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

The δ13C value of pore water methane produced in a Michigan peatland varied by 11‰ 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 δ13C value of methane from CO2reduction (average = −71.4 ± 1.8‰) was depleted in13C compared to that produced from acetate (−44.4 ± 8.2‰). The dissolved methane at the site remained heavy (approximately −51‰) during most of the year. Tracer experiments using14C‐labeled CO2indicated that during January 110 ± 25% of the methane was produced by CO2reduction. Because of low‐methane production rates during the winter, this13C‐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 δ13C value of the dissolved methane in shallow peat was influenced by the isotopically light methane and approached −61‰. Peat incubation experiments conducted at 15°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 δ13C value of −48.7 ± 5.6‰. 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‰) similar to that produced in the incubation experiments.