154 EHHALT 



between recent biologic and fossil CH 4 sources. Similarly, the content of the 

 stable isotopes deuterium and 13 C in CH 4 may be used to identify the major 

 sources of atmospheric CH 4 . This is possible if the isotope content of CH 4 from 

 various sources is sufficiently different. The isotope data now available are 

 summarized in Table 5. The isotope contents are given as 6 values in % o; 5 

 represents the relative deviation of the isotope ratios, deuterium/hydrogen or 

 1 C/ 12 C, in a sample from that of a standard. For deuterium, the standard 

 generally used is "standard mean ocean water" (SMOW). 20 For 13 C, Pee dee 

 belemnite (PDB), a limestone, is used as the standard. A 5 value of — 100% 

 means that the sample contains 100% less deuterium or l 3 C than the standard. 

 From Table 5 it appears that the isotope contents are sufficiently different to 

 distinguish between the various sources, such as marshes, geothermal areas, or 

 natural-gas wells. Unfortunately, however, none of the sources agrees iso- 

 topically with the atmospheric CH 4 . In particular, the presumably largest 

 sources, paddy fields and marshes, emit CH 4 that has an average l 3 C content of 

 — 65/ 00 , which is much lighter than the average for atmospheric CH 4 , — 41°/ 00 . 

 This difference can be explained by isotopic fractionation in the reaction of CH 4 

 with OH. Both the collision frequency and the kinetic effects would favor the 

 reaction of the lighter molecule 12 CH 4 , so that the CH 4 remaining in the 

 atmosphere is enriched in ' C. This explanation unfortunately also means that 

 the C content of atmospheric CH 4 cannot now be used to identify its source 

 and more work on the fractionation during reaction is required. It is interesting 

 to note in this context 21 that the average l C content of atmospheric CO is 

 about — 27% . If CH 4 is indeed the major source of CO, this would mean that 

 the carbon isotopes must be also fractionated in the oxidation of CO. 



Atmospheric CH 4 has a relatively high content of tritium, which is probably 

 due to the release of tritiated CH 4 from nuclear industry. Thus reaction 

 products of CH 4 which contain hydrogen should also have a high tritium 

 content. The molecular H 2 in the atmosphere has a high tritium content anyway 

 because of the release of tritiated H 2 from thermonuclear explosions and nuclear 

 industry. The only other reaction product that is long-lived enough to be 

 collected and analyzed for tritium is formaldehyde, which has been found in 

 rain. 22 Musgrave 23 enriched the formaldehyde from large amounts of rain and 

 indeed observed high tritium contents. 24 This finding lends more support to the 

 above-mentioned reaction scheme. 



CONCLUSION 



From the preceding descriptions, the following picture of the CH 4 cycle 

 emerges. The atmospheric CH 4 originates at the earth's surface, and about 80% 

 results from the anaerobic decay of recent organic matter. Upon entering the 

 troposphere, it reacts with the hydroxyl radical. This and the subsequent 

 reactions constitute the major sink of CH 4 and a major source of CO. There is 



