Radiocarbon determination of atmospheric methane at Baring Head, New Zealand

Abstract

Although methane in the atmosphere is clearly increasing, its sources are still poorly defined. Measurement of carbon isotope ratios allows constraints to be placed on relative source strengths of atmospheric methane because different potential sources have different isotope ratios. Unfortunately, interpreting 13C/12C ratios of atmospheric methane is subject to uncertainty because a correction for isotope fractionation in the oxidation of methane is not well determined. Interpreting 14C/12C ratios is also complicated by the need to correct for release of 14CH4 from nuclear power plants using rough estimates. Simultaneous use of both carbon isotope ratios, however, improves the confidence of interpretation. Here we show that measurements of the 14C/12C ratio, using accelerator mass spectrometry, and 13C/12C ratio of methane from clean Southern Hemisphere air are consistent with current estimates of both types of correction and imply that about 32% of atmospheric methane is derived from fossil carbon sources.