SEASONAL VARIATIONS IN EBULLITIVE FLUX AND CARBON ISOTOPIC COMPOSITION OF METHANE IN A TIDAL FRESHWATER ESTUARY

Methane carbon isotopic composition ranged from -76.9 to -62.6 degrees/infinity in a tidal freshwater estuary (the White Oak River, North Carolina, United States) with site specific seasonal variations ranging from 6 to 10 degrees/infinity. During warmer months, tidally induced bubble ebullition actively transported this methane to the atmosphere. At two sites, these seasonally varying fluxes ranged from 1.2 +/- 0.3 to 1.3 +/- 0.3 mol CH4 m(-2)yr(-1) (19.2 to 20.8 g CH(4)m(-2)yr(-1)), with flux-weighted average isotopic compositions at two sites of -66.3 +/- 0.4 and -69.5 +/- 0.6 degrees/infinity. The carbon isotopic composition of naturally released bubbles was shown to be indistinguishable from the sedimentary methane bubble reservoir at three sites, leading to the conclusion that isotopic fractionation did not occur during the ebullition of methane. The hypothesis was developed that ebullitive methane fluxes are depleted in (CH4)-C-13 relative to fluxes transported via molecular diffusion or through plants, as zones of 13C enriching microbial methane oxidation are bypassed. Copyright 1988 by the American Geophysical Union.