METHANE FLUX AND STABLE HYDROGEN AND CARBON ISOTOPE COMPOSITION OF SEDIMENTARY METHANE FROM THE FLORIDA EVERGLADES

Methane flux and the stable isotopic composition of sedimentary methane were measured at four locations in the Florida Everglades system. Individual estimates of methane flux ranged over more than 3 orders of magnitude, from about 0.001 to 2.6 g CH4 m(-2) d(-1). Significant interstation differences in total methane flux were also observed and are judged most likely attributable to differences in the size and spacing of emergent aquatic vegetation, and possibly differences in the type (i.e., vascular plant versus algal) of organic matter incorporated into the sediments. On the basis of measurements presented here and by other investigators, the Everglades system appears to be a relatively weak source of atmospheric methane, probably contributing less than 0.5 Tg CH4 yr(-1). Emergent aquatic plants appear to be capable of indirectly affecting the stable isotopic composition of sedimentary methane by stimulating methane oxidation via root aeration. A significant positive correlation between delta D-CH4 and delta C-13-CH4 was observed for samples collected from sediments covered by tall, dense stands of emergent plants. In contrast, a significant negative correlation between the delta D-CH4 and delta C-13 of sedimentary methane was observed for samples collected at an open water site where ebullition dominated methane transfer to the atmosphere. The mean delta C-13 of sedimentary methane samples measured in the Everglades system (mean delta C-13 = -61.7 degrees / (00), s.d. = 3.6 degrees / (00), n = 51) is not significantly different from the estimated average delta C-13 of all natural sources (-58.3 degrees / (00)). The mean D of Everglades sedimentary methane (mean delta D = 293 degrees / (00), s.d. = 14 degrees / (00), n = 50) appears to be slightly less D -depleted than the estimated average methane (delta D = -360 +/- 30 degrees / (00)) from all sources.