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