64 



MARINE BOTTOM SAMPLES OF LAST CRUISE OF CARNEGIE 



to note, however, that in all cases except sample 21 the 

 silica sesquioxide ratio is narrower than in the whole 

 samples, owing usually to a large increase in alumina. 

 Except for sample 17, the silica ferric oxide ratios are 

 much wider than in the whole samples. 



Base Exchange Capacity 



The base exchange capacities of three whole sam- 

 ples of clay and of four fine fractions (two from Globig- 

 erina oozes and two from north Pacific red clays) were 

 determined by Professor W. P. Kelley. By base ex- 

 change capacity is meant the amount of cations, usually 

 expressed as milliequivalents per 100 grams, contained 

 by a given material and exchangeable for the cations of a 

 neutral salt solution. This quantity was determined by 

 leaching with neutral normal ammonium acetate, and the 

 total base exchange capacity represents the quantity of 

 NH4 ions absorbed by the sample. The amounts of cati- 

 ons removed from the sample in all cases exceed the 

 amounts of NH4 ions absorbed from the solution. This 

 shows that the samples analyzed contained substances 

 which merely dissolved in the salt solutions. The results 

 are as follows: 



Table 14. Replaceable and soluble bases and 

 base exchange capacities 



Milliequivalents per 100 grams base exchange 



The base exchange capacities of the colloids sepa- 

 rated from samples 73 and 77, which are noncalcareous 

 red clays, are close to those of three soil colloids, nos. 

 431, 3232, and 7083, investigated by Kelley, Dore, and 

 Brown (1931). (The smaller base exchange capacities of 

 the Globigerina oozes, nos. 17 and 19, and of entire sam- 

 ples 34, 72, and 77 are owing to the lesser amounts of 

 clay and colloids present in them.) Soil no. 431 is de- 

 signated as Ramona clay loam taken near La Habra, Cali- 

 fornia, and represents a soil of granitic origin recently 

 formed under semiarid conditions. Soil no. 7083, called 

 Dublin clay adobe, taken near Gilroy, California, was 

 also formed under semiarid conditions, whereas no. 3232 

 is a glacial drift soil from Indiana. Optical and other 

 studies indicate that nos. 431 and 7083 contain in addition 

 to quartz a beidellite-like mineral. 



The mineral beidellite was found by these authors to 

 have a base exchange capacity of about 51 milliequiva- 

 lents per 100 grams, whereas in three bentonites inves- 

 tigated by them, the exchangeable bases varied in amount 

 between 110 and 35 milliequivalents. The base exchange 

 capacities of kaolinite and halloysite are much lower 

 than any of these values, and those of zeolites are a great 

 deal higher, the amounts of exchangeable bases in natro- 

 lite and stilbite, for example, being respectively 221 and 



312 milliequivalents per 100 grams. A soil colloid con- 

 taining halloysite, investigated by Kelley, Dore, and 

 Brown, had a base exchange capacity of only 18 milli- 

 equivalents. 



Kelley and Liebig (1934), among others, have shown 

 that the chief replaceable base of soils which have been 

 in contact with sea water is magnesium. Correspond- 

 ingly, it may be seen from table 14 that, except for the 

 Globigerina oozes in which large amounts of calcium 

 carbonate were dissolved by the ammonium acetate, 

 magnesium is the principal cation removed from the 

 analyzed samples by the leaching solution. 



Nitrogen and Organic Matter 



In addition to the values for nitrogen in table 9 (de- 

 termined by Dr. Trask), calculated values for organic 

 carbon and organic matter are also given. These calcu- 

 lations are based on the fact that both in soils and in 

 marine sediments there has been found to be a more or 

 less constant ratio between organic carbon and nitrogen, 

 and, at least in soils, between organic carbon and total 

 organic matter. The values of these ratios are depend- 

 ent on the chemical nature of the source material, on 

 the conditions under which decomposition takes place, 

 and on the microorganisms which are active in the de- 

 composition process. Trask (1932) found that with near- 

 shore sediments the carbon nitrogen ratio is approxi- 

 mately 8.4, but this ratio would be expected to be greater 

 in deep-sea sediments in which the rate of accumulation 

 of organic matter is probably very slow, thus allowing 

 decomposition under oxidizing conditions to proceed un- 

 til only the most resistant constituents of the marine hu- 

 mus remain. These, as pointed out by Waksman (1933), 

 are probably lignins and related substances which have a 

 high carbon nitrogen ratio. Waksman found a rather 

 large variation in the carbon nitrogen ratio of four deep- 

 sea samples from the Atlantic, but the average is close 

 to that of soils, namely, 10, and this factor has been used 

 in calculating the organic carbon. The ratio of 1.9 be- 

 tween organic carbon and total organic matter is also 

 that suggested by Waksman. 



A study of chart 3, which gives the distribution of 

 nitrogen in the Carnegie deposits, shows that in pelagic 

 deposits the highest contents of nitrogen are found in 

 fine-grained noncalcareous red clays and siliceous oozes, 

 whereas the nitrogen content of normal and ferruginous 

 Globigerina oozes is quite low. The northeast Pacific 

 red clays contain between a third and a half as much ni- 

 trogen as the average fine-grained near -shore sediments 

 of the American coast (see Trask, 1932). In general, the 

 amounts of nitrogen (and therefore, presumably, the 

 amounts of carbonaceous and nitrogenous organic mat- 

 ter) are of about the same order of magnitude in red 

 clays and in siliceous oozes, even though the latter are 

 built up, in large part, of the siliceous remains of organ- 

 isms. This fact supports the suggestion made previous- 

 ly, that the organic matter in pelagic deposits represents 

 an undecomposable residue, perhaps, in part, of continen- 

 tal origin, the amount of which in a sediment is largely a 

 function of the rate of deposition of inorganic material. 

 The very low nitrogen content of south Pacific Globiger- 

 ina oozes, however, probably is owing not only to dilution 

 by the shells of pelagic foraminifera but also to the small 

 supply of humus from surrounding land and the low 

 plankton population of the surface waters. 



