870 



RIEDEL 



B. Diatoms 



[chap. 33 



Members of this ubiquitous group have siKceous frustules of a variety of 

 shapes (Fig. 2), most of them ornamented with characteristic areolae or other 

 surface patterns. The forms most obvious in sediment preparations are discoidal, 

 elliptical, and, much less commonly, triangular or quadrangular in the usual 

 plan view, generally 10-100 \j. in diameter, but fragments of elongate rod- or 

 thread-like forms up to several hundred microns in length are common at some 

 localities. Some species form frustules so delicate as to dissolve soon after the 

 death of the organism, in many cases apparently during their descent from the 

 photic zone to the sea floor. Highly diatomaceous sediments are especially 

 characteristic of high latitudes, and occur also under some areas of coastal 

 upwelling along the western coasts of Africa and America. In tropical regions, 

 radiolarians generally exceed diatoms in the mass of silica contributed to the 



50u 



Y^ 



Fig. 2. Some Quaternary planktonic diatoms. 



sediments, but under conditions especially favorable to the preservation of 

 delicate sihceous skeletons (relatively shallow depth and high rate of sediment 

 accumulation) the contribution of biogenous silica by the diatoms approaches 

 or exceeds that of the radiolarians. Well defined diatom oozes in some areas 

 contain as much as 30-40% biogenous silica, but it must not be assumed that 

 such a high content of siliceous skeletons is characteristic of all of the areas 

 marked as diatom ooze on charts of Recent sediments : the sediments of a part 

 of the North Pacific usually mapped as diatom ooze are now estimated to 

 contain only up to about 10% biogenous silica (Riedel, 1959; N. S. Skornyakova, 

 in litt.). The floral assemblages of most diatomaceous sediments comprise a 

 large number of species : the Ethmodicus oozes reported from some tropical and 

 temperate localities of the open oceans represent a special case in which frag- 

 ments of only one genus of large diatom dominate the assemblage (Kolbe, 1957; 

 Jouse, Petelin and Udintsev, 1959). 



Patrick (1948) comprehensively reviewed earlier work elucidating the factors 

 affecting distribution of living diatoms, and it is possible here to mention only 

 a few of the biogeographic studies which have broadened our understanding of 



