Rates of anaerobic oxidation of methane and authigenic carbonate mineralization in methane-rich deep-sea sediments inferred from models and geochemical profiles

Pore water chemical data obtained from a 10.5-m long giant gravity core collected in methane-rich sediments from 647 in water depth in the northern Gulf of Mexico (N 28 degrees 04.00' W 89 degrees 43.15') defines sub-bottom gradients in unprecedented detail. This core penetrated the sulfate-methane interface (SMI) at similar to 300 cm below the seafloor (cmbsf. At the SMI dissolved inorganic carbon (DIC) concentrations reach a maximum (13.5 mM) and pore water delta C-13 DIC (-63.2%. PDB) and delta C-13 methane (-89.5%o PDB) values are most negative. Below the SMI pore water sulfate is nearly depleted, methane concentrations rise sharply with simultaneous occurrence of a bubble-textured sediment, and fine-grained methane-derived authigenic carbonate nodules and cements are common. The sharp peaks in DIC concentration and isotope values centered at the SMI indicate that DIC is being produced by anaerobic oxidation of methane (AOM) within a narrow zone centered at the SMI. The detailed sulfate and DIC concentration profiles, and DIC delta C-13 values have enabled geochemical models to be constructed that explore the rate of DIC formation by AOM and its effect on pore water DIC delta C-13 values. Model results closely match measured DIC concentration and delta C-13 isotope profiles and indicate that microbiological conversion of methane carbon to DIC is rapid in geologic terms and that AOM is occurring at the present position of the SMI. Isotope values for authigenic carbonate found immediately below the present-day SMI (delta C-13 = -60.2 +/- 0.7%. PDB at 440 cmbsf) are consistent with derivation of the carbonate carbon from methane via AOM at the former location of a SMI. These observations and model results suggest that AOM is occurring at rates that would generate the observed profiles and begin the precipitation of methane-derived carbonate occur on time-scales of centuries. Model results also show that the time needed to produce the resulting authigenic cements is an order of magnitude greater than that for AOM to produce the observed DIC profiles. The metabolic rates for DIC production by AOM inferred from modeling the geochemical profiles compare favorably with available rate data obtained from laboratory microbial incubations and radiolabeled tracer experiments. (c) 2007 Elsevier B.V. All rights reserved.