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Effect of seasonal changes in the pathways of methanogenesis on the δ 13 C values of pore water methane in a Michigan peatland
Author(s) -
Avery G. Brooks,
Shan Robert D.,
White Jeffrey R.,
Martens Christopher S.,
Alperin Marc J.
Publication year - 1999
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/1999gb900007
Subject(s) - methanogenesis , methane , peat , environmental chemistry , chemistry , dissolved organic carbon , ecology , organic chemistry , biology
The δ 13 C 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 δ 13 C value of methane from CO 2 reduction (average = −71.4 ± 1.8‰) was depleted in 13 C 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 using 14 C‐labeled CO 2 indicated that during January 110 ± 25% of the methane was produced by CO 2 reduction. Because of low‐methane production rates during the winter, this 13 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 δ 13 C 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 δ 13 C 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.