Upward advection rates and methane fluxes, oxidation, and sources at two Gulf of Mexico brine seeps

Pore water chloride, sulfate, and methane profiles were measured at two Gulf of Mexico brine seeps to determine upward fluid advection rates and to quantify the advective methane flux from these sediments. We also measured δ13C–CH4 values to constrain the source of methane and determine whether anaerobic methane oxidation can occur at high chloride concentrations. Chloride concentrations were near seawater values at the sediment–water interface and reached as high as 4500 mM by 25 cm below seafloor (cmbsf). Advection rates were determined by a 1-D advection–diffusion model. Sulfate concentrations were near seawater values of 28 mM at the sediment–water interface and quickly decreased to ~ 1 mM by the bottom of the cores. The brine fluids were also charged with CH4 enriched water, reaching up to 3 mM, compared to background cores and could be an important source of methane to the overlying water column. At one site, dissolved methane had an average δ13C value of − 54 ± 0.2‰ and − 49 ± 6‰. Our main conclusions are: 1) advection rates are heterogeneous and range from 7 to 65 cm/year, 2) assuming brine fluids are devoid of sulfate, the sulfate depletion depth in these sediments is controlled by brine advection, 3) methane contained in the brine fluid has either a mixed biogenic/thermogenic source or a biogenic source that has been oxidized, and 4) the advective flux of methane from these brine seeps ranges from 2 to 58 mol m− 2 y− 1.