SECT. 3] BASIN SEDIMENTATIOISr AND DIAGENESIS 603 



soluble (120-140 ppm below pH 9) as well as being an important source con- 

 tinually provided by diatoms (Siever, 1957). The oceans on an average contain 

 about 5 ppm and so are highly undersaturated. Deep waters, especially in the 

 higher latitudes, contain considerably larger quantities. Interstitial waters from 

 the basins in southern California (Rittenberg, Emery and Orr, 1955) showed a 

 tenfold increase in silicon relative to the overlying basin waters. There is a steady 

 increase with depth and in the three basins measured a maximum was reached 

 at about 120-150 cm. It cannot be concluded at the present stage whether these 

 depths constitute a real maximum until further and deeper measurements are 

 made. 



The actual mechanism influencing silica solution is not known. It is possible 

 that a time factor is necessary. It is further possible that bacterial processes 

 result in the breakdown of a protective film which encases the diatom frustule. 

 Lewin (1959) found that removal of this film with nitric acid increased the 

 solubility of the tests in dilute alkali by 50%. 



Sulfur in the ocean is not usually considered in a discussion of nutrients 

 since it is normally abundant (880 mg/kg as the sulfate ion) but nevertheless 

 it is an essential constituent of all living matter. In the aerobic zones of the 

 ocean the concentration of sulfate varies only slightly. In reducing basins, such 

 as the Black Sea, the disappearance of sulfate, as well as the presence of other 

 forms of sulfur (Danil'chenko and Chigirin, 1929), has been reported for the 

 waters. The sediments of basins may present a complex mixture of sulfur 

 compounds. 



Studies on the sediments from the southern California basins show that 

 apart from sulfate, free sulfide, acid volatile iron monosulfide (hydrotroilite), 

 fixed sulfide (pyrite), elementary sulfur and organic sulfur can occur in varying 

 concentrations. In the highly reducing sediments where sulfide formation is 

 active, the sulfate concentration decreases rapidly in the first 50 or 60 cm and 

 then more slowly, down to one-tenth or less of the concentration in sea- water at a 

 depth of about 6 m. In the Santa Catalina Basin, where oxidizing layers overlie 

 reducing ones, there is good evidence to suggest that a reoxidation of bottom 

 sulfides has occurred to form sulfates at a higher concentration than initially 

 present in the sea- water. 



The concentration of elementary sulfur is directly proportional to the content 

 of free and acid volatile sulfide. In the oxidizing basins, or those with small 

 sulfide content, native sulfur is absent. Organic sulfur and pyrite ajopear to be 

 proportional to the content of organic matter. Thus, in oxidizing or slightly 

 reducing sediments having high organic contents and low free sulfide and hydro- 

 troilite (e.g. Santa Catalina Basin), the content of pyrite is high. The mechanism 

 of pyrite formation is as yet unknown, although most observations indicate that 

 it probably has an authigenic origin. 



d. Oxidation-reduction potentials and pH 



The oxidation-reduction potential (Eu) of a marine sediment is influenced 

 more by the activity of organisms than by any other factor. Microorganisms, 



