432 S. S. VISHER SALINITY AS CAUSE OP CLIMATIC CONTRASTS 



saline seas of the past than at present. This condition, coupled with the 

 fact that evaporation, a cooling operation, is favored by the absence of 

 salt, ma)' have been one reason why, when for some reason it was cold 

 enough on the earth for glaciers to be widespread, the glaciated areas of 

 the earlier ice ages, such as the Proterozoic and Permian, were located 

 farther south than were the ice-sheets of the recent Glacial period. If the 

 ocean were ice-covered down to middle latitudes, a lack of glaciation in 

 high latitudes would not be any more surprising or harder to explain 

 than is the lack of Pleistocene glaciation in the northern parts of Alaska 

 and Asia. In both cases the coldness of the ice-sheet of lower latitudes 

 apparently induced prompt condensation of the wind-borne moisture. 

 Hence not enough moisture was carried into higher latitudes to produce 

 glaciers there. 



It is not known how saline the ocean was at any given time in the past, 

 but the fact that salt dej)osits are lacking in the rock formations before 

 the Paleozoic, according to Barrell,^ suggests that in the Proterozoic it 

 was decidedly less than now. Schuchert surmises that the salinity of the 

 Proterozoic was perhaps 1 per cent instead of the present 3.5 per cent. 



Salinity affects the Kate of deep-sea Circulation 



The third great effect of the lower salinity of the past than at present 

 is on the deep-sea circulation of the ocean. The vertical circulation is 

 now dominated by cold water from subpolar latitudes. All but the sur- 

 face of the ocean is almost freezing cold, because cold water sinks in high 

 latitudes by reason of its greater density, due to its low temperature. It 

 then "creeps" to low latitudes, where it rises and replaces either the 

 water moved poleward by surface currents produced by the trade winds 

 or that wliich has evaporated from the surface. During past ages, when 

 the sea-water was less salty than at present, the circulation was pre- 

 sumably more rapid than now. This is because in tropical regions the 

 rise of cold water is interfered with by the sinking of warm surface water, 

 which is relatively dense because evaporation has removed part of the 

 Avater and caused an accumulation of salt. 



According to Krlimmel and Mill,*^ the surface salinity of the subtrop- 

 ical belt of the North Atlantic commonly exceeds 3.7 per cent and some- 

 times reaches 3.77 per cent, whereas the underlying waters have a salinity 

 of less than 3.5 per cent and locally as little as 3.44 per cent. The other 

 oceans are slightly less saline than the North Atlantic at all depths, so 



= T. Barrel! : The origin of the earth, in Lull and others : The evolution of the earth 

 and its inhabitants, 1918, p. 32. 



» The ocean. Encyclopedia Britannica, 11th edition. 



