ABSTRACT

Pore waters at the depths of 0-590 cm below seafloor (bsf) were collected from four core samples at four different sites in a mud volcano off Tanegashima Island between Ryukyu trench and Ryukyu arc of Japan. Concentrations of Cl^-, SO₄^2-, CH₄, C₂H₆ and stable isotopic composition of δ¹³C_CH4, δ¹⁸O_H2O, δD_H2O in the pore waters vary as a function of distance from seafloor. This paper reports and discusses the pore waters collected at the summit (CV) site. The concentrations of Cl^- decrease from 540 at the seafloor to 375 mmol/kg at a depth of ∼200 cm and remain constant at around 350 mmol/kg (64% of the concentration of seawater) below the depth. The concentrations of CH₄ are two to three orders of magnitude higher than those at other sites and have a maximum value of 715 μmol/kg at around 120-140 cm bsf. Core samples collected at depths deeper than 180 cm bsf show collapsing gas bubbles and empty voids when they were split open. It was also observed that liquid seeped out from the surface of the split core. Considering the physical condition is favorable for the formation of methane hydrate, the observations suggest the existence of methane hydrates. High concentration of C₂H₆, which had similar depth profile to that of CH₄, was also observed. C₂H₆/CH₄ ratio remained larger than 10^-3 and δ¹³C_CH4 also remained around -45‰ below 180 cm bsf. The data suggest presence of thermogenic methane in the CV site. δ¹⁸O_H2O and δD_H2O profiles exhibited an opposite depth dependence, and only δD_H2O showed a decreasing depth profile similar to the concentration profile of Cl^-. They were inversely correlated with the concentration of Cl^-. The data of these two isotope compositions suggest a dilute fluid originates mainly from clay mineral dehydration but meteoric water. A simple mixing model of fluids from three sources (ambient seawater, water dissociated from methane hydrates, and diagenetic water ascending from deeper depth) with isotopic fraction during methane hydrate dissociation was applied for the observation result below 280 cm bsf to constrain ranges of δ¹⁸O_H2O and δD_H2O of diagenetic water. Using the observed depth profile of Cl^- as a conservative component of ambient seawater, contribution of ambient seawater is estimated to be 64% whereas 36% from other two sources. Considering an isotopic fractionation during methane hydrate dissociation and using the estimated source fractions and observed isotopic composition of pore water, δ¹⁸O_H2O and δD_H2O of the diagenetic water were estimated to range from +15 to +22‰ and from -103 to -43‰, respectively, which are in good agreement with isotopic compositions of water formed from clay minerals during their dehydration but quite different from those of meteoric water, supporting negligible contribution of meteoric water in the Tanegashima mud volcano fluid.