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Concentrations and δS and δC values were determined on SC^", HCO3", CO2, and CH4 in interstitial water and gas samples from the uppermost 400 m of sediment on the Blake Outer Ridge. These measurements provide the basis for detailed interpretation of diagenetic processes associated with anaerobic respiration of electrons generated by organic-matter decomposition. The sediments are anaerobic at very shallow depths (< 1 m) below the seafloor. Sulfate reduction is confined to the uppermost 15 m of sediment and results in a significant outflux of oxidized carbon from the sediments. At the base of the sulfate reduction zone, upward-diffusing CH4 is being oxidized, apparently in conjunction with SC^~ reduction. CH4 generation by CO2 reduction is the most important metabolic process below the 15-m depth. CO2 removal is more rapid than CO2 input over the depth interval from 15 to 100 m, and results in a slight decrease in HCO3~ concentration accompanied by a 40‰ positive shift in δ C. The differences among coexisting CH4, CO2, and HCOf are consistent with kinetic fractionation between CH4 and dissolved CO2, and equilibrium fractionation between CO2 and HCO3". At depths greater than 100 m, the rate of input of CO2 (δ C = -25‰) exceeds by 2 times the rate of removal of CO2 by conversion to CH4 (δ C of 60 to 65‰). This results in an increase of dissolved HCO3" concentration while maintaining δ C of HCO3" relatively constant at -IlO‰. Non-steady-state deposition has resulted in significantly higher organic carbon contents and unusually high (70 meq I") pore-water alkalinities below 150 m. These high alkalinities are believed to be related more to spontaneous decarboxylation reactions than to biological processes. The general decrease in HCOf concentration with constant δC over the depth interval of 200 to 400 m probably reflects increased precipitation of authigenic carbonate. Input-output carbon isotope-mass balance calculations, and carbonate system equilibria in conjunction with observed CO2-CH4 ratios in the gas phase, independently suggest that CH4 concentrations on the order of 100 mmol kg" are present in the pore waters of Blake Outer Ridge sediments. This quantity of CH4 is believed to be insufficient to saturate pore waters and stabilize the CH4 6H2O gas hydrate. Results of these calculations are in conflict with the physical recovery of gas hydrate from 238 m, and with the indirect evidence (seismic reflectors, sediment frothing, slightly decreasing salinity and chlorinity with depth, and pressure core barrel observations) of gas-hydrate occurrence in these sediments. Resolution of this apparent conflict would be possible if CH4 generation were restricted to relatively thin (1-10 m) depth intervals, and did not occur uniformly at all depths throughout the sediment column, or if another methanogenic process (e.g., acetate fermentation) were a major contributor of gas.

Anoxic Diagenesis and Methane Generation in Sediments of the Blake Outer Ridge, Deep Sea Drilling Project Site 533, Leg 76