364 THE DEEP SEA 



more to the pelagic sediments in the North Pacific than in the 

 South Pacific. 



Sources and rate of pelagic clay accumulation in the North 

 Pacific thus seem more similar to those of the Atlantic Ocean than 

 in the South Pacific, and these differences seem clearly reflected in 

 the color and other features of the pelagic clays of the two regions. 



A distinct discontinuity between this surface clay of the North 

 Pacific and an underlying clay much like that in the South Pacific 

 is encountered in many cores at depths of about 3 to less than 1 m, 

 with the upper bed thinner on topographic highs. The upper clay 

 shows the usual character of modern pelagic sediments of this 

 region, including any remains of the plankton preserved, and is 

 doubtless all of Quaternary age. The clay below the marked 

 discontinuity is commonly dark brown, and similar in this color 

 and other features to the surface clay of the South Pacific. Goldberg 

 and Arrhenius (1958, p. 203) show the chemical composition of the 

 lower clay is also more like that of the South Pacific. 



All this sediment below the marked change is probably of 

 Tertiary age, as indicated where evidence from fossils is adequate. 

 Some of the differences in sources of sediment and rates of its 

 accumulation that result in the marked differences of modern clay 

 in the North Pacific from that beneath it, may thus indicate 

 certain post-Tertiary influences greater in the North Pacific than 

 in the South Pacific where no such difterence in the later clay is 

 evident. Goldberg has indicated that the windborne fine-grain 

 quartz is much less common below the discontinuity. Both this, 

 and any possible contributions from areas of turbidity current 

 deposition in the northwest Pacific, might be more conspicuous 

 with advent of the glacial times. 



The great segregation of manganese in pelagic sediments 

 presents many problems, but only one may be considered here. 

 Vast amounts are disseminated as fine particles or micronodules 

 in the slowly accumulated pelagic clay. Very great amounts are 

 also obvious in the large nodules over large areas of the ocean floor 



Menard and Shipek (1958) have estimated, from such photo- 

 graphs as shown in Fig. 9b and many dredge and core samples, 

 that these nodules cover from one-fourth to one-half of large areas 

 of the floor, especially in the southwest Pacific. Their estimates of 



