Deep-biosphere methane production stimulated by geofluids in the Nankai accretionary complex

Microbial life inhabiting subseafloor sediments plays an important role in Earth’s carbon cycle. However, the impact of geodynamicprocesses on the distributions and carbon-cyclingactivities of subseafloor life remains poorly constrained. We explore a submarine mud volcanoof the Nankai accretionary complexby drilling downto 200 m below the summit. Stable isotopic compositions of water and carbon compounds, including clumped methane isotopologues, suggest that ~90% of methane is microbially produced at 16° to 30°C and 300 to 900 m below seafloor, corresponding to the basin bottom, where fluids in the accretionary prism are supplied via megasplay faults. Radiotracer experiments showed that relatively small microbial populations in deep mud volcanosediments (10^2 to 10^3 cells cm^(−3)) include highly active hydrogenotrophic methanogensand acetogens. Our findings indicate that subduction-associated fluid migration has stimulated microbial activity in the mud reservoir and that mud volcanoesmay contribute more substantially to the methane budget than previously estimated.