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

Analyses for C 1 ‐C 4 hydrocarbon concentrations and the 13 C/ 12 C ratio in CH 4 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 CH 4 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 / 12 C ratio in this biogenic methane is depleted in 13 C (−64‰) relative to atmospheric CH 4 (−47‰), as is CH 4 from almost all other biogenic sources. Because laboratory measurements to date indicate only a very small 13 C/ 12 C isotope effect in the reaction of CH 4 with HO, an apparent discrepancy remains between the 13 C/ 12 C ratios of the known CH 4 sources and that of atmospheric CH 4 . Five other hydrocarbons (C 2 H 6 , C 2 H 4 , C 3 H 8 , i ‐C 4 H 10 , n ‐C 4 H 10 ) were also found at the 1 to 35 ppbv level in the air sample with 4100 ppmv CH 4 . These concentrations are not large enough to indicate any major importance for this source in C 2 ‐C 4 hydrocarbon budgets on either a global or regional basis.