ABSTRACT

The spatial variation of methane contents and its stable carbon isotopic composition of sediments from a mesotrophic lake (Lake Biwa) were determined. Methane concentrations in the sediments (0–10 cm), ranging from 0.04 to 2.41 μmol ml^-1, were weakly correlated with the amounts of organic carbon in the surface sediments (0–2 cm), but were poorly correlated with the nitrogen isotopic composition of the bulk sediments used as an indicator of the origin of the organic matter (autochthonous vs. allochthonous). The carbon isotopic composition of methane ranged from −61 to −80‰ and exhibited positive correlation with methane concentration and the carbon isotopic composition of total carbonate (ΣCO₂). The apparent fractionation factor (αcCH₄-CO₂) between methane and ΣCO₂ was in the range between 1.07 and 1.085, which were rather high compared to previously reported values in freshwater sediments. The results suggested that major parts of methane were produced via CO₂-H₂ in the Lake Biwa sediments.