DETERMINATION OF THE ISOTOPIC COMPOSITION OF ATMOSPHERIC METHANE AND ITS APPLICATION IN THE ANTARCTIC

A procedure for the determination of the C-13/C-12 ratio and the C-14 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 CO2 is collected. Also CO is removed prior to oxidation. The C-13/C-12 ratio is determined on the CO2 sample by stable isotope ratio mass spectrometry. The C-14 content is determined by means of accelerator mass spectrometry. The overall precision of the technique is 0.1 parts per thousand for delta-C-13 and 1.5 pMC for C-14. 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 delta-C-13 value at Baring Head was -47.13 +/- 0.20 parts per thousand for 1989 which includes seasonal effects probably due to OH variations and local meteorology. The annual mean C-14 value at Baring Head in 1989 was 118.3 percent modern (pMC).