A history of delta C-13 in atmospheric CH4 from the Cape Grim air archive and antarctic firn air

Marine (baseline) air from Cape Grim (41 degrees S), collected and archived in high-pressure metal containers, provides a history of delta(13)C in atm ospheric methane from 1978. A similar history is obtained from air pumped f rom different layers of the firn at Law Dome, Antarctica, after correction for diffusion and gravitational settling effects in the firn. The archive r ecords are linked to measurements since 1992 using 5-L glass flasks filled at Cape Grim, and compared to data since 1989 from a comparable site at Bar ing Head, New Zealand. Over 18 years the delta(13)C of atmospheric methane in the extratropical Southern Hemisphere has increased by similar to 0.6 pa rts per thousand while the methane mixing ratio increased by similar to 200 ppb. The delta(13)C growth rate decreases over the 18-year period, but by relatively less than the simultaneous decrease in mixing ratio growth rate. The overall increase in delta(13)C is significantly smaller than, and the recent slowing is in conflict with, previous estimates [Stevens and Engelke meir, 1989]. The long-term trend in delta(13)C, and the different shape to the trend in mixing ratio, are shown to be consistent with constant global methane sources and sinks since 1982. The slower equilibration of observed delta(13)C, compared to that of the mixing ratio, is an example of an effec t pointed out recently by Tans [1997], The data presented here constrain ch anges in the relative mix of isotopically heavy and light sources to be sma ll and suggest that there was little change in the ratio of anthropogenic t o natural sources in the 1978 to 1995 period.