The isotopic composition of atmospheric methane

Measurements of the C-13/C-12, D/H and C-14 composition Of atmospheric methane (CH4) between 1988 and 1995 are presented. The C-13/C-12 measurements represent the first global data set with time series records presented for Point Barrow, Alaska (71 degrees N), Olympic Peninsula, Washington (48 degrees N), Mauna Loa, Hawaii (20 degrees N), American Samoa (14 degrees S), Cape Grim, Australia (41 degrees S), and Baring Head, New Zealand (41 degrees S). North-south trends of the C-13/C-12 and D/H of atmospheric CH4 from air samples collected during oceanographic research cruises in the Pacific Ocean are also presented. The mean annual delta(13)C increased southward from about -47.7 parts per thousand at 71 degrees N to -47.2 parts per thousand at 41 degrees S. The amplitude of the seasonal cycle in delta(13)C ranged from about 0.4 parts per thousand at 71 degrees N to 0.1 parts per thousand at 14 degrees S. The seasonal delta(13)C 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 delta(13)C Of th, CH4 source were needed to explain the seasonal cycle. The higher delta(13)C value in the southern (-47.2 parts per thousand) versus northern (-47.4 parts per thousand) hemisphere was a result of interhemispheric transport of CH4. A slight interannual delta(13)C increase of 0.02+/-0.005 parts per thousand yr(-1) was measured at all sites between 1990 and 1995. The mean delta D of atmospheric CH4 was -86+/-3 parts per thousand between 1989 and 1995 with a 10 parts per thousand depletion in the northern versus southern hemisphere. The C-14 content of CH4 measured at 48 degrees 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 C-14 content of CH4.