THE INFLUENCE OF METHANE OXIDATION ON THE STABLE ISOTOPIC COMPOSITIONOF METHANE EMITTED FROM FLORIDA SWAMP FORESTS

This study reports the first measurements of the delta C-13 of CH4 emi tted from seasonally flooded swamp forests in the southeastern United States. The seasonally averaged delta C-13 of CH4 emitted from a north Florida swamp forest located in the St. Marks National Wildlife Refug e was -52.7 +/- 6.11 parts per thousand (error is +/- one standard dev iation throughout, n = 28), a value C-13-enriched, relative to typical wetland emissions. In an Everglades cypress dome, the average delta C -13 of emitted CH4 was -52.5 +/- 6.7 parts per thousand (n = 3). Consi stent with attenuation of CH4 emission by CH4 oxidation in these envir onments, CH4 emitted via diffusion from the St. Marks swamp forest was enriched in C-13 by 6.4 +/- 5.8parts per thousand (n = 28) and D by 5 7 +/- 36parts per thousand (n = 6) relative to sedimentary CH4. Methan e emitted from the cypress dome had also been altered by oxidation, as it was enriched in C-13 by 12.1 +/- 4.3 parts per thousand relative t o sedimentary CH4. Emission experiments, performed in situ with inhibi tors of aerobic CH4 oxidizing bacteria, were used to calculate the fra ctionation factors (alpha) 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 an d hydrogen alpha values were 1.020 and 1.068, respectively. The delta values of produced (sedimentary) CH4 were relatively constant in the S t. Marks subtropical swamp forest. Additionally, because the transport of CH4 to the atmosphere was dominated by molecular diffusion, variat ions in the magnitude of CH4 oxidation appeared to be the primary fact or controlling the delta values of emitted CH4. This contrasts with sy stems dominated by bubble ebullition, where variations in CH4 producti on mechanisms have been hypothesized to be the primary factor controll ing the delta values of emitted CH4.