Determination of the isotopic composition of atmospheric methane and its application in the Antarctic

Abstract

A procedure for the determination of the 13 C/ 12 C ratio and the 14 C abundance in atmospheric methane is presented. The method is based on the collection of air samples in stainless steel tanks at a pressure of 7 bar. The air is processed in the laboratory by cryogenic removal of condensibles, followed by oxidation of the methane content after which the resulting CO 2 is collected. Also CO is removed prior to oxidation. The 13 C/ 12 C ratio is determined on the CO 2 sample by stable isotope ratio mass spectrometry. The 14 C content is determined by means of accelerator mass spectrometry. The overall precision of the technique is 0.1‰ for δ 13 C and 1.5 pMC for 14 C. The method has been used to determine the carbon isotopic composition of methane in air collected at Baring Head, New Zealand, and also in air collected on aircraft flights between New Zealand and Antarctica. No gradient in the carbon isotopic composition between Baring Head and South Pole station was detected. The annual mean δ 13 C value at Baring Head was −47.13±0.20‰ for 1989 which includes seasonal effects probably due to OH variations and local meteorology. The annual mean 14 C value at Baring Head in 1989 was 118.3 percent modern (pMC).