Sources of Low-Chloride Fluids in Sediments beneath Biological Communities on the Nankai Accretionary Wedge off Kumano

We report chemical and isotopic compositions of interstitial waters extracted from surface sediments inside and outside of dense biological communities on the seafloor of the Nankai accretionary prism off Kumano. Particularly those taken within bacterial mat at the Oomine Ridge site show the following characteristics: (1) CH₄ enrichment (more than 600 μ mol/kg), (2) chloride depletion (up to 10% depletion from bottom seawater), and (3) δ D_H2O and δ ¹⁸O_H2O depletion (more than 4‰ and 0.7‰ depletion, respectively), compared with bottom seawater. The highest CH₄ value observed at the Oomine Ridge samples is compatible with that previously reported at one of the most active seep areas within Nankai Trough, suggesting that also this site should be one of the most active seep sites in Nankai Trough. The fluid chemistries of these samples show that the fluids of CH₄ enrichment, chloride depletion, and low δ D_H2O and δ ¹⁸O_H2O value relative to bottom seawater are ascending from deeper zone to sediments beneath the Oomine Ridge site. Assuming simple two-component mixing of ambient seawater with fresh water of Cl^- = 0 mmol/kg, the estimated end-member δ D_H2O and δ ¹⁸O_H2O values were -46+/-7‰ SMOW and -6.3+/-0.7‰ SMOW, respectively. The δ D_H2O and δ ¹⁸O_H2O values coincide with those of groundwater at nearby land area. Land-derived groundwater could be one of the possible sources for the low chloride fluids, while we donOt have any idea for such underground lateral flow system of groundwater to the depth of ca. 2,500 m and ca. 100 km off the Japan Islands. Alternative possible source to produce the fluid of low Cl^-, δ D_H2O and δ ¹⁸O_H2O is phase separation of seawater. Phase separation would lower Cl^-, δ D_H2O and δ ¹⁸O_H2O of vapor phase by a fractionation process during boiling, while we have no idea for a heat source to induce phase separation at the area. Fault slips during large earthquake could possibly be result in such source of heat.