230 



Sediments 



posited at a more uniform rate than is the 

 detrital component of the sediment. When 

 only the green muds are considered, organic 

 matter shows a downward decrease in per- 

 centage which is rapid for the first meter or 

 so and gradual at greater depth. The transi- 

 tion between rapid and slow rate of decrease 

 occurs near the depth of zero oxidation- 

 reduction potential {Eh), signifying that oxi- 

 dation must be slower after all the dissolved 

 oxygen originally present in the interstitial 

 water has been consumed by oxidation of 

 organic matter soon after deposition while 

 it is still in the upper, recent, layers of sedi- 

 ment. Approximately one-third the total 

 content of organic matter is gone by the time 

 it becomes buried to the depth of zero Eh, 

 zero to more than 6 meters, depending on 

 the particular basin. 



Reference to Figure 86 reveals that the 

 standing crop of diatoms (the chief pro- 

 ducers of organic matter) varies by a factor 

 of 1000 in the region, much more than the 

 variation of organic matter in the basin sedi- 

 ments; moreover, the areas of greatest pro- 

 duction are not necessarily the areas of 

 greatest abundance in the sediments. Evi- 

 dently, the areal and depth variation of or- 

 ganic matter in the sediments is more a func- 

 tion of dilution and oxidation than of 

 production in the overlying water. 



The control of organic content by grain 

 size of whole sample is borne out by analy- 

 ses of organic nitrogen present in different- 

 size fractions of samples. Samples from the 

 tops and bottoms of ten basin cores were 

 separated by decantation into five size frac- 

 tions. After carbonates were determined 

 (Fig. 189, Table 15), nitrogen was measured 

 by Kjeldahl analysis on each of the 100 frac- 

 tions. The results were somewhat erratic, 

 probably because of some solution and other 

 minor losses of organic matter during han- 

 dling of the samples, but average nitrogens 

 for the size fractions are as follows: 



>62 microns 



62 to 16 



16 to 4 



4 to 1 



<1 



0.25 per cent 



0.29 



0.33 



0.42 



0.48 



The progressive increase of average nitrogen 

 content with decreasing grain size of the 

 fractions shows the existence of a definite 

 affinity of organic matter for fine-grained 

 sediments. This may be a result of similarity 

 in their settling velocities or of adsorption 

 of organic matter on clay minerals. Al- 

 though the finest grain-size fraction contains 

 the highest percentage of nitrogen, the total 

 weight of this size fraction is small compared 

 to the weight of coarser fractions (Fig. 189). 

 As a result, about 65 per cent of the nitrogen 

 is associated with grains having diameters 

 between 1 and 16 microns. 



In addition to variations in the percentage 

 of total organic matter, the basins also pres- 

 ent variations in composition of organic 

 matter. Since these are rather involved, they 

 are described in a separate and later section. 



F. Mineral composition. The bulk of the 

 sediments of clay size (<4 microns) usually 

 consists of clay minerals, members of the 

 kaolinite, montmorillonite, illite, or chlorite 

 groups. It is generally considered that kaoli- 

 nite is characteristic of an acid environment 

 in which leaching of calcium, magnesium, 

 and iron is active. Montmorillonite forms 

 in an alkaline environment having magne- 

 sium, calcium, and ferrous iron. Illite re- 

 quires a similar environment but one that 

 includes potassium; chlorite needs a similar 

 one but with especially high magnesium 

 (Grim, 1951, 1953; Keller, 1953). According 

 to G. Millot (Grim, 1951), in lake sediments 

 kaolinite is dominant, except that if much 

 calcium carbonate is present illite may be 

 important instead; in brackish lagoonal sedi- 

 ments ilhte or montmorillonite is dominant; 

 and in marine sediments illite is dominant. 

 Because of the presence of abundant potas- 

 sium and magnesium in sea water, some of 

 the kaolin and montmorillonite brought to 

 the ocean is converted during diagenesis to 

 illite and chlorite. 



Although these are the general environ- 

 mental relationships that can be expected 

 through physical-chemical factors of the en- 

 vironments, many complications prevent 

 strict interpretations of environments from 

 clay mineralogy alone. Perhaps chief of the 

 complicating factors in the marine environ- 



