Circulation of water at moderate depths in subduction zones is dominantly driven by clay mineral dehydration over distinct pressure and temperature gradients. The signature of these dehydration reactions is found in mud volcano pore waters, however, it is largely unknown, how much of the deep-seated fluids are emitted at mud volcanoes. To unravel this relation for the region off the Kii Peninsula, Japan, we calculated the water volume that is subducted in the Nankai Trough using input data from IODP holes C0011 and C0012 and the correspondent water volume released from the subducted plate under the Kumano Basin, in an area where 13 mud volcanoes are located. According to our model, water released at depth in the mud volcano area is derived almost entirely from basaltic saponite and sedimentary smectite transformation (up to 96%). Nonetheless, the mud volcanoes themselves expel ≪1% of the total volume. To test the contribution of the accreted strata and the Kumano Basin fill to the water budget, we run a second model. Water loss due to compaction of sediments and smectite-illite transition below the basin floor have been calculated. The results were compared with salinity measurements on background cores scattered in the study area to extrapolate the volume of water loss at depth. The comparison of the two methods yielded similar results and led us to conclude that the bulk part of the deep-seated fluid re-enters the hydrosphere via the basin floor, a mechanism rarely taken into account in fluid budgets in the literature.
