602 KAPLAN AND RITTEXBERG [CHAP. 23 



bacterial decom])ositioii of the organic matter; thus, a decrease in organic and 

 an increase in inorganic phos])horns with depth in the sediment should be 

 found if these forms could be distinguished analytically. The solubility of the 

 phosphate ion is very susceptible to pH changes and the presence of divalent 

 cations. At a low ]iH it exists as the acid H2PO4" whose salts are relatively 

 soluble. At more alkaline pH, above 7.2, HP042~ is dominant and may even 

 dissociate to VO^^^, which forms insoluble phosj^hates with divalent cations. 

 Thus the phos])hate content of the interstitial water is not necessarily directly 

 related to the state of decomposition of organic matter. In fact, the sediments 

 in the southern California basins show concentrations of soluble phosphate up 

 to fifty times the values reported for sea-water. The content varies from basin 

 to basin and \\itli depth in the sediment column. 



Although phosphorus has only one known valence state in the sediments — 

 pentavalent — its concentration in solution appears to be markedly affected by 

 the oxidation-reduction potential. This is very probably only a secondary 

 l^henomenon; more directly the phosphate is proportional to the sulfide content 

 of the sediment. Baas Becking and MacKay (1956) found that treatment of 

 suspensions of metal phosphates in sea-water with H2S released HPO42- and 

 H2P04^ causing a marked drop in pH. They also demonstrated that marine 

 algae (e.g. Eiiteromorpha) are capable of releasing bound phosphate from the 

 sediment as a result of liberating organic acids which have a marked solubilizing 

 action. As mentioned earlier, the stagnant waters of the Black Sea, Lake Mara- 

 caibo and some Norwegian fjords contain abnormally high phosphate. Sugawara, 

 Koyama and Kamata (1957) demonstrated the direct relationship between the 

 H2S content and the phosphate in the interstitial water of a series of freshwater 

 and brackish lakes in Japan. The same relationship is found in the southern Cali- 

 fornia basin sediments; phosphate is abundant throughout the Santa Barbara 

 Basin where free H2S is available at all depths so far cored, and increases in the 

 Santa Catalina Basin where the reducing environment is reached. In the 

 Santa Monica Basin, where no free H2S is available, the phosphate content 

 of the interstitial water is low at all depths. 



The distrilnition of phosphate in the interstitial water can be explained as 

 follows. Bacterial sulfate reduction is responsible for the formation of H2S and 

 CO2 in the sediments. These molecules effectively bind Fe-+ as FeS and Ca2+ 

 as CaCOa, which are highly insoluble and preci]iitate preferentially to phos- 

 phates. The removal of the metal cations enables the ])hosphate ion to remain 

 in solution. In the Santa Monica Basin the supply of detrital material rich in 

 iron exceeds the sulfide ])roduction. There is, therefore, no competition for the 

 calcium and iron binding the phosphate and it remains in the insoluble state. 



Silicon, like phosphorus, is known to have only one valence state in the natural 

 environment. It seems to be little affected by the oxidation state but is in- 

 fluenced by the pH. According to Krauskopf (1956) and White, Brannock and 

 Mm*ata (1956), working on the solubility of opal, silica solubility increases 

 rapidly at a pH above 9, but is little affected below it. Of the various forms of 

 silicon dejiositing on the ocean bottom, amorphous silica is probably the most 



