The influence of methane oxidation on the stable isotopic composition of methane emitted from Florida swamp forests

Abstract

This study reports the first measurements of the δ 13C of CH 4 emitted from seasonally flooded swamp forests in the southeastern United States. The seasonally averaged δ 13C of CH 4 emitted from a north Florida swamp forest located in the St. Marks National Wildlife Refuge was −52.7 ± 6.11%. (error is ± one standard deviation throughout, n = 28), a value 13C-enriched, relative to typical wetland emissions. In an Everglades cypress dome, the average δ 13C of emitted CH 4 was −52.5 ± 6.7%.( n = 3). Consistent with attenuation of CH 4 emission by CH 4 oxidation in these environments, CH 4 emitted via diffusion from the St. Marks swamp forest was enriched in 13C by 6.4 ± 5.8%. ( n = 28) and D by 57 ± 36%. ( n = 6) relative to sedimentary CH 4. Methane emitted from the cypress dome had also been altered by oxidation, as it was enriched in 13C by 12.1 ± 4.3%. relative to sedimentary CH 4. Emission experiments, performed in situ with inhibitors of aerobic CH 4 oxidizing bacteria, were used to calculate the fractionation factors (α) for stable carbon and hydrogen isotopes of CH 4 undergoing transport and oxidation. Values ranged from 1.003 to 1.021 and 1.050 to 1.129, respectively. The best estimates for carbon and hydrogen α values were 1.020 and 1.068, respectively. The δ values of produced (sedimentary) CH 4 were relatively constant in the St. Marks subtropical swamp forest. Additionally, because the transport of CH 4 to the atmosphere was dominated by molecular diffusion, variations in the magnitude of CH 4 oxidation appeared to be the primary factor controlling the δ values of emitted CH 4. This contrasts with systems dominated by bubble ebullition, where variations in CH 4 production mechanisms have been hypothesized to be the primary factor controlling the δ values of emitted CH 4.