146 EHHALT 



14 C from nuclear explosions and should represent the natural 1 C level in CH 4 . 

 The average 14 C content is 80% of that of recent wood, which indicates that 

 80% of the CH 4 is of recent biologic origin and 20% is "dead" CH 4 . This 20% 

 represents an upper limit. The measured CH 4 samples have been provided by 

 air-liquefaction plants, which are usually situated in heavily industrialized areas. 

 Hence these samples might have been subjected to local contamination by 

 industrial (dead) CH 4 . 



The fact that most of the atmospheric CH 4 must be produced by the 

 anaerobic decay of recent organic matter simplifies considerably the search for 

 its sources. One can concentrate on the biologic sources. Table 2 lists the known 

 biologic sources and their annual production. The average source strengths and 

 the total areas of the sources are also included. Clearly, humid or marshy areas 

 that provide anaerobic conditions constitute the major sources. There the source 

 strength can be as high as 210 tons km -2 year -1 or 210 g m -2 year -1 . This is 

 nearly 10% of the annual net production of dry organic matter in a swamp in 

 temperate latitudes, which is 2500 g m" 2 year -1 . All other sources are con- 

 siderably weaker. By multiplying the source strengths with the global area, we 

 obtain the annual CH 4 production of the different ecological systems. Also on a 

 global scale, marshy areas are of major importance. There are a few discrepancies 

 in Table 2 which need to be discussed. First, we have two estimates for CH 4 

 production in the digestive tract of animals. Hutchinson's estimate 8 is 25 years 

 older, and Singer's larger estimate 9 reflects mainly an increase in cattle 

 population (1.5% per year). The difference between the two estimates for the 

 production of swamps is more serious. Robinson and Robbins 1 assumed that 

 swamps and paddy fields have the same production rate per unit area. A rather 

 careful estimate by Koyama 1 r for the CH 4 production rate of paddy fields is 

 based on the mean CH 4 productivity of paddy soils and the mean depth and 

 temperature of the paddy fields (Table 2). However, extrapolation of the 

 production rate per unit area in paddy fields to that in swamps implies that the 

 average depth, temperature, and chemical character of marshy and paddy soils 

 are the same. This is unlikely, and one would like to have an independent value 



