662 ARRHENIUS [CHAP. 25 



state of the sediment is mainly determined l^y the duration of contact with the 

 oxidizing bottom water. This property consequently reflects the total rate of 

 deposition, and Bramlette suggests a high degree of oxidation as one criterion 

 for pelagic sediments. 



Any single one of the criteria for pelagic versus high-rate sedimentation 

 might be insufficient when a])])lied alone. In cases where several indications 

 occur jointly, conclusions regarding the order of magnitude of the sedi- 

 mentation rate can apparently be safely drawn. The reliability of such con- 

 clusions is illustrated by the fact that estimates of the rate of deposition of 

 different types of pelagic sediments, made before the advent of nuclear age- 

 determination methods, have in general proved correct within surprisingly 

 narrow limits. Even if the number of absolute age determinations will remain 

 relatively limited, the suggested definition of pelagic sediments on the basis of a 

 maximal value for the rate of deposition of the terrigenous component thus 

 appears rational and practical. 



2. Composition 



Attempts to classify pelagic sediments have been based either on appearance 

 and composition, or on the ultimate origin of the components. A rigorous appli- 

 cation of the latter scheme has been attempted by Grabau (1904) and Schott 

 (1935), and recently in a modified form by Goldberg (1954), further applied by 

 Arrhenius (1959). However, at this time not enough is known of the ultimate 

 origin and mode of accretion of some of the minerals making up the sediment 

 (cf. Petti John, 1949, p. 184). To indicate the general distribution of different 

 types of pelagic sediments it is consequently necessary to limit genetic classi- 

 fications to major groups, except in the case of biotic sediments which can be 

 identified morphologically. The origin of the major inorganic component in 

 most areas has not been established with certainty; the term "red clay" has 

 come into general use, although a red hue is seldom dominant and clay minerals 

 sometimes are not the major mineral group. 



Although introducing additional technical terminology is deprecable, the 

 discussion of pelagic sediments on a genetic basis warrants some modification 

 of existing terms. It appears feasible to distinguish minerals which crystallized 

 in sea-water from those which formed in magmas, in hydrothermal solution, or 

 by weathering under acidic conditions. This distinction is important, inasmuch 

 as only the first group can be used to interpret the physicochemical state of 

 the ocean in the past. These minerals are here described as halmeic (from 

 aXfirj; sea-water). Conversely, the properties of the third group of components 

 frequently furnish information on the processes acting on the lithosphere and 

 on the transport of this group of minerals into the pelagic environment. The 

 minerals derived from the exposed surface of the lithosphere are here designated 

 as terrigenous. Minerals and mineraloids deposited into the ocean by volcanic 

 eruption are described as pyroclastic, Sohds secreted by living organisms are 

 referred to as biotic. 



