Stable carbon isotope ratios in atmospheric methane and some of its sources

Ratios of 13 C/ 12 C have been measured in atmospheric methane and in methane collected from sites and biota that represent potentially large sources of atmospheric methane. These include temperate marshes (∼−48‰ to ∼−54‰), landfills (∼−51‰ to ∼−55‰), and the first reported values for any species of termite (−72.8 ± 3.1‰ for Reticulitermes tibialis and −57.3 ± 1.6‰ for Zootermopsis angusticollis ). Numbers in parentheses are δ 13 C values with respect to PDB (Peedee belemnite) carbonate. Most methane sources reported thus far are depleted in 13 C with respect to atmospheric methane (−47.0 ± 0.3‰). Individual sources of methane should have 13 C/ 12 C ratios characteristic of mechanisms of CH 4 formation and consumption prior to release to the atmosphere. The mass‐weighted average isotopic composition of all sources should equal the mean 13 C of atmospheric methane, corrected for a kinetic isotope effect in the OH attack of CH 4 . Assuming the kinetic isotope effect to be small (∼−3.0‰ correction to −47.0), as in the literature, the new values given here for termite methane do not help to explain the apparent discrepancy between 13 C/ 12 C ratios of the known CH 4 sources and that of atmospheric CH 4 .