The vertical chloride ion profile at the IODP Site C0002, Kumano Basin, off coast of Japan

The vertical profiles of chloride ion (Cl−) concentration and oxygen and hydrogen isotopic ratios in pore fluids of basin-fill sediments in the southern Kumano Basin were examined. A model has been proposed to explain those profiles in conjunction with published data by invoking diffusion and advection of freshwater from methane hydrate dissolution. The model requires a low initial Cl− concentration in the old accretionary prism below the basin before the basin began to fill with sediments. It also requires little or no methane hydrate formation in the old accretionary prism. Given these requirements, an upward fluid advection rate of 1 × 10−12 m/s has been estimated which is very low compared to other accretionary prisms. Our data suggests that the tectonic compaction of the basin-filling sediments and fault activity under the basin sediments have not been that active recently in the Kumano Basin.