Dissolved Nutrients 



269 



less in the anaerobic zone at depth. Nitrate 

 was doubtfully present near the top of the 

 core from Santa Monica Basin and was not 

 detected at Santa Barbara Basin. More 

 abundant by 15 to 1000 times than the total 

 nitrogen present in all dissolved compon- 

 ents is the nitrogen of particulate organic 

 matter. It decreases in abundance with 

 depth, causing the total for all nitrogen con- 

 stituents also to decrease with depth. 



Interpretation of this depth distribution 

 of nitrogen components is simplified by as- 

 suming the existence of steady-state con- 

 ditions throughout the period of deposition 

 of the cores; that is, in a relatively constant 

 environment a study of the change in a 

 property with depth of burial is essentially 

 the same as a study of the alteration of the 

 property with time in an isolated mass of 

 sediment. By this method changes pro- 

 duced in the sediments by bacterial activity 

 can be traced for thousands of years, 

 rather than for the brief periods that are 

 possible in laboratory experiments. As 

 soon as sediment containing organic matter 

 is deposited, ammonia begins to form as 

 the first stage of oxidation. Where ammonia 

 is released into an aerobic environment 

 such as the tops of some of the cores, it is 

 further oxidized to nitrite and thence to 

 nitrate by a specific group of bacteria, 

 Nitrobacteriacea, obligate aerobic forms 

 that cannot nitrify in reducing environments. 

 Because aerobic conditions exist only at the 

 top of the core from Santa Catalina Basin, 

 this is the only core having appreciable dis- 

 solved nitrate. The amount of new nitrate 

 formed here is greatly amplified by the in- 

 troduction of large new supplies of oxygen 

 by the activities of burrowing animals. 

 These supplies total many thousand times 

 the original supply of oxygen as shown by 

 Emery and Rittenberg (1952), and thus the 

 newly produced nitrate is many thousand 

 times the original amount. Nitrate in Santa 

 Catalina Basin persists to all depths reached, 

 although there is only a small remnant below 

 the depth of zero Eh, which is reached by a 

 layer of sediment about 5000 years after it 

 it first deposited. Under the reducing con- 



ditions of sediment in Santa Barbara Basin 

 nitrate is not formed, and even the nitrate 

 present in the sea water that is originally 

 trapped within the interstices of the sedi- 

 iment is soon reduced or denitrified to am- 

 monia. Because the sediments of Santa 

 Monica Basin are less anaerobic, the origi- 

 nal supply of nitrate may remain for several 

 years before it is reduced. In addition to re- 

 verting back to ammonia, nitrate may also 

 be denitrified to molecular nitrogen by ac- 

 tivity of some bacteria and by abiogenic re- 

 actions. This denitrification is slow and re- 

 quires nitrate or nitrite as starting materials, 

 so it can be expected only in sediments of 

 Santa Catalina Basin or of similar basins. 



The decrease with depth of the total nitro- 

 gen present in all forms means that nitrogen 

 must have escaped from the sediment into 

 the overlying water if steady-state conditions 

 existed during the time of deposition of the 

 entire length of core. In other words, the 

 original nitrogen content of the bottom- 

 most annual layer must have been the same 

 as that of the layer that is being deposited 

 now. From the sediments of Santa Barbara 

 Basin nitrogen could have escaped only in 

 the form of ammonia, but from Santa Cat- 

 aUna Basin it could have escaped as ammonia, 

 nitrite, nitrate, and molecular nitrogen. 

 Possibly some in both basins could also 

 have escaped as dissolved organic matter. 



Typically, the basin waters contain about 

 40 jag-atoms /liter of nitrate and no detect- 

 able nitrite or ammonia. Near the bottom, 

 however, nitrate may be present in greater 

 or smaller concentrations than this, and 

 small amounts of nitrite may also be present, 

 as shown by Rittenberg, Emery, and Orr 

 (1955). At the bottom of Santa Catalina 

 Basin nitrate reaches a value of 45 jUg-atoms/ 

 Hter, having been augumented by supplies 

 from the sediment by diffusion or perhaps 

 by some bodily movement of interstitial 

 water through channels (Fig. 218). At the 

 bottom of Santa Monica and Santa Barbara 

 Basins nitrate is impoverished to concen- 

 trations of only 35 and 30 jUg-atoms/liter, 

 respectively, indicating its passage from 

 overlying water into the sediment. Reduc- 



