272 



Sediments 



'TTTTT/Z'y^wy/////, 



GROUND WATER: WELLS, SPRINGS, GEYSERS, MINES 

 (5 J analyses J 



STREAMS 



(249 analyses) 



SALT LAKES 



(S6 analyses) 



FRESH WATER LAKES 

 (40 analyses) 



CLEAR WATER ABOVE CORES 



INTERSTITIAL WATER OF CORES 

 (107 analyses) 



10,0 



Ol 

 Mc Atoms Si per Liter 



0.01 



0.00 



Figure 220. Concentration of dissolved silica in various kinds of waters. Note high concentrations in interstitial waters 

 of basin sediments. From Emery and Rittenberg (1952, Fig. 29). 



concentrated only where it is not rapidly re- 

 moved from solution by plant growth. Pos- 

 sibly, therefore, it has reached saturation 

 values in the interstitial waters of the basin 

 sediments. 



Although not a nutrient, strictly speaking, 

 sulfur in sediments has a cycle of regenera- 

 tion somewhat like those of nutrients, and 

 so it is worthy of mention here. Sulfur is 

 deposited in the sediments chiefly in the form 

 of dissolved sulfate ion of interstitial water 

 and as sulfur in the protenaceous and fatty 

 parts of organic matter. In the aerobic zone 

 it changes little, but at greater depths, be- 

 ginning at the top of the anaerobic zone, the 

 sulfur-reducing bacteria Desulfovibho re- 

 duces it to hydrogen sulfide. Some sulfide 

 ion reacts with iron of organic matter (Is- 

 satchenko, 1929; Le Calvez, 1951) or of 

 minerals, depositing iron sulfides. Iron 

 monosulfide, hydrotroilite, has not been 

 found in the basin sediments, although it 

 imparts the characteristic dark-gray-to-black 

 color to marsh sediments. The control as 

 to whether hydrotroilite or pyrite is present 

 may be rate of deposition, whereby more 

 iron is made available in marshes than in 

 basin sediments to react with the hydrogen 

 sulfide to form the monosulfide; in addi- 



tion, if pyrite is formed from hydrotroilite 

 as an intermediary, the slower rate of depo- 

 sition in basins may allow complete conver- 

 sion to pyrite in the time required for only 

 a few centimeters of sediments to be de- 

 posited, whereas great depth of burial may 

 be required in marshes before the conver- 

 sion occurs. 



Most of the hydrogen sulfide works its 

 way toward the surface, probably by dif- 

 fusion. When it enters the aerobic zone of 

 sediment or the overlying water, some of 

 the hydrogen sulfide is oxidized to molecular 

 sulfur, which is deposited, and some is oxi- 

 dized to sulfate ion. Some sulfate doubt- 

 lessly escapes to the overlying water into 

 which it mixes. Probably precise work will 

 show a higher sulfate-chloride ratio in the 

 bottom water than in shallower water in re- 

 sponse to this movement— just as is the 

 situation for regenerated nitrate and phos- 

 phate. A sulfate-chloride ratio in deep-sea 

 sediments off Brazil twice as high as normal 

 (F. W. Locher in Ahrens, Rankama, and 

 Runcorn, 1956, p. 206) may well be the re- 

 sult of such oxidation. In estuarine areas 

 some sulfate ion also replaces carbonate in 

 shells, converting them to gypsum, and 

 authigenic gypsum crystals form locally. 



