Effect of seasonal changes in the pathways of methanogenesis on the δ13C values of pore water methane in a Michigan peatland

The delta(13)C value of pore water methane produced in a Michigan peatland varied by 11 parts per thousand during the year. This isotopic shift resulted from large seasonal changes in the pathways of methane production. On the basis of mass balance calculations, the delta(13)C value of methane from CO2 reduction (average = -71.4 +/- 1.8 parts per thousand) was depleted in C-13 compared to that produced from acetate (-44.4 +/- 8.2 parts per thousand). The dissolved methane at the site remained heavy (approximately -51 parts per thousand) during most of the year. Tracer experiments using C-14-labeled CO2 indicated that during January 110 +/- 25% of the methane was produced PY CO2 reduction. Because of low-methane production rates during the winter, this C-13-depleted methane had only a slight effect on the isotopic composition of the methane pool. In early spring when peat temperatures and methane production rates increased, the delta(13)C value of the dissolved methane in shallow peat was influenced by the isotopically light methane and approached -61 parts per thousand. Feat incubation experiments conducted at 15 degrees C in May and June (when the peat reaches its maximum temperature) indicated that an average of 84 +/- 9% of the methane production was from acetate and had an average delta(13)C value of -48.7 +/- 5.6 parts per thousand. Rising acetate concentrations during April-May (approaching 1 mmol L-1(mM)) followed by a rapid decrease in acetate concentrations during May-June reflected the shift toward methane production dominated by acetate fermentation. During this period, dissolved methane in shallow peat at the site returned to heavier values (approximately -51 parts per thousand) similar to that produced in the incubation experiments.