SECT. 3] PELAGIC SEDIMENTS 681 



water the solubility of celestite is 587 ^mole/1, at 25°C and 1 bar^ as compared 

 with 9.5 (jimole/l. for barite. Under the assumption of a linear increase in solu- 

 bility with substitution, the observed 5.4 mole per cent of celestite in the barite 

 structure would result in a solubility of 40.7 [xmole/l., or 4.3 times the solubility 

 of barite. As is seen in Table IV, the other substitutions observed further in- 

 crease the solubility of the crystal. 



Table IV 

 Solubility and Substitution Relations 



Gjrj^jsum has been observed in several instances in pelagic sediments with a 

 high content of organic matter. Whether these crystals were formed on the 

 ocean floor or during storage of the samples, as a result of sulfide oxidation, is 

 not known. 



C. Phosphates 



The phosphate concentration in surface sea-water is limited by photo- 

 synthetic organisms. These settle toward the bottom or are consumed by higher 

 members of the food chain, and a fraction of the phosphorus is ultimately 

 used by vertebrates for building an apatite skeleton. A continuous transport of 

 phosphorus from the surface layer of the ocean to the bottom is thus maintained. 

 The solubility of the ajDatite series is not accurately known (Sillen, 1959, p. 

 566); but it is a well-established fact that skeletal apatite is slowly dissolving 

 on the deep-ocean floor (Koczy, 1952; Arrhenius, 1959), and that no precipita- 

 tion occurs there of the phosphate formed by hydrolysis of organic matter. It is 

 thus obvious that the solubility of apatite at the temperature and hydrogen 

 ion concentration of present deep-ocean water is higher than the phosphate 

 concentration observed in bottom water and in interstitial water of deep-sea 

 sediments (5.0 [xmole/1., Arrhenius and Rotschi, 1953; 5.5 mole/1., Bruejewicz 

 and Zaytseva, 1958). Consequently, all the organic phosphorus and a large 

 fraction of the skeletal apatite reaching the deep-ocean floor are returned to 

 solution in the deep water and ultimately brought back to the upper layer of 

 the ocean. The recycling of phosphate in the ocean due to temperature and pH 

 control of the solubihty leads to partial removal of phosphate from the deep- 

 ocean floor and to permanent accumulation of skeletal apatite and of authigenic 



1 Owing to the presence of sulfate ion the solubility is somewhat lower in sea-water 

 (approx. 500 (xmole/l. at 25°C and 1 bar) than in pure water (Miiller and Puchelt, 1961). 



