PORE FLUIDS AND AUTHIGENIC CARBONATE PRODUCTS FROM BIOGENIC AND THERMOGENIC SOURCED GAS HYDRATES IN THE GULF OF MEXICO

The Gulf of Mexico (GOM) slope is a fitting place for the study of extant gas hydrates because of their abundance, variable stability, and varying composition. We have initiated a study of hydrate-bearing sediments in the GOM in order to test the relation between biogenic and thermogenic-derived hydrates, pore fluids and carbonate products in order to examine the first-order effects of gas hydrate dissociation on host sediments.

Gas hydrates in the Green Canyon area of the GOM (lease block 272, 24º41’N;91º32’W) outcrop on the seafloor at 700 m water depth at an ambient temperature of 7º C. Paired push cores (~25 cm long, collected with a submersible robot arm) for pore fluid and sediment study were taken through Beggiatoa spp. bacterial mats in areas suspected of containing clathrates. The presence of hydrates in sediments from two proximal sites (Sites I and II, 300 m apart), is evidenced by visual observations and the geochemistry of pore fluids which show the effects of hydrate dissociation on Cl^- concentrations and oxygen isotopes. Site I pore fluids have near-seawater salinities and show strong bacterial sulfate reduction. The sediments are composed of fine-grained hemipelagic clays and authigenic carbonates (up to 40% carbonate) composed of centimeter-size, buff colored high-Mg calcite and aragonite pelloids with no traces of crude oil. The d¹³C of the pore fluid dissolved inorganic carbon (DIC) is between -43 and -52 ‰ PDB, which attests to a biogenic source for the gas hydrate. Site II sediments and pore fluids have the same general characteristics as the biogenic gas sediments from Site I, but the d¹³C of the pore fluid DIC is -35 ‰ PDB, which suggests the presence of a thermogenic source for those hydrates. Stable carbon and radiocarbon isotope assays of pore fluids and authigenic carbonate pelloids further substantiate our interpretations concerning the methane hydrate sources.