ATMOSPHERIC METHANE AND ITS CARBON ISOTOPES IN THE SOUTHERN-HEMISPHERE - THEIR TIME-SERIES AND AN INSTRUCTIVE MODEL

We report data from the clean air monitoring station at Baring Head, New Zealand on concentrations of atmospheric methane and its C-13/C-12 and C-14/C-12 ratios. The record for methane concentration features a recent (post-July 1991) and persistent elevation above expected levels. The 2-year deltaC-13 record shows a surprisingly large seasonal cycle (ca 0.3 parts per thousand peak to peak) about a mean value of -47.14 +/- 0.03 parts per thousand (2 standard deviations) with no discernible trend. The C-14/C-12 record is relatively featureless, corresponding to 11 9.7 +/- 0.7 percent modern carbon (pMC) in January 1990 with a small upward trend of 1.3 +/- 0.8 pMC/yr over the past three years. A simple model depicting the southern hemisphere atmosphere as a single methane reservoir with seasonally-modulated sources and sinks is used to examine methane dynamics in this hemisphere. According to this model, approximately half the methane in the southern hemisphere atmosphere is delivered from the northern hemisphere. The model can interpret the deltaC-13 seasonal cycle only as a large injection of isotopically heavy methane during the austral spring, such as might result from biomass burning,