MARINE BOTTOM SAMPLES OF LAST CRUISE OF CARNEGIE 



A division of the sea floor on the basis of the pre- 

 vailing conditions of oxidation or reduction has been 

 suggested by Murray and Hjort (1912) who state: "We 

 may indeed divide the floor of the sea, according to the 

 relative abundance or paucity of dissolved oxygen in the 

 bottom waters, into oxidizing and reducing areas. Re- 

 ducing conditions will prevail wherever there is a larg- 

 er excess of putrifiable organic matter than can be 

 coped with by whatever supply of oxygen (depending on 

 the circulation of the area) may be available. In gener- 

 al, therefore, the coast-lines of continents are girdled 

 by reducing areas, and it is here that blue muds char- 

 acteristically occur. Oxidation of the organic matter is 

 here effected at the expense of ferric iron, probably by 

 bacterial agency." 



In areas near shore, where there is a large amount 

 of deposition of organic matter, the surface layers of 

 fine-grained sediments may be light colored, brown," or 

 black, depending on the amount of destruction of organic 

 matter by scavengers and bacteria (seeTwenhofel, 1932) 

 and on the circulation and oxygenation of the water. For 

 example, Petersen and Jensen (1911) found that the sea 

 bottom off the coast of Denmark is covered by two dif- 

 ferent types of organic mud, a brown layer and a black 

 one. Both originate from the breaking up of the sea- 

 weeds and Zostera along the shore. The brown layer 

 forms in the open sea where there is sufficient circula- 

 tion and oxygenation of the water. It is the most impor- 

 tant sourceof bottom life. The black, foul-smelling bot- 

 tom layer is most common in the inner fjords and 

 represents an early stage of the condition which, in its 

 extreme development, is found in the Black Sea. Like- 

 wise, Hadding (1932) finds that the change in fossil sed- 

 iments from bituminous muds carrying graptolites and 

 brachiopods with phosphatic shells, but without glauco- 

 nite (that is, extremely reducing muds), to glauconitic 

 sands with autochthonous glauconite, formed according 

 to this author under nearly neutral conditions, does not 

 indicate an alteration of depth or other general condi- 

 tions, but merely the admission of a stronger, probably 

 cold, ocean current to the sheltered inlets and embay- 

 ments where fine mud is being deposited with its plank- 

 togenous organic material. 



On the other hand, Zenkewitch (1931) believes that 

 the brown deposits high in ferric iron and manganese di- 

 oxide and low in organic matter and bottom fauna which 

 have been described by Gorshkova (1931) from the cen- 

 ters of the White, Barents, and Kara seas owe their ex- 

 istence to prolonged periods of stagnation and oxygen 

 depletion during which the benthonic fauna is reduced or 

 killed, carbonic acid is developed, and conditions are 

 favorable for the development of iron bacteria. A more 

 likely explanation seems to be that there is a very small 

 benthonic fauna because there is little accumulation of 

 edible organic material, and that the small amounts of 

 organic material allow the maintenance of an oxidizing 

 potential. 



That In some areas near continental margins the rel- 

 ative rate of accumulation of easily decomposable organ- 

 ic matter is slow compared with that of ferric hydroxide 

 and clay minerals containing Fe203 is shown by the ex- 

 istence of red muds of perhaps considerable thickness, 

 such as those described by the Challenger from off the 

 Brazilian coast. Somewhat similar deposits found by 

 the Siboga expedition in the Dutch East Indies (Boggild, 

 1916) were found to be bluish in color at a depth 15 cm 

 below the surface. Certain sediments also, notably vol- 



canic muds, owe their colors in part to the presence of 

 unaltered colored materials, such as dark colored vol- 

 canic glass, pyroxenes, amphiboles, and olivine. 



In general, however, the colors of deep-sea sedi- 

 ments are an index of the relative rates of deposition of 

 their components and thus of the sum total of conditions 

 under which they are formed; furthermore color is a 

 more or less quantitative, easily determinable property. 



Texture . In comparison with color the texture of 

 deep-sea sediments is often a complicated and mislead- 

 ing quality, since it is the result either of mechanical 

 sorting, by currents and otherwise, of clastic particles, 

 or of the building up of authigenic mineral grains, or of 

 the varying sizes of the skeletal remains and coprolitic 

 or other agglomerations of organisms (see Buchanan, 

 1890; Murray and Philippi, 1908; Vaughan, 1924; Thorp, 

 1931; Takahashi and Yagi, 1929; Moore, 1931,1933). In 

 consequence, Thoulet's (1894) attempted size classifica- 

 tion of deep-sea sediments on the basis of grain size 

 yields such anomalies as the designation of a Globiger- 

 ina ooze as a calcareous sand. 



Nevertheless, the grain sizes of the individual detri- 

 tal or volcanic mineral particles in deep-sea sediments 

 can usually at least roughly be determined, if only by 

 microscopic examination, and are of fundamental impor- 

 tance in classification. 



Chemical Composition . Certain of the chemical char- 

 acters of sediments may be of use in classification. For 

 example, the calcium carbonate content, as will be shown 

 in a subsequent section, reflects many of the conditions 

 of formation. Likewise, sediments might be separated 

 on the basis of their silica sesquioxide ratios into fer- 

 ruginous or basic, argillaceous, and siliceous types. 

 Similarly, according to Caspari (1910), sediments of 

 low silica-base ratio contain large amounts of unweath- 

 ered materials, whereas those in which the ratio is high 

 consist of products of weathering, such as quartz and 

 clay minerals. Gripenberg (1934) has recently shown 

 that the sediments of the north Baltic Sea may be satis- 

 factorily classified on the basis of their content of or- 

 ganic matter, and use might be made of this quality in 

 the classification of deep-sea sediments. The role of 

 organic matter, however, perhaps depends as much on 

 its composition- -particularly the nature and amount, of 

 decomposable constituents --as on the actual total 

 amount present in the sediments. 



Physical Composition . The chief materials which 

 constitute marine sediments may be classified accord- 

 ing to origin as follows: 



I. Substances of terrigenous origin 



A. Relatively unaltered minerals and rock fragments 



1. Detrital materials. The products largely of me- 



chanical weathering of continental land; trans- 

 ported by wind, water, ice, and organic agen- 

 cies 



2. Volcanic ejecta. The explosive products of vul- 



canism; transported by the initial force of the 

 explosion, by wind, and by marine currents 



B. The ultimate products of the chemical weathering 



of rocks on land. These include certain clay 

 minerals, iron and aluminium hydroxides, and 

 free silica, either as quartz or as colloidal sili- 

 ca; transported from land chiefly in suspension 

 by marine currents 



II. Substances precipitated from solution in the sea, 



either by organisms or otherwise 



