Analysis of methane and sulfate flux in methane-charged sediments from the Mississippi Canyon, Gulf of Mexico

Abstract Sediment porewater geochemical data (SO4−2, CH4, DIC, δ13C-DIC and Cl−) were obtained from piston cores collected in Atwater Valley, Gulf of Mexico, prior to 2005 drilling to study gas hydrates in the region. The geochemical data were used for a study of shallow sediment CH4 cycling on a seafloor mound (mound F) where an apparent upward deflection of the bottom simulating reflector (BSR) suggested vertical fluid advection. Fifteen sediment cores, ranging from 300 to 800 cm long, were collected from locations on top of the mound and across a transect up to 3.5 km off the mound. The sulfate–methane transition (SMT) was determined in each core from porewater SO4−2 and CH4 concentration profiles and occurred at depths ranging from 0 to 410 cm below the seafloor (cmbsf). The shape of porewater SO4−2 profiles plotted against depth also varied from linear to non-linear along the transect. Diffusion rates estimated from linear SO4−2 concentration gradients ranged from −20.4 to −249.1 mmol m−2 a−1 with the greatest rate measured in sediments on the mound. The large variation in SMT depth and SO4−2 profiles along the transect indicates lateral differences in total vertical CH4 flux between locations. Results suggest steady-state and non-steady-state CH4 fluxes both on the mound and transitioning off the mound and likely differences in the relative contribution of fluid advection to local shallow sediment CH4 cycling. Cores collected from on the mound had high porewater headspace CH4 concentrations (up to 8.34 mM) coupled with elevated Cl− concentrations (up to 956.5 mM) at shallow depths suggesting that salt diapirism in deep sediments may be inhibiting hydrate stability and increasing vertical CH4 flux.

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