SECT. 3] 



PELAGIC SEDIMENTS 



693 



sediments in a few instances. Another part of the dissolved siHca is returned to 

 the bottom water by diffusion. Thus, a marked vertical gradient is maintained 

 in the concentration of silica in the interstitial solution of the sediment, and 

 at the water interface (Table VI and Fig. 21). 



Table VI 



Concentration of Dissolved Monosilicate in the Water immediately above the 

 Sediment Surface and in the Interstitial Solution of the Sediment in the South 



Pacific. (Arrhenius and Rotschi, 1953.) 



Wind-transported diatom frustules have been observed in quantity in eolian 

 dust derived from arid areas with exposed diatomaceous deposits (Hustedt, 

 1921; Radczewski, 1937). Such transport appears to be the most probable 

 explanation of the observation by Kolbe (1955) of freshwater diatoms in 

 equatorial Atlantic sediments. The same author has reported the presence in 

 these sediments of silicified grass cells, presumably suspended in the atmosphere 

 by fires, an explanation which is further indicated by the similar distribution of 

 charcoal fragments (Radczewski, op. cit.). 



Quartz was recognized as a significant component in deep-sea sediments by 

 Murray and Renard (1884), who drew attention to the importance of wind 

 transport of this mineral. Radczewski (1937, 1939) investigated quantitatively 

 the distribution of eolian dust and found that eolian quartz in pelagic sediments 

 of the equatorial Atlantic frequently exceeded 30% of the total sediment. He 

 further established the regional variations in concentration of eolian quartz and 

 demonstrated a decrease in median size and concentration of the eolian 

 component with increasing distance from the continent. Furthermore, Rad- 

 czewski found windborne quartz in sediments of glacial and interglacial age 

 as well as in the Recent. Subrounded and pitted quartz has been further re- 

 ported by Norin (1958) as, together with calcite, the quantitatively most 

 important non-volcanic mineral in the eolian component of Mediterranean 

 sediments. 



Revelle (1944) observed notable amounts of silt-size quartz in Pacific pelagic 



