Thermogenic methane and hydrogen generation in subducted sediments of the Nankai Trough

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<jats:title>Abstract</jats:title><jats:p>Active and widespread CH<jats:sub>4</jats:sub> accumulations and emissions in the Nankai Trough subduction zone are attested by numerous mud volcanoes, gas plumes, and gas hydrates containing biogenic and thermogenic CH<jats:sub>4</jats:sub>. However, the source rocks of the thermogenic CH<jats:sub>4</jats:sub> and the geological source of H<jats:sub>2</jats:sub> for microbial CH<jats:sub>4</jats:sub> production by methanogens remain uncertain. Here, we reveal the timing and rate of thermogenic CH<jats:sub>4</jats:sub> and H<jats:sub>2</jats:sub> generation from shales and metapelites associated with oceanic plate subduction in the Nankai Trough by gas and geochemical analyses. The results show that the thermogenic CH<jats:sub>4</jats:sub> and H<jats:sub>2</jats:sub> are generated mainly in the underthrust sediments below the décollement. The sustainable H<jats:sub>2</jats:sub> supply from the underthrust sediments can potentially contribute to microbial CH<jats:sub>4</jats:sub> production. The findings enhance our understanding of the active CH<jats:sub>4</jats:sub> emission, large-scale gas hydrate formation, and subseafloor biosphere in the oceanic plate subduction zone.</jats:p>

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