Dissolved Nutrients 



267 



hematite near the sediment surface, siderite 

 at intermediate depth, and pyrite nearer the 

 bottoms of many cores. Organic matter 

 should be stable at all negative Eh's. In 

 reality, however, conditions are not so sim- 

 ple as suggested by the geological fence dia- 

 grams, because of the presence of micro- 

 environments so small as to defy precise 

 measurements with existing equipment and 

 because of the activity of biogenic processes 

 that cause organic matter, for example, to 

 continue to break down in spite of its 

 presence in an environment of negative Eh. 

 A good example of the importance of 

 microenvironments is given by pyrite. In 

 the tops of cores above the level of zero Eh 

 pyrite forms internal casts of foraminiferans, 

 diatoms, and radiolarians. These casts are 

 highly porous and very fragile. Evidently, 

 local spots of negative Eh produced by de- 

 composing protoplasm of these organisms 

 are more or less insulated from the positive 

 Eh of the matrix. Issatchenko (1929) also 

 noted the development of minute crystals of 

 pyrite within cells of sulfate-reducing bac- 

 teria, and Le Calvez (1951) found iron sul- 

 fides forming within foraminiferal tests, 

 some developing into muriform masses like 

 those pictured in Figure 216. At depths 

 below the level of zero Eh in the cores the 

 pyrite forms in addition large irregular 

 masses as though it had grown beyond the 

 confines of shells and filled the interstices of 

 grains as a type of cement. Both sorts of 

 pyrite are grouped in Figure 216. A few 

 octahedrons served to identify the material 

 as pyrite rather than marcasite, agreeing 

 with experiments of Allen, Crenshaw, 

 Johnston, and Larsen (1912) which show the 

 characteristic deposition of iron disulfide as 

 pyrite in alkaline-to-neutral media and as 

 marcasite in acid media. 



Dissolved Nutrients 



Nutrients are chemical compounds that 

 are needed for plant growth but are some- 

 times present in such small concentrations 

 in sea water that growth is limited. Chief 

 of these are dissolved nitrate, phosphate, 

 and silica. Although plants require much 



Figure 216. Grains of pyrite from zone of negative Eh in 

 Core 1983 from East Cortes Basin. Present are large 

 irregular masses and internal casts of foraminiferans F, 

 diatoms D, and radiolarians R. 



larger quantities of carbon, oxygen, and 

 hydrogen, these elements are always so 

 abundant that they do not limit growth. 

 Still other elements such as zinc, iodine, 

 and copper are also needed by organisms, 

 but in such minute quantities that growth is 

 not limited even though they are very minor 

 constituents of sea water. Animals need 

 the same building materials, but they obtain 

 them mostly from plants and plant debris. 

 The ultimate source of the nutrients is the 

 land (Emery, Orr, and Rittenberg, 1955), 

 but the plants each year use more nutrients 

 than are newly contributed to the ocean. A 

 balance is provided by the fact that most of 

 the nutrients extracted from the water by 

 plants are returned on the death of the 

 organisms. According to Riley (1951), 90 

 per cent of the organic matter produced by 

 plants is regenerated in the top 200 meters. 

 And additional regeneration occurs at 

 greater depth in the water column and from 

 the bottom sediments during the period 

 after deposition and before burial to great 

 depth. 



An examination of the distribution of 

 nutrients in basin sediments and overlying 

 waters was made by Rittenberg, Emery, and 

 Orr (1955). Three basins were selected as 

 representative of all basins: Santa Catahna 

 because of its change from aerobic to an- 

 aerobic conditions at a depth of about 2 



