Origin of Low Salinity Pore Fluids: Kumano Forearc Mud Volcanoes, Nankai Trough

Several seafloor mud volcanoes have been found in the Kumano forearc basin located in the northern part of the Middle Nankai Trough (Kuramoto, 1998 etc.). From 2000 to 2002, we took five piston core samples (length 1~3.9 m) were retrieved from four of the mud volcanoes. In this study, we report the origin of pore fluids of the Kumano mud volcanoes based on the results of chemical analyses of the pore waters squeezed from these core samples. We found that the pore waters in the Kumano mud volcanoes can be characterized by slight Cl^- depletion relative to seawater. The maximum anomaly is ca. 5%. Besides, the isotopic ratio ratios of the pore fluids become enriched in 18-O and depleted in D with the decrease of Cl^- concentration. Assuming that the isotopic compositions of pore water have been decided by the simple mixing of seawater and freshwater (Cl^-=0), the δ ¹⁸O and δ D values of fresh water can be estimated to be approximately +14 (‰ SMOW) and -41 (‰ SMOW). These values correspond to those of the clay minerals. On the other hand, the ratio of methane to ethane in pore water and stable carbon isotope ratios of methane (C1/C2: <100, δ ¹³C: ca. -60~-30‰ suggest that the thermogenic-originated hydrocarbons mixed in biogenic-originated methane.