Geophysical and geochemical signatures of Gulf of Mexico seafloor brines

Geophysical, temperature, and discrete depth-stratified geochemical data illustrate differences between an actively venting mud volcano and a relatively quiescent brine pool in the Gulf of Mexico along the continental slope. Geophysical data, including laser-line scan mosaics and sub-bottom profiles, document the dynamic nature of both environments. Temperature profiles, obtained by lowering a CTD into the brine fluid, show that the venting brine was at least 10°C warmer than the bottom water. At the brine pool, two thermoclines were observed, one directly below the brine-seawater interface and a second about one meter below the first. At the mud volcano, substantial temperature variability was observed, with the core brine temperature being either slightly (~2°C in 1997) or substantially (19°C in 1998) elevated above bottom water temperature. Geochemical samples were obtained using a device called the "brine trapper" and concentrations of dissolved gases, major ions and nutrients were determined using standard techniques. Both brines contained about four times as much salt as seawater and steep concentration gradients of dissolved ions and nutrients versus brine depth were apparent. Differences in the concentrations of calcium, magnesium and potassium between the two brine fluids suggests that the fluids are derived from different sources or that brine-sediment reactions are more important at the mud volcano than the brine pool. Substantial concentrations of methane and ammonium were observed in both brines, suggesting that fluids expelled from deep ocean brines are important sources of methane and dissolved inorganic nitrogen to the surrounding environment.