Pore-water geochemistry of two contrasting brine-charged seep sites in the northern Gulf of Mexico continental slope

Two brine-charged sites (GB425 and GC233) at the northern Gulf of Mexico (GOM) continental slope were studied for their pore-water advective flow rates and stable carbon isotope geochemistry. Using chloride as a conservative tracer, a pore-water diffusion–advection model suggests that advection dominates pore-water flow in the upper 17 cm sediments at both sites. Moreover, modeled results indicate that there is a downward pore-water flow in GB425 and an upward one in GC233, presumably caused by nearby brine-seep activities. Profiles of pore-water solutes (dissolved inorganic carbon, or DIC, sulfate, and ammonium) can be largely explained by conservative mixing between bottom water and underlying brines, which contain dissolved species that are produced at or below a subsurface reaction front, where sulfate reduction coupled with organic carbon oxidation occurs. Stable carbon isotope data indicate that organic carbon being remineralized has a similar δ13C value (− 40 to − 60‰) as that of thermogenic and biogenic methane found in this area. However, without further evidence, the exact carbon source and reaction pathway remain unclear.