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 CO₂ reduction (average =-71.4 ± 1.8%) was depleted in ¹³C compared to that produced from acetate (-44.4 ± 8.2%o). The dissolved methane at the site remained heavy (approximately-51%o) during most of the year. Tracer experiments using ¹⁴C-labeled CO₂ indicated that during January 110 ± 25% of the methane was produced by CO₂ reduction. Because of low-methane production rates during the winter, this C-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-5 l%o) similar to that produced in the incubation experiments.