710 



ARRHENIUS 



[chap. 25 



Sw 74 



Sw 60 



Sw 39 



Depth 

 cm 



100 



200 



300 



400 



500 



600 



700 



800 



900 



1000 



Fig. 37. Size distribution of frustules of the diatom Coscinodiscus nodulifer in three widely 

 spaced sedimentary^ sequences from the equatorial Pacific (Sw 39, long. 93°W; Sw 60, 

 long. 135°W; and Sw 74, long. 150°W). The flat cylindrical frustvile of this diatom 

 consists of two halves, the upper one fitting over the lower like a top over a box. At 

 cell division, each daughter cell retains half a frustule as a top and secretes a new 

 bottom (smaller) half. Continuous reproduction by division leads to a distribution of 

 frustule sizes from a few hundred microns to approximately 13 [j,. In the majority of 

 cases the frustules are shed and sexual reproduction takes place when the frustule 

 has been reduced to 40 [x. A naked auxospore is then formed, which grows out to 

 maximum size before a new frustule is secreted. 



During times of maximum circulation, as indicated by the rate of accimiulation of 

 calcareous, siliceous, and phosphatic fossils (cf. Fig. 38), a secondary maximum at 

 large size occurs in the frustule frequency distribution, indicating a modification of 

 the reproductive cycle. This phenomenon is employed to confirm the correlation of 

 strata over wide distances in equatorial facies. The distance between Sw 39 and Sw 74 

 amounts to approximately one earth radian. Stratigraphic stages are indicated by 

 numbers in the graphs. (For the age-and-productivity relations of these stages, see 

 Figs. 38 and 35. Also, compare D. Ericson, Chapter 31 of this volume, and Ericson etal., 

 1955, 1956.) 



