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

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 δ D 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 δ 13 C ranged from −52‰ to −61‰, relative to the porewater CO 2 it had a consistent fractionation about α c = 1.05. With the combination of H isotopes, we can interpret previously ambiguous methane δ 13 C values (−50 to −60‰) as being of biogenic origin.