The determination of δ13C in atmospheric methane in the Southern Hemisphere

A technique to make measurements of δ13C in atmospheric methane (δ13CH4) to a precision of 0.02 per mille (1 sigma) is presented. Measurements of the mixing ratio and δ13CH4 from Baring Head, New Zealand and Scott Base, Antarctica are reviewed for the period 1989 to 1998. The data show persistent but irregular seasonal cycles in δ13CH4 ranging from 0.1-0.3 per mille superimposed on an increasing secular trend of about 0.04 per mille.year-1 . In 1990/91 a 0.2 per mille positive anomaly appeared in this trend. The causes of the anomaly remain unresolved but, during 1992 when it disappeared, the decrease in δ13C was approximately coincident with a decrease in growth rate consistent with a decrease in biomass burning sources, which are relatively enriched in 13CH4. δ13CH4 data are also reported from large clean air samples collected every 2.5 deg. to 5 deg. of latitude on four voyages across the Pacific between New Zealand and the West Coast of the USA in 1996 and 1997. These data show that the inter-hemispheric gradient for δ13CH4 was highly dependent on season, and varied from less than 0.1 per mille in June 1996 to more than 0.5 per mille in November 1996 with an estimated annual mean of 0.2-0.3 per mille. (author)