The isotopic composition of atmospheric methane

Measurements of the 13C/12C, D/H and 14C composition of atmospheric methane (CH4) between 1988 and 1995 are presented. The 13C/12C measurements represent the first global data set with time series records presented for Point Barrow, Alaska (71°N), Olympic Peninsula, Washington (48°N), Mauna Loa, Hawaii (20°N), American Samoa (14°S), Cape Grim, Australia (41°S), and Baring Head, New Zealand (41°S). North‐south trends of the 13C/12C and D/H of atmospheric CH4 from air samples collected during oceanographic research cruises in the Pacific Ocean are also presented. The mean annual δ13C increased southward from about −47.7 ‰ at 71°N to −41.2 ‰ at 41°S. The amplitude of the seasonal cycle in δ13C ranged from about 0.4 ‰ at 71°N to 0.1 ‰ at 14°S. The seasonal δ13C cycle at sites in tropical latitudes could be explained by CH4 loss via reaction with OH radical, whereas at temperate and polar latitudes in the northern hemisphere seasonal changes in the δ13C of the CH4 source were needed to explain the seasonal cycle. The higher δ13C value in the southern (−47.2 ‰) versus northern (−47.4 ‰) hemisphere was a result of interhemispheric transport of CH4. A slight interannual δ13C increase of 0.02±0.005 ‰ yr−1 was measured at all sites between 1990 and 1995. The mean δD of atmospheric CH4 was −86±3 ‰ between 1989 and 1995 with a 10 ‰ depletion in the northern versus southern hemisphere. The 14C content of CH4 measured at 48°N increased from 122 to 128 percent modern between 1987 and 1995. The proportion of CH4 released from fossil sources was 18±9% in the early 1990s as derived from the 14C content of CH4.