Water samples collected by the French submersible Nautile from macrofauna flourishing in cold seepage zones of the eastern Nankai Trough accretionary prism were analyzed for major chemical components on board Nadir and in shore-based laboratories in order to characterize pore fluid chemistries. Extremely high CH4 concentrations of up to 4.01 μM confirmed the active supply of CH4-rich fluids from the bottom. The CH4 C2H6 ratio ranged from 113 to 3875, suggesting a biogenic rather than thermogenic origin of the hydrocarbons. It was inferred that the oxidation of biogenic CH4 occurs in surface sediments at a rate that is approximately twice as fast as that of C2H6, based on the positive correlation between CH4 concentrations and ( CH4 C2H6) ratios. The existence of NH4+ (10-50 μM) and high alkalinity values ( > 3 mM) in some water samples were indicative of microbial organic matter decomposition (sulfate reduction and/or methane generation) in the sediments. By extrapolating the ΣCO2-δ13C(ΣCO2) relationship, δ13C(ΣCO2) of an unknown fluid end member was estimated to range from -30 to -47‰, indicating a contribution of CH4-derived CO2 with low δ13C values.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science