Abstract
Geological sources of H2 and abiotic CH4 have had a critical role in the evolution of our planet and the development of life and sustainability of the deep subsurface biosphere. Yet the origins of these sources are largely unconstrained. Hydration of mantle rocks, or serpentinization, is widely recognized to produce H2 and favour the abiotic genesis of CH4 in shallow settings. However, deeper sources of H2 and abiotic CH4 are missing from current models, which mainly invoke more oxidized fluids at convergent margins. Here we combine data from exhumed subduction zone high-pressure rocks and thermodynamic modelling to show that deep serpentinization (40–80 km) generates significant amounts of H2 and abiotic CH4, as well as H2S and NH3. Our results suggest that subduction, worldwide, hosts large sources of deep H2 and abiotic CH4, potentially providing energy to the overlying subsurface biosphere in the forearc regions of convergent margins.
Highlights
Geological sources of H2 and abiotic CH4 have had a critical role in the evolution of our planet and the development of life and sustainability of the deep subsurface biosphere
Geochemical data from forearc mud volcanos and hydrothermal seeps suggest that life exists as deep as 15 km below the surface at convergent margins[14,17,18], and that the essential carbon to sustain deep microbiological habitats in the forearc of convergent plate margins is provided by the metamorphic recycling of subducting slabs[2]
As serpentinization affects the mantle wedge above subducting slabs[24,30], our results suggest that subduction zones may represent large source regions of H2 and abiotic CH4 on Earth, with important consequences for the mobility of deep C and the genesis of high-pressure sources of energy
Summary
Geological sources of H2 and abiotic CH4 have had a critical role in the evolution of our planet and the development of life and sustainability of the deep subsurface biosphere. Thanks to the anomalous geothermal regimes of subduction zones which stabilize serpentine minerals to depths of ~100 km[21]; serpentinization of deep-seated mantle rocks may represent a suitable environment for the genesis and migration of deep H2 and abiotic CH4 The delivery of these gases to the subsurface biosphere from the deep Earth could dramatically change our understanding of deep carbon cycling at convergent margins and the distribution and magnitude of deep life on Earth[1,22], and potentially other planetary bodies. As serpentinization affects the mantle wedge above subducting slabs[24,30], our results suggest that subduction zones may represent large source regions of H2 and abiotic CH4 on Earth, with important consequences for the mobility of deep C and the genesis of high-pressure sources of energy. In addition to H2 and CH4, our data show that other strongly reduced compounds such as H2S and NH3 can form in deep serpentinization fluids, a result that has implications for the diversity and distribution of deep subsurface communities at convergent margins
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