Carbon and hydrogen isotopic composition of bacterial methane in a shallow freshwater lake

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Abstract
Hydrogen and carbon isotopes measured in biogenic gas from a shallow freshwater lake indicate the acetate‐reduction pathway to be responsible for over 70% of the methanogenesis in these sediments. Methane δD was strongly depleted (up to 307‰) relative to the associated formation water (ca. −25‰ vs. SMOW) and was similar to fermentation methane from sewage sludge digesters. Although the methane δ13C ranged from −52‰ to −61‰, relative to the porewater CO2 it had a consistent fractionation about αc = 1.05. With the combination of H isotopes, we can interpret previously ambiguous methane δ13C values (−50 to −60‰) as being of biogenic origin.