DETERMINATION OF SEDIMENTATION RATES IN GULF OF MEXICO GAS HYDRATE-BEARING SEDIMENTS USING RADIOCARBON AND STABLE CARBON ISOTOPES

Sedimentation rates at bathyal depths are generally poorly known and in gas hydrate-bearing sediments of the Gulf of Mexico (GOM) continental slope they are essentially unknown. In normal deep sea sediments, the carbonates are dominated by a pelagic component consisting of foraminifera calcite tests, whereas hydrate-bearing sediments on the Louisiana continental slope contain up to 70% authigenic carbonate which forms as a result of bacterially-mediated coupled anaerobic hydrocarbon oxidation/sulfate reduction reactions. The different hydrocarbons consumed in these processes imprint distinctive stable carbon isotope signatures on the resulting carbonate phases which provide insight into the different microbial processes occurring in the sediments. In this study, we use coupled radiocarbon-stable carbon isotopes to determine sediment accretion rates in Gulf of Mexico deepwater hydrate-bearing sediments, carbon sources in authigenic seep carbonates, and fluxes of hydrocarbon-rich fluids into the seep sediments.

Gas hydrates in the Green Canyon area of the GOM outcrop on the seafloor at 600 m water depth at a temperature of approximately 7 degrees C. Push cores (~ 25 cm long, collected with a submersible) were taken through Beggiatoa spp. bacterial mats on hydrate mounds in Green Canyon lease blocks 185, 232, and 272. The presence of gas hydrates at the sample sites is evidenced by visual observations and from the geochemistry of pore fluids.

Paired samples of deep-sea gastropod shells and carbonate concretions were taken at the top, middle, and base of 6 sediment cores. Carbonate concretions have anomalously depleted D¹⁴C values ranging from –704 to –910 per mil, the result of their incorporation of ¹⁴C free carbon from hydrocarbons. The shells show D¹⁴C values ranging from –96 to –735 per mil, which are dependant on the shells’ age and amount of authigenic carbonate overgrowth. Correction of the shell ages for carbonate overgrowth allows for the determination of true sediment ages and sedimentation rates which range from 12.4-27.1 cm/ka. Mass balance calculations show that the carbonates contain 60-90% “dead” carbon with d¹³C of approximately –26 per mil, PDB, which suggests a carbon source in crude oil or higher-weight (ethane though pentane) hydrate forming gases instead of more isotopically depleted methane.