174 HILL 



circulation of sulfur through the biosphere, and this compound was therefore 

 the one put forth whose emission would make up for apparent emission deficits. 

 It was and is a convenient choice since its fate is thought to be known and to be 

 consistent with other notions about the sulfur cycle, i.e., it is thought to be 

 oxidized to S0 2 and sulfates, the other principal forms of sulfur in the 

 atmosphere. 



However, in possible contradiction of this view, evidence has recently been 

 obtained which offers the possibility that other volatile compounds of sulfur of 

 biogenic origin, such as dimethyl sulfide, may serve in place of H 2 S in 

 contributing to the apparent source deficit. If these other compounds should in 

 fact be found to constitute the principal biogenic contributions to the 

 atmosphere and if, in addition, they are not converted in the atmosphere largely 

 to S0 2 and sulfate, then it will not be possible to account for the large amounts 

 of these latter materials which are removed from the atmosphere, and present 

 concepts of the sulfur cycle will be called into question. 



In the following discussion we will, at least at the outset, continue to regard 

 H 2 S as the principal compound of biogenic origin emitted into the atmosphere. 

 Toward the end of the section, we will present the evidence for emission of 

 other biogenic sulfur compounds. 



In recently prepared estimates of the atmospheric sulfur budget, a number of 

 different approaches were used to arrive at the biogenic H 2 S transfer rate. 

 Junge 7 derived a terrestrial emission rate of 70 million tons S/year by assuming 

 that the sulfur absorbed from the atmosphere by soil and vegetation was released 

 to the atmosphere as H 2 S. By requiring the inputs and outputs of the 

 pedosphere to be in balance, Eriksson 5 estimated a terrestrial emission of 110 

 million tons/year. On a similar basis, Robinson and Robbins 2 estimated 68 

 million tons/year, and Friend 8 estimated 58 million tons/year. 



Estimates of emission from the oceans range from 170 (Ref. 5) to 30 million 

 tons/year (Ref. 2). These figures follow from invoking a balance of total sulfur 

 in the atmosphere over the oceans or within the oceans themselves. Kellogg et 

 al. 3 assert that the oceans cannot be a source of H 2 S, since its oxidation within 

 seawater would be too rapid to permit appreciable escape to the atmosphere. 

 Instead, they suggest that emissions must be terrestrial in origin and that 

 shoreline emission may be important. In separately balancing total sulfur inputs 

 and outputs in the northern and southern hemispheres, they estimate a total 

 global emission of 90 million tons/year. 



The overall emission process to be characterized has the following features. 

 As mentioned earlier, hydrogen sulfide is generated by bacteria in two ways. In 

 one way, dissimilatory sulfa te-reducing bacteria mediate the reduction of sulfate 

 ion to H 2 S, obtaining the necessary reducing power from organic material. 

 The organisms involved are strictly anaerobic and may be found in water and 

 sediments and in terrestrial soils. In the second way, the source of sulfur is the 

 reduced organic form found in dead plant and animal matter. Sulfur in living 

 cells is predominantly in the form of protein sulfur. The release of sulfide from 



