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

Abstract

Concentrations and δ 34 S and δ 13 C values were determined on SC^", HCO 3 ", CO 2 , and CH 4 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 CH 4 is being oxidized, apparently in conjunction with SC^~ reduction. CH 4 generation by CO 2 reduction is the most important metabolic process below the 15-m depth. CO 2 removal is more rapid than CO 2 input over the depth interval from 15 to 100 m, and results in a slight decrease in HCO 3~ concentration accompanied by a 40‰ positive shift in δ 13 C. The differences among coexisting CH 4 , CO 2 , and HCOf are consistent with kinetic fractionation between CH 4 and dissolved CO 2 , and equilibrium fractionation between CO 2 and HCO 3 ". At depths greater than 100 m, the rate of input of CO 2 (δ 13 C = -25‰) exceeds by 2 times the rate of removal of CO 2 by conversion to CH 4 (δ 13 C of -60 to -65‰). This results in an increase of dissolved HCO 3 " concentration while maintaining δ 13 C of HCO 3 " relatively constant at -I-lO‰. Non-steady-state deposition has resulted in significantly higher organic carbon contents and unusually high (70 meq I" 1 ) 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 δ 13 C 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 CO 2 -CH 4 ratios in the gas phase, independently suggest that CH 4 concentrations on the order of 100 mmol kg" 1 are present in the pore waters of Blake Outer Ridge sediments. This quantity of CH 4 is believed to be insufficient to saturate pore waters and stabilize the CH 4 6H 2 O 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 CH 4 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.

Isotopic Composition of Interstitial Fluids and Origin of Methane in Slope Sediment of the Middle America Trench, Deep Sea Drilling Project Leg 84

“…The 77% 0 δ 13 C difference between CH 4 (-41‰) and ΣCO 2 ( + 36%o) is essentially the same as the δ 13 C difference in DSDP cores from other sites where CH 4 with δ 13 C as negative as -90‰ has been observed (e.g., Hole 533, Leg 76; Claypool and Threlkeld, 1983). The observed <5 13 C difference between coexisting CH 4 and ΣCO 2 is interpreted as resulting from a kinetic isotope effect associated with bacterial CO 2 reduction (Rosenfeld and Silverman, 1959).…”

“…These unusual aspects of pore-fluid chemistry are undoubtedly related to the presence of gas hydrate, but the nature of the relationship is unclear. We know from observations on samples from Hole 533 (Claypool and Threlkeld, 1983) and Hole 565 (this chapter) that gas hydrate occurrence is not always accompanied by such degrees of 13 C-enrichment in CH 4 and CO 2 . This suggests that the isotope effects are somehow related to the unusually organic-carbon-rich nature of these upper-rise and slope sediments off Guatemala.…”

“…A more quantitative interpretation of kinetic isotope effects involved with early diagenetic CH 4 generation was given in connection with Leg 76, Hole 533 pore-fluid data (Claypool and Threlkeld, 1983). This treatment uses the open-system Rayleigh equations developed by Wigley et al (1978) to model the concentration and δ 13 C change of dissolved CO 2 reservoirs with depth (or time).…”

Isotopic Composition of Gases and Interstitial Fluids in Sediment of the Vøring Plateau, ODP Leg 104, Site 644

“…Similar trends in both the concentration and 8 13 C of the dissolved bicarbonate and methane 5 13 C were observed for samples from DSDP Hole 533 on the Blake Outer Ridge (Claypool and Threlkeld, 1983). At Hole 533, these pore fluids trends were modeled using open-system Rayleigh distillation equations (Wigley et al 1978), and carbon-isotope mass balance calculations.…”

Modeling sulfate reduction in methane hydrate-bearing continental margin sediments: Does a sulfate-methane transition require anaerobic oxidation of methane?

“…[2] Methane-bearing continental margin sediments have characteristic near-seafloor geochemical profiles, schematically illustrated in Figure 1 [e.g., Claypool and Threlkeld, 1983;Borowski et al, 1996Borowski et al, , 2000Burns, 1998;Moore et al, 2004;Treude et al, 2005;Claypool et al, 2006;Sivan et al, 2007;Snyder et al, 2007;Kastner et al, 2008a;Pohlman et al, 2008;Ussler and Paull, 2008]. These profiles are strongly influenced by microbially mediated sulfate reduction processes that oxidize methane and more complex organic compounds to carbon dioxide.…”