A history of δ¹³C in atmospheric CH₄ from the Cape Grim Air Archive and Antarctic firn air

Marine (baseline) air from Cape Grim (41°S), collected and archived in high‐pressure metal containers, provides a history of δ 13 C in atmospheric methane from 1978. A similar history is obtained from air pumped from different layers of the firn at Law Dome, Antarctica, after correction for diffusion and gravitational settling effects in the firn. The archive records 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 Baring Head, New Zealand. Over 18 years the δ 13 C of atmospheric methane in the extratropical Southern Hemisphere has increased by ∼0.6‰ while the methane mixing ratio increased by ∼200 ppb. The δ 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 δ 13 C is significantly smaller than, and the recent slowing is in conflict with, previous estimates [ Stevens and Engelkemeir , 1989]. The long‐term trend in δ 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 δ 13 C, compared to that of the mixing ratio, is an example of an effect pointed out recently by Tans [1997]. The data presented here constrain changes in the relative mix of isotopically heavy and light sources to be small and suggest that there was little change in the ratio of anthropogenic to natural sources in the 1978 to 1995 period.