The isotopic composition of atmospheric methane

Abstract

Measurements of the 13 C/ 12 C, D/H and 14 C composition of atmospheric methane (CH 4 ) between 1988 and 1995 are presented. The 13 C/ 12 C 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 13 C/ 12 C and D/H of atmospheric CH 4 from air samples collected during oceanographic research cruises in the Pacific Ocean are also presented. The mean annual δ 13 C increased southward from about −47.7 ‰ at 71°N to −41.2 ‰ at 41°S. The amplitude of the seasonal cycle in δ 13 C ranged from about 0.4 ‰ at 71°N to 0.1 ‰ at 14°S. The seasonal δ 13 C cycle at sites in tropical latitudes could be explained by CH 4 loss via reaction with OH radical, whereas at temperate and polar latitudes in the northern hemisphere seasonal changes in the δ 13 C of the CH 4 source were needed to explain the seasonal cycle. The higher δ 13 C value in the southern (−47.2 ‰) versus northern (−47.4 ‰) hemisphere was a result of interhemispheric transport of CH 4 . A slight interannual δ 13 C increase of 0.02±0.005 ‰ yr −1 was measured at all sites between 1990 and 1995. The mean δD of atmospheric CH 4 was −86±3 ‰ between 1989 and 1995 with a 10 ‰ depletion in the northern versus southern hemisphere. The 14 C content of CH 4 measured at 48°N increased from 122 to 128 percent modern between 1987 and 1995. The proportion of CH 4 released from fossil sources was 18±9% in the early 1990s as derived from the 14 C content of CH 4 .