276 



Sediments 



and for those at 300 cm 13.0. Both values 

 exhibit a progressive increase over Trask's 

 8.0, which was for samples mostly between 

 and about 10 cm. This increase of C/N 

 ratio continues through the Pliocene and 

 Miocene strata of Los Angeles Basin (Table 

 23) and through older sedimentary rocks 

 elsewhere. For general computations of the 

 content of total organic matter in sediments 

 the C/N ratio at the surface has been taken 

 as 10, an approximate average of Trask's and 

 our values. 



Table 23 



Average C/N of Various Sedimentary 

 Materials 



* Table 22, combustion. 



-Trask (1932, pp. 282-284), average of 13 analyses — 

 titration. 



^ Emery and Rittenberg (1952), average of 10 analyses 

 — combustion. 



^ Trask and Patnode (1942, pp. 423-461), average of 

 3600 analyses — titration. 



^ Trask and Patnode (1942, p. 34), average of 6865 

 analyses — titration. 



'^ZoBell(1945). 



Carbons by combustion for the surface 

 and the 3-meter levels of nine of the cores 

 of Figure 222 plus three later cores (Orr and 

 Emery, 1956(3) yielded averages of 4.23 per 

 cent carbon at the surface and 2.76 at depth. 

 If steady-state conditions obtained during 

 the period of deposition of these materials, 

 the sediment now at 3 meters originally 

 contained 4.23 per cent organic carbon. 

 Evidently, during its burial to 3 meters, 35 

 per cent has been lost by regeneration. 

 Since the C/N ratio increases with depth, 

 more nitrogen than carbon has been regener- 



ated, 45 per cent. Lacking data on losses 

 of oxygen and hydrogen, we might round 

 off the regeneration of total organic matter 

 in the top 3 meters of sediment at one-third 

 of the near-surface concentration for basin 

 sediments (Table 14, Fig. 196). This is 

 probably a better basis for estimating re- 

 generation than the carbonate-to-nitrogen 

 ratio used by Emery and Rittenberg (1952) 

 by which a 50 per cent regeneration was 

 computed. The latter method depends on 

 the now-realized change in rate of deposi- 

 tion of carbonate during the past and on 

 the greater rate of regeneration of nitrogen 

 than of carbon. 



As pointed out by Trask (1932, p. 181), 

 Bader (1955), and others, the C/N ratio is 

 controlled by the kind of materials deposited 

 in the sediments and by the nature of its 

 decomposition. The C/N ratio alone then 



% NITROGEN 



Figure 222. Organic carbon versus nitrogen (per cent by 

 dry weight of total sediment) for samples from basins. 

 Triangles and circles are for carbon based on wet com- 

 bustion for samples at depths between 0-35 cm and 

 275-325 cm below water-sediment interface, respectively. 

 Dots are for carbon based on titration for samples at all 

 depths. For comparison, crosses show data obtained by 

 Trask (1932, pp. 282-284) for samples in basins mostly 

 between and 10 cm for which carbon was determined 

 by titration. The hne is the best fit for 3600 analyses 

 based on carbon titration for Los Angeles Basin (Trask 

 and Patnode, 1942, pp. 423-461). 



