Carbonate records of submarine hydrocarbon venting: northern Gulf of Mexico

Hydrocarbon seep sediments are examined from 3 sites in the Gulf of Mexico (Green Canyon Lease Blocks 232, 185, and 272) to relate the effects of gas hydrates and hydrocarbons on microbial process, pore fluid chemistry, and the processes of authigenic carbonate formation. Hydrocarbon-fueled microbial reactions result in pore fluids with lowered SO42- (all consumed by –10 cm) and enrichments in H2S, alkalinity, and DIC (up to 20 mmol, 30 meq/L, and 18 mmol/L, respectively) which promote carbonate formation. Pore fluid d13CDIC (PDB) is influenced by thermogenic hydrocarbons and crude oil in GC 232 and GC 185 (ranging from –9 to –39‰) and methane formed in situ via fermentation. GC 272 sediments experience seepage from biogenic methane and thermogenic hydrocarbons (d13CDIC as negative as –52‰). Radiocarbon measurements of pore fluid DIC in GC 272 provide evidence of fossil biogenic methane. Pore fluid chloride concentrations influenced by gas hydrate formation and decomposition show that hydrates occupy from 5-30% of the sediment pore space. The calculated saturation state of pore fluids with respect to carbonate minerals show that they are supersaturated with respect to calcite, aragonite and dolomite. Pore fluid calcium, magnesium, and strontium, show the effects of recent carbonate precipitation. The sediments contained elevated carbonate levels (up to 70wt.%) due to either aragonite/Mg-calcite nodules (GC 232 and GC 185) or dolomite/Mg-Calcite nodules (GC 272) formed near the sediment-water interface. The GC 272 mud volcano site represents the first described occurrence in the Gulf of Mexico of pseudomorphs after the carbonate mineral, ikaite. GC 232 and GC 185 carbonate d18O averages 3.4‰ and indicates that the nodules were precipitated from fluids with the present day temperature and stable isotope composition. GC 272 carbonates have d18O indicative of formation at elevated temperatures (as negative as –6.7‰ PDB). The carbonates have d13C that ranges from –12 to -40‰, reflective of the pore fluid d13CDIC and indicates that non-methane hydrocarbons are the ultimate source for the carbonates. Sediments from GC 232 , GC 272 and GC 185 were shown to have sedimentation rates in the top 30 cm of 13 cm/ka, 15 cm/ka, and 27 cm/ka, respectively