This study reports the first measurements of the δ13C of CH4 emitted from seasonally flooded swamp forests in the southeastern United States. The seasonally averaged δ13C of CH4 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 δ13C of emitted CH4 was -52.5 ± 6.7%.(n = 3). Consistent with attenuation of CH4 emission by CH4 oxidation in these environments, CH4 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 CH4. 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 CH4. Emission experiments, performed in situ with inhibitors of aerobic CH4 oxidizing bacteria, were used to calculate the fractionation factors (α) for stable carbon and hydrogen isotopes of CH4 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) CH4 were relatively constant in the St. Marks subtropical swamp forest. Additionally, because the transport of CH4 to the atmosphere was dominated by molecular diffusion, variations in the magnitude of CH4 oxidation appeared to be the primary factor controlling the δ values of emitted CH4. This contrasts with systems dominated by bubble ebullition, where variations in CH4 production mechanisms have been hypothesized to be the primary factor controlling the δ values of emitted CH4.
ASJC Scopus subject areas
- Geochemistry and Petrology