270 



Sediments 



Figure 218. Diagram showing 

 the control exerted by pH and 

 Eh on the movement of certain 

 ions across the sediment-water 

 interface. Width of arrows in- 

 dicates relative quantities for 

 different conditions. From 

 Emery (1958^7, Fig. 14). 



tion of nitrate by the sediment should also 

 be reflected by a concentration of ammonia 

 and nitrite in the bottom waters of these 

 basins; however, surveys of these eff'ects are 

 incomplete. 



If the decrease of total nitrogen at depth 

 in the sediment of Santa Barbara Basin is 

 ascribed to ammonia alone, the rate of es- 

 cape of ammonia from the top 4 meters of 

 sediment is 1.6 jiig-atoms/sq cm/yr. If aU 

 is oxidized to nitrate in the basin water be- 

 tween bottom and sill, it would increase the 

 nitrate content of the water 5 jUg-atoms/hter 

 in 20 years. Because the nitrate of this 

 water is actually within 5 /xg-atoms/liter of 

 the concentrations in the water that enters 

 at sill depth, the water of the basin must be 

 completely replaced in less than 20 years. 

 Ammonia from layers deeper than 4 meters 

 in the sediment must also escape, so the 

 turnover time of the basin water may be 

 much shorter than 20 years. This short 

 period of overturn is supported by the re- 

 lated depletion of dissolved oxygen in oxi- 

 dation of organic matter in the sediment as 

 discussed by Emery and Rittenberg (1952). 

 About one-third of the organic carbon 

 originally deposited in an annual layer (3.5 

 per cent by dry weight) is oxidized to car- 

 bon dioxide by the time it has been buried 

 to a depth of 4 meters, about 2300 years. 



At this rate of oxidation, 2.0 ml of oxygen 

 must annually diffuse through each square 

 centimeter of sediment from the overlying 

 basin water. Loss of this much oxygen 

 from the basin water corresponds to 0.13 

 ml/liter/yr for the whole water column be- 

 tween sill and bottom depth. Since the 

 oxygen content of the basin water is within 

 0.1 ml/liter of that which enters, the basin 

 water must be completely replaced at least 

 once each year. 



The cycle of phosphate in the interstitial 

 waters is less complex than the cycle for 

 nitrogen, because there are no intermediate 

 forms between the phosphorus of organic 

 matter and the orthophosphate dissolved in 

 the water. Again, however, an examination 

 of the amount of dissolved phosphate in 

 interstitial water shows differences between 

 the three basins (Fig. 219). By far the 

 greatest concentration occurs in Santa 

 Barbara Basin where phosphate reaches 

 nearly 200 /xg-atoms/liter in the sediment in 

 contrast to typical values of only 3.0 to 3.5 

 jLig-atoms/liter in the basin water. In sed- 

 iments of Santa Monica Basin phosphate 

 ranges from slightly more to slightly less 

 concentrated than in basin water. In sedi- 

 ment of Santa Catalina Basin the dissolved 

 phosphate of the aerobic zone has a con- 

 centration similar to that in Santa Monica 



