CONCENTRATION AND C-13 RECORDS OF ATMOSPHERIC METHANE IN NEW-ZEALAND AND ANTARCTICA - EVIDENCE FOR CHANGES IN METHANE SOURCES

Measurements of C-13 in atmospheric methane made at Baring Head, New Z ealand (41-degrees-S), over the 4-year period, 1989-1993, display a pe rsistent but highly variable seasonal cycle. Values for deltaC-13 peak in summer at about -46.9 parts per thousand and drop to around -47.5 parts per thousand in the late winter. Methane concentration shows a s imilar cycle, with winter peaks and summer minima. Similar features ar e observed at the New Zealand Antarctic station, Scott Base, at 78-deg rees-S. While the phase of the deltaC-13 cycle is consistent with a ki netic isotope effect that preferentially leaves methane enriched in C- 13 in the atmosphere after oxidation by OH, the amplitude of the cycle is much larger than expected from published laboratory measurements o f the effect. We interpret the origin of this cycle and its interannua l variability to be due to episodic southward transport of isotopicall y heavy methane from large-scale tropical biomass burning, possibly in conjunction with changes in the rate of interhemispheric transport in the troposphere. The Baring Head C-13 data show no significant secula r trend from 1989 to mid-1991, followed by a rapid trend toward methan e less enriched in C-13. This indicates a major shift in the balance o f the sources of atmospheric methane and precludes an increased sink s trength. The trend in C-13 since mid-1991 coincided with significant c hanges to the methane growth rate observed at Baring Head and at Scott Base: an elevated growth rate of about 15 parts per billion by volume (ppbv) during 1991 gave way to less than 3 ppbv yr-1 thereafter. A 2- box model of atmospheric methane (one box per hemispheric reservoir) s uggests that (1) the recent decline in C-13 in methane observed at Bar ing Head and Scott Base cannot have a solely northern hemispheric orig in and (2) the most plausible origin is a recent reduction in methane released by biomass burning in the southern hemisphere, combined with a lower release rate of fossil methane in the northern hemisphere.