SECT. 3] 



PELAGIC SEDIMENTS 



705 



of organic carbon and nitrogen first decreases rapidly with depth in the sedi- 

 ment and then slowly as the remainder contains an increasingly larger proportion 

 of refractory compounds (Fig. 31) (Mironov and Bordovsky,1959). Correspond- 

 ingly, less resistant structures, such as scales of fish and the chitinous beaks of 

 cephalopods, are found only in the surface layer of the sediment. The decom- 

 position of proteins and aminosugar in the sediment produces ammonia and 

 nitrate, which distribute themselves between the interstitial solution and 

 sorption sites on the minerals. Typical values range between 0.1 and 4 mmole 

 of NH4+ per kg of solid, and similar amounts per liter of interstitial solution 



Ratio C/N 



c 

 E 



Qi 



Q. 

 CD 

 Q 



Fig. 33. Frequency distribution of the carbon/nitrogen ratio as a function of depth in 

 Pacific north equatorial clay sediments. The right-hand graph shows the number of 

 samples measured within each of the depth classes limited by horizontal lines. (From 

 Arrhenius, 1952.) 



(Fig. 32b) (Bruejewicz and Zaytseva, 1959; Shiskina, 1958). The oxidation state 

 of the interstitially dissolved nitrogen compounds changes rapidly with depth 

 in the sediment, so that at the surface a considerable portion is nitrate whereas 

 at depth ammonium ion predominates in solution (Fig. 32d), In the progressive 

 decomposition of pelagic organic matter, carbon is lost more rapidly than 

 nitrogen and consequently the ratio of carbon to nitrogen in the sediment de- 

 creases with depth below the sediment surface, i.e. with geological time (Fig. 

 33). The combined influence of origin and diagenesis of the organic matter on 

 its composition is illustrated in Fig. 34. 



Most of the refractory solid organic matter consists of the original and de- 

 composed organic molecules interstratified with, and protected by, the apatite 



