5. INSOLUBLES 



R. W. Rex and E. D. Goldberg 



The atmosphere acts as a temporary reservoir for dust derived from volcanic, 

 continental and extra-terrestrial sources. The wind systems transport these 

 materials before their fall-out and their subsequent distribution on the earth's 

 surface can be expected to reflect their atmospheric paths. Slowly depositing 

 deep-sea sediments accumulate these materials and to a certain extent preserve 

 a sequential record of fall-out in contrast to the continents where dilution of 

 the fall-out with similar-appearing terrestrial solids, combined with weathering 

 processes, usually destroys any continuous record. Hence, deep-sea deposits 

 provide excellent source areas for paleometeorological investigations provided 

 eolian materials can be recognized. 



1. Meteorology of Transport 



Dust-transport processes occur predominantly in the troposphere, although 

 some transport is found in the stratosphere followed by settling to the tropo- 

 sphere where it behaves similarly to other tropospheric fall-out. The tropo- 

 spheric circulation is dominantly zonal with three main zones of air-mass 

 movement in each hemisphere — the equatorial easterlies, the temperate 

 westerlies and the polar easterlies (Fig. 1). Each of these zones of air circulation 

 can carry dust, contributed primarily by the arid areas of the continents. 

 Precipitation scrubbing appears to be the most important mechanism of fall-out 

 from the troposphere (Wilkins, 1958). 



One of the first records of the transport of atmospheric dusts to oceanic 

 areas was made in 1160 by the Arabian geographer Edrisi (as quoted in 

 Radczewski, 1937a). Dust from North Africa is carried across the Mediterranean 

 and far into Europe by winds called siroccos (Ehrenberg, 1847; Clerici, 1901, 

 1902; Hellman and Meinardus, 1901; Walther, 1903; Glawion, 1938). The 

 reddish sirocco dust consists predominantly of quartz and clay, and is primarily 

 removed from the atmosphere by the scrubbing effects of rain and snow. 



Brown (1952) reported volcanic dust over the Carribean to an elevation of 

 over 4000 meters which was derived from a volcano in the Cape Verde Islands 

 off the African coast. The ash moved across the Atlantic seaboard of the United 

 States and was last observed over Bermuda after traveling about 7000 miles, 

 all in the troposphere. Fall-out of this ash essentially covered the entire north 

 equatorial and north temperate regions of the Atlantic. 



The strong westerly winds of temperate latitudes have carried volcanic ash 

 from eruptions in Iceland across the Atlantic far into Northern Europe (Salmi, 

 1948; Thorarinsson, 1954) and ash from Chilean eruptions out into the South 

 Atlantic (Larsson, 1937). 



In the Pacific the meteorological conditions are similar to those in the 

 Atlantic, and, in spite of the larger areas, there are records of desert dust 

 storms crossing broad expanses of ocean. 



[MS received August, 1960] 295 



