13C/12C ratio in methane from the flooded Amazon forest

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Abstract
Analyses for C1‐C4 hydrocarbon concentrations and the 13C/12C ratio in CH4 were performed on two air samples collected in the Amazon jungle (3.5°S, 59°W) after the nearby release of biogenic gas bubbles. The CH4 concentrations of each sample were greatly enhanced (4100 and 310 ppmv) over the background concentration (1.6 ppmv) for remote locations at that latitude and time. The 13/12C ratio in this biogenic methane is depleted in 13C (−64‰) relative to atmospheric CH4 (−47‰), as is CH4 from almost all other biogenic sources. Because laboratory measurements to date indicate only a very small 13C/12C isotope effect in the reaction of CH4 with HO, an apparent discrepancy remains between the 13C/12C ratios of the known CH4 sources and that of atmospheric CH4. Five other hydrocarbons (C2H6, C2H4, C3H8, i‐C4H10, n‐C4H10) were also found at the 1 to 35 ppbv level in the air sample with 4100 ppmv CH4. These concentrations are not large enough to indicate any major importance for this source in C2‐C4 hydrocarbon budgets on either a global or regional basis.