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

Active and widespread CH 4 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 4 . However, the source rocks of the thermogenic CH 4 and the geological source of H 2 for microbial CH 4 production by methanogens remain uncertain. Here, we reveal the timing and rate of thermogenic CH 4 and H 2 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 4 and H 2 are generated mainly in the underthrust sediments below the décollement. The sustainable H 2 supply from the underthrust sediments can potentially contribute to microbial CH 4 production. The findings enhance our understanding of the active CH 4 emission, large-scale gas hydrate formation, and subseafloor biosphere in the oceanic plate subduction zone.