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Interstitial sulfate and methane profiles measured from a piston core taken near the Mississippi River mouth indicate that sulfate is reduced to near-zero concentrations between 1.5 and 2.0 m depth and that methane consequently increases. In the sulfate-reducing zone, ^dgr13C values of the methane decrease downward from -53 to -80 parts per thousand versus PDB. From these observations, a comprehensive kinetic model has been developed incorporating the effects of diffusion, the sediment accumulation, and the effects of bacteria on sulfate and methane. The model, in which isotopic variations of methane are considered to be the key, indicates that methane is microbially consumed in the sulfate-reducing zone and is extensively produced below the zero-sulfate depth. Rates of production and consumption of methane, as well as reduction rates of sulfate, can be estimated from the model.
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