Controls on the stable carbon isotopic composition of biogenic methane produced in a tidal freshwater estaurine sediment

The delta(13)C value Of methane in sediments from a tidal freshwater site in the White Oak River Estuary, North Carolina, exhibited a relatively small, but consistent, seasonal variation (similar to 3 parts per thousand) with isotopically heavier values occurring during the warmer months (-66.1 parts per thousand summer, -69.2 parts per thousand winter). These isotopic shifts could have resulted from changes in: (1) isotopic compositions of precursor molecules; (2) kinetic isotope effects associated with methane production; or (3) pathways of methane production. Methane production rate and isotopic data from sediment incubation experiments and field measurements were used to determine the relative contributions of these factors to the observed seasonal variations. Although changes in delta(13)C values of biogenic methane are typically thought to result from changes in pathways of methane production, this study showed that a significant amount (36 +/- 22%) of the seasonal variations between the delta(13)C value of methane produced in sediment incubation experiments could be attributed to changes in the delta(13)C value of the CCO, pool. This was due to increased methane production rates and removal of (CO2)-C-12 with increasing temperature, a prevalent feature of methanogenic systems that may account for some of the frequently observed C-13 enrichment in methane during warmer months. Combining the change in the delta(13)C value of the Sigma CO2 pool with temperature-controlled changes in fractionation (alpha) resulting from kinetic isotope effects accounted for (53 +/- 22%) of the C-13 enrichment observed during summer sediment incubation experiments. Although large pathway changes were not observed in sediment incubation experiments, the remaining differences in delta(13)C values could have resulted from smaller, undetectable changes in the percentage of methane production from acetate (similar to 14%) and/or a shift in the delta(13)C values of methane produced from acetate (similar to 4 parts per thousand). Copyright (C) 1999 Elsevier Science Ltd.