SECT. 3] 



PELAaiC SEDIMENTS 



695 



report low concentrations of quartz in the Tertiary pelagic sediments of the 

 North Pacific, as, compared to the unconformably overlying Pleistocene, 

 indicating a smaller arid source area or weaker transporting wind currents dur- 

 ing Tertiary time. Another indication of low intensity of the Tertiary circulation 

 is the low organic j)roductivity in the Equatorial Divergence during the 



Fig. 23. Tropospheric transport of dust, as indicated by average frequency of haze (dry 

 aerosol) during the northern winter (upper map) and summer (lower map). Fre- 

 quencies are given in per cent of total niumber of observations. (From Arrhenius, 1959.) 



Pliocene, followed by conditions suggesting a sequence of epochs with markedly 

 increased intensities of zonal winds during the Pleistocene (Fig. 24). 



Eolian dust contains little material in the size range below one or a few 

 microns (Fig. 25), though pelagic sediments often display a secondary maxi- 

 mum in this range. Most of this fine-grained pelagic quartz was probably 

 transported together with the aluminosilicate suspensoid in sea-water from 

 the continents to the areas of deposition. 



Mixed assemblages of mainly plagioclase feldspar with a grain-size distri- 

 bution similar to that of quartz are ubiquitous in pelagic sediments. The optical 

 properties indicate that the dominant species range between labradorite and 



