Bacteria sulfate reduction is a major pathway for organic carbon oxidation in marine sediments. Upward diffusion of methane from gas hydrate deep in the sedimentary strata might be another important source of carbon for sulfate reducing bacteria and subsequently induce higher rates of sulfate reduction in sediments. Since abundant gas may migrate upward to the surface as a result of tectonic activity occurring in the accretionary wedge, this study investigates the effect of methane migration on the sulfate reduction process in continental margin sediments offshore southwestern Taiwan. Piston and gravity core samples were taken in order to evaluate vertical and spatial variations of sulfate and methane. Pore water sulfate, sulfide, methane, sediment pyrite, and organic carbon were extracted and analyzed.
Rapid sulfate reduction, high pyrite contents in sediments and higher concentrations of dissolved sulfide and methane were found in the study area with relatively low organic carbon concentrations. Up to 300 μmole g-1 of pyrite, 10 mM dissolved sulfide and 9 mM methane were found in the study region. Sulfate depletion was as shallow as ~1 m beneath the sediment/water interface. At similar water depths, however, low methane and pyrite were found in some areas with almost no sulfate reduction.
Sulfate flux in the study region is among the highest reported. Large spatial variations were a result of high methane flux, probably originating from gas hydrate deeper in sediments. Co-existence of rapid sulfate reduction and high methane concentrations in sediments indicated that methane is controlling sulfate reduction in the region. Downward diffusion of sulfate is linearly correlated with upward methane flux in the Good Weather Ridge region. Methane becomes an important source of carbon for the sulfate reducer and subsequently controlling sulfate reduction and pyrite formation.