The Carbon Isotope Biogeochemistry of Methane Production in Anoxic Sediments: 2. A Laboratory Experiment
The factors that contribute to variability in the isotopic composition of methane produced by anoxic sediment were evaluated by monitoring stable carbon isotope ratios in methane produced by a sediment microcosm. Anoxic marine sediment from Cape Lookout Bight, North Carolina, was placed in a 2-L pyrex incubation syringe for 114 d. Sediment samples were removed at weekly intervals and analyzed for sulfate, acetate, and methane concentrations as well as13C/12C ratios in the dissolved CO2 and methane pools. In addition, rates of sulfate reduction, methane production from acetate, and methane production from CO2 were measured by radiotracer techniques. The isotopic composition of the methane pool varied by more than 40%0 over the course of the experiment. The changes appear to be the result of at least two factors: (1) changes in the isotopic composition of the dissolved CO2 pool and (2) changes in the fraction of the methane production derived from acetate. A comparison of the laboratory experiment with field data suggests that similar biogeochemical processes operate to control carbon isotope ratios of methane in both undisturbed and laboratory-incubated sediment.
KeywordsPorosity Methane Fermentation Dioxide Geochemistry
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