfate) for the deep biosphere. No major hydrothermal events are thought to have occurred in the Kidd Creek area after 2.60 Ga (ref. 20)
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