Fluid Dynamics along the Nankai Trough: He Isotopes Reveal Direct Seafloor Mantle-Fluid Emission in the Kumano Basin (Southwest Japan)

We collected bottom water samples and unconsolidated sediment samples for noble-gas analysis in the water column as well as in the pore water phase along the Nankai Trough to assess the sources and fluxes of seafloor fluid emission at the local/regional scale. The results indicate that terrigenic He emission is rather variable in terms of fluxes and isotope signatures. The largest He fluxes are detected at a previously investigated cold seep site where terrigenic He is mainly of crustal origin. High He isotope ratios detected at shallower depths in the Kumano Basin indicate that mantle fluids are able to migrate from the deep sediment column up to the seafloor. Our findings are in line with the detection of mantle He in fluids recovered by IODP (International Ocean Discovery Program) deep-drilling operations and make the case that even seafloor volatile screening and monitoring might provide useful insights on the relation between fluid dynamics and tectonic/seismic activity in the investigated region for future forecasting of earthquakes.