Origin of ¹³C-enriched methane in the crater lake Towada, Japan

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Concentration and stable carbon isotopic composition (δ¹³C) of CH₄ are determined in a water column of an 327 m deep oxic oligotrophic crater lake, Lake Towada in Japan. The results of CTD measurements show relatively high temperature and high conductivity in the lower part of the column, possibly derived from hot springs at the crater wall approximately 150 m deep. The vertical profile of CH₄ concentration shows two sharp maxima of 68 and 69 nmol/kg at depths of 20 and 150 m, respectively. The δ¹³C value of CH₄ at 20 m is -55‰PDB, suggesting microbial production in and around the lake. In contrast, the δ¹³C value of CH₄ at 150 m is +11‰PDB, which suggests some secondary isotopic alternation processes peculiar to hot spring-derived CH₄ in the oxic water column, such as rapid aerobic microbial oxidation.