Controls on the stable carbon isotopic composition of biogenic methane produced in a tidal freshwater estaurine sediment |
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| Institution: | 1. Department of Agricultural, Forest and Food Sciences (D.I.S.A.F.A.), University of Torino, Largo Braccini, 2, 10095 Grugliasco, Torino, Italy;2. NATRISK, University of Torino, Largo Braccini, 2, 10095 Grugliasco, Torino, Italy;3. Bangladesh Water Development Board, Ministry of Water Resources, Dhaka 1000, Bangladesh;4. Bangladesh Australia Centre for Environmental Research, Department of Soil, Water and Environment, University of Dhaka, Dhaka 1000, Bangladesh |
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| Abstract: | The δ13C 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 (～3‰) with isotopically heavier values occurring during the warmer months (?66.1‰ summer, ?69.2‰ 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 δ13C 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 δ13C value of methane produced in sediment incubation experiments could be attributed to changes in the δ13C value of the ΣCO2 pool. This was due to increased methane production rates and removal of 12CO2 with increasing temperature, a prevalent feature of methanogenic systems that may account for some of the frequently observed 13C enrichment in methane during warmer months. Combining the change in the δ13C value of the ΣCO2 pool with temperature-controlled changes in fractionation (α) resulting from kinetic isotope effects accounted for (53 ± 22%) of the 13C enrichment observed during summer sediment incubation experiments. Although large pathway changes were not observed in sediment incubation experiments, the remaining differences in δ13C values could have resulted from smaller, undetectable changes in the percentage of methane production from acetate (～14%) and/or a shift in the δ13C values of methane produced from acetate (～4‰). |
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