Effect of seasonal changes in the pathways of methanogenesis on the delta C-13 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 result ed from large seasonal changes in the pathways of methane production. On th e 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 thousa nd). The dissolved methane at the site remained heavy (approximately -51 pa rts per thousand) during most of the year. Tracer experiments using C-14-la beled CO2 indicated that during January 110 +/- 25% of the methane was prod uced PY CO2 reduction. Because of low-methane production rates during the w inter, 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 an d approached -61 parts per thousand. Feat incubation experiments conducted at 15 degrees C in May and June (when the peat reaches its maximum temperat ure) indicated that an average of 84 +/- 9% of the methane production was f rom 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 mmo l L-1(mM)) followed by a rapid decrease in acetate concentrations during Ma y-June reflected the shift toward methane production dominated by acetate f ermentation. During this period, dissolved methane in shallow peat at the s ite returned to heavier values (approximately -51 parts per thousand) simil ar to that produced in the incubation experiments.