SECT. 1] THE INFLUENCE OF ORGANISMS ON THE COMPOSITION OF SEA-WATER 



45 



Cariaco Trench and Black Sea, Richards and Vaccaro (1956) have shown that 

 organic decomposition proceeds in proportion to the apparent oxygen utiliza- 

 tion until the supply of oxygen is exhausted. After this, the phosphorus con- 

 tinues to increase at a rate which indicates that in its regeneration each sulfide 

 ion is equivalent to approximately four oxygen atoms in accordance with 

 the stoichiometrv of the equation for sulfate reduction. 



The reduction of carbon dioxide to methane is well known to occur under 

 anoxic conditions in freshwater systems, where it gives rise to the production 

 of marsh gas. We know of no direct evidence that this takes place in sea-water. 

 Perhaps this is because sulfate is present in sea-water in excess of the amounts 

 required for the oxidation of the organic matter. 



Carbon dioxide is produced by the oxidation of organic matter by any of the 

 hydrogen acceptors. Because the accumulation of organic matter is greater in 

 anoxic basins, its decomposition produces larger changes in the total concentra- 

 tion than is usual in ocean waters. In the Black Sea, the total carbon present 

 as HC03~ and CO3 2 " increases from 3275 mg atoms/m 3 in the surface water to 

 4263 mg atoms/m 3 at a depth of 2000 m (see Fig. 7). This is an increase of 30%. 



100 200 



S 2 " (mg atoms/m 3 ) 



300 



Fig. 7. Relation of sulfide sulfur and total carbonate carbon in waters of the Black Sea. 

 Numbers indicate depth of samples. Slope of line corresponds to ZlS 2- /zJC = 0.36. 

 (From data of Skopintsev et al., 1958.) 



In contrast, the oxidation of organic matter in the depths of oceanic waters 

 increases the total carbonate by only 10%. It follows that the quantity of 

 organic matter which has decomposed in the deep water of the Black Sea is 

 about three times that characteristic of the open ocean basins. 



The change in total carbonate carbon with increasing depth is proportional 

 to the change in sulfide content and proceeds in the ratio JS/zlC = 0.36 (see 

 Fig. 7). This is somewhat less than the ratio of 0.5 given by the equation for 

 sulfate reduction. 



When separate components are considered, the changes in chemical composi- 

 tion of sea-water in anoxic basins thus appear to proceed in about the pro- 

 portions required by the equations shown on page 43. If it is assumed that 



