Detection of low-chloride fluids beneath a cold seep field on the Nankai accretionary wedge off Kumano, south of Japan

Abstract

Chemical and isotopic characteristics were determined for interstitial waters extracted from surface sediments in and around dense biological communities on the seafloor of the Nankai accretionary prism off Kumano, south of Japan. We found the following unique features when compared with usual interstitial water samples of normal seafloor in those of samples from bacterial mats on the Oomine Ridge, one of the outer ridge in the Nankai accretionary prism: (1) significant depletion of chloride concentration (maximum 10% depletion from bottom seawater), (2) high concentrations of CH4 and ΣCO2 (more than 660 μmol/kg and 60 mmol/kg, respectively), (3) sulfate depletion (more than 90% depletion compared to bottom seawater), and (4) δDH2O and δ18OH2O depletion [more than 4‰ and 0.7‰ depletion, respectively, compared to standard mean ocean water (SMOW)]. The highest CH4 value among these samples was comparable to the highest value so far reported at one of the most active seep areas in the Nankai Trough, suggesting that these sites should also be regarded as one of the most active seep sites in the Nankai Trough. The chemical compositions of the samples taken from the Oomine Ridge strongly suggest that the fluid originates not from normal sediment–seawater interaction at the sediment surface of hemipelagic environments, but from active seepage of fluids that are rich in CH4 and ΣCO2, depleted in Cl− and SO42−, and low in δDH2O and δ18OH2O compared to normal seawater. Values for the carbon isotopic composition (δ13CCH4) of the dissolved methane in the interstitial fluid [less than −70‰ PeeDee Belemnite (PDB)] and for the C2H6/CH4 ratio (less than 10−3) suggest that the methane originates from microbial production in a relatively shallow layer of sediment, not from the deep sedimentary layer of higher temperature than 60 °C at the depth of more than 300 m below the seafloor. The Cl−=0 mmol/kg extrapolated end-member δDH2O and δ18OH2O values of low-chloride fluids were −46±7‰ and −6.3±0.7‰ SMOW, respectively, suggesting that land-derived groundwater could be one of the possible sources for the low-Cl− fluids. Depth profiles of chloride concentrations of interstitial fluids show the heterogeneity of end members and upward fluid flow velocities suggest that active fluid seepage on the Oomine Ridge seems to be a localized phenomenon. Assuming steady-state emission of fluid from the cold seep vent, upward fluid flow velocities from the seeping vent are estimated to be 40–200 cm year−1, comparable to the previously reported values within the bacterial mats in the Nankai Trough accretionary wedge. Development of bacterial mat might favor slower advection, which might allow longer time for diagenetic reactions in the vent conduits, and consequently, carry more reductive compounds in the fluids.