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

Concentration and stable carbon isotopic composition (delta(13)C) of CH4 ar e determined in a water column of an 327 m deep oxic oligotrophic crater la ke, Lake Towada in Japan. The results of CTD measurements show relatively h igh temperature and high conductivity in the Lower part of the column, poss ibly 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 delta(13)C value of CH4 at 20 m is -55 parts per thousand PDB, suggesting microbial production in and around the lake. In contrast, the delta(13)C value of CH4 at 150 m is +11 parts per thousand PDB, which suggests some secondary isotopic alter nation processes peculiar to hot spring-derived CH4 in the oxic water colum n, such as rapid aerobic microbial oxidation.