Spatial and temporal patterns in diurnal variations of carbon isotope ratios of early-diagenetic methane from freshwater sediments

Methane was collected from the sediments of two freshwater lakes in SW Poland, Lake Sulistrowiczki and Lake Nowa Cerekiew, on three separate occasions in 1994, 1995, and 1997. Samples were collected from sediments at various shallow depths in the lakes (0.15 to 0.8 m) over the course of the day, and analyzed for carbon isotope ratios. A diurnal variation in the carbon isotopic composition of the methane was evidenced by changes in δ13C ratios of up to 14.5‰, with the most 13C-depleted gases collected at night and the least 13C-depleted gases collected in the early afternoon. The magnitude of the diurnal variation decreases with increasing water column depth as well as with a decrease in the period of daylight. The lowest δ13C values are believed to result from increased production of methane from carbon dioxide and hydrogen. The highest δ13C values are believed to occur when the pool of carbon dioxide available as a substrate is most enriched in 13C and when acetic acid fermentation becomes a more important methanogenic pathway. The observed changes in diurnal carbon isotope composition suggest that increased photosynthetic activity accompanied by carbon dioxide exchange between the sediment pore waters, overlying water column, and the atmosphere are more important factors than variations in temperature or oxidation of methane. Seasonal variations in the carbon isotopic composition of sediment methane and seasonal variations in the magnitude of the observed diurnal variation in isotopic composition may be related to changes in the light/dark cycle and latitude. However, the observed magnitudes of the diurnal variations are as large or larger than the seasonal changes and/or spatial variability that have previously been reported for the carbon isotopic composition of methane from wetlands. Thus, existing isotopic data on biogenic methanogenesis may need to be re-evaluated in light of such diurnal variations.