CARBON-ISOTOPE EVIDENCE FOR DIURNAL-VARIATIONS IN METHANOGENESIS IN FRESH-WATER LAKE-SEDIMENTS

Diurnal variations in the C-13/C-12 ratio in bubble methane have been observed in shallow water lake sediments at Lake Piaseczno, poland. Measurements in August 1993 and September 1992 show that values are lowest in the early morning, and highest in the afternoon, ranging from a minimum of -63.1 at 03:30 on August 30, 1993 to a maximum of -47.5 at 15:30 on August 29, 1993. The minimum values were apparently caused by an enhanced CO2-H-2 methanogenesis pathway. The maximum values occur when the CO2 pool decreases and acetic acid fermentation becomes relatively more important. Seasonal variation and seasonal variations in the magnitude of the diurnal variations in the delta(13)CH(4) value may depend on temperature and the day-to-night time ratio, of the diurnal cycle.