Sustainable thermogenic CH4 and H2 generation in the Nankai Trough subduction zone

Abstract Mud volcanoes, gas plumes, and gas hydrates comprising thermogenic and biogenic CH4 are widely distributed in the Nankai Trough subduction zone, showing ongoing significant CH4 activity. However, the source rocks of the thermogenic CH4 and the geological source of H2 for microbial CH4 production remain uncertain. Here, we reveal the timing and amount of the thermogenic CH4 and H2 generation in shales and metapelites during diagenesis to metamorphism and estimate their current generation in the Nankai Trough from the movements of the oceanic plate and the accretionary prisms. 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 be another potential H2 contributing 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.