Geochemical constraints on the origin of the pore fluids and gas hydrate distribution at Atwater Valley and Keathley Canyon, northern Gulf of Mexico

Abstract

Pore fluids from Atwater Valley (AT 13/14) and Keathley Canyon (KC 151) in the northern Gulf of Mexico are surprisingly similar with respect to ionic concentrations and oxygen and strontium isotope values, as well as hydrocarbon geochemistry, suggesting that these widely separated localities share common deep subsurface fluid origins. Seafloor mounds with focused fluid migration pathways and inferred near-seafloor gas hydrates characterize the AT 13/14 region, whereas the KC 151 region has a bottom simulating reflector (BSR) at ∼310 mbsf, which is rather uncommon in the Gulf of Mexico (GOM). At these sites seafloor gas hydrates were not observed but the sediment surface in the vicinity and particularly at the mounds is populated with chemosynthetic communities that are commonly associated with seafloor gas emission. The geochemical results, together with the pressure core data, suggest that at the AT region methane hydrate mostly occurs in near-surface sediments at mounds, consistent with focused migration pathways. In the KC region methane hydrate mostly occurs deeper in the section, in highly fractured silty-clayey sediments from ∼220 to 300 mbsf. The pore fluids at the AT mounds and KC 151 are characterized by higher than seawater salinity. The more saline pore fluids at the AT mound and at KC151 sites, located ∼350 km apart, are almost chemically indistinct. Ionic ratios indicate that this distinct high salinity fluid is not from in situ salt dome halite dissolution. Rather, this fluid is a subsurface brine derived from Jurassic or Cenozoic evaporite formation, modified by fluid-sediment reactions, and migrated to the two sites analyzed. Despite porewater salinities elevated above that of seawater, the sediment temperatures are within the range of methane hydrate stability for each of the sites. Based on Cl − dilutions the maximum gas hydrate pore volume occupancy at the AT mound sites would be ∼9%. At KC, Cl − concentrations in pressure cores imply that in situ hydrate is unevenly distributed, with pore volume occupancy of 1–12%. Significant variations in sulfate gradients were observed, with the sulfate-to-methane transition zone (SMTZ) at or near the seafloor at the AT mound sites. At AT 13#2 the well-defined SMTZ is at ∼8 mbsf, and at KC 151#3 it is at ∼9 mbsf. There is no coincidence between the steepness of the sulfate gradients and the presence or depth of a BSR, suggesting that the SMTZ interfaces are measuring different aspects of the subsurface methane hydrology. At both AT and KC the δ 13C-DIC values clearly indicate that anaerobic oxidation of methane (AOM) is the dominant reaction responsible for sulfate reduction and the increased alkalinities observed. The most negative δ 13C-DIC values obtained are −46.3‰ and −49.6‰ at the SMTZs at AT 13#2 and KC 151#3, respectively.