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

Abstract

Active and widespread CH4 accumulations and emissions in the Nankai Trough subduction zone are attested by numerous mud volcanoes, gas plumes, and gas hydrates containing biogenic and thermogenic CH4. However, the source rocks of the thermogenic CH4 and the geological source of H2 for microbial CH4 production by methanogens remain uncertain. Here, we reveal the timing and rate of thermogenic CH4 and H2 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 CH4 and H2 are generated mainly in the underthrust sediments below the décollement. The sustainable H2 supply from the underthrust sediments can potentially contribute to microbial CH4 production. The findings enhance our understanding of the active CH4 emission, large-scale gas hydrate formation, and subseafloor biosphere in the oceanic plate subduction zone.