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

Journal Article · Thu Apr 01 04:00:00 EST 1999 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/S0016-7037(98)00315-9· OSTI ID:680059

Avery, Jr, G B; Martens, C S ^[1]

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1. Univ. of North Carolina, Chapel Hill, NC (United States). Marine Sciences Dept.

The {delta}{sup 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 ({approximately}3{per_thousand}) with isotopically heavier values occurring during the warmer months ({minus}66.1{per_thousand} summer, {minus}69.2{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}{sup 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}{sup 13}C value of methane produced in sediment incubation experiments could be attributed to changes in the {delta}{sup 13}C value of the {Sigma}CO{sub 2} pool. This was due to increased methane production rates and removal of {sup 12}CO{sub 2} with increasing temperature, a prevalent feature of methanogenic systems that may account for some of the frequently observed {sup 13}C enrichment in methane during warmer months. Combining the change in the {delta}{sup 13}C value of the {Sigma}CO{sub 2} pool with temperature-controlled changes in fractionation ({alpha}) resulting from kinetic isotope effects accounted for (53 {+-} 22%) of the {sup 13}C enrichment observed during summer sediment incubation experiments. Although large pathway changes were not observed in sediment incubation experiments, the remaining differences in {delta}{sup 13}C values could have resulted from smaller, undetectable changes in the percentage of methane production from acetate ({approximately}14%) and/or a shift in the {delta}{sup 13}C values of methane produced from acetate ({approximately}4{per_thousand}).

• OSTI ID: 680059
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 7-8 Vol. 63; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
BIOSYNTHESIS
CARBON 13
EMISSION
ESTUARIES
ISOTOPE RATIO
METHANE
NORTH CAROLINA
SEASONAL VARIATIONS
WETLANDS