SALINITY AFFECTS RATE OF DEEP-SEA CIRCULATION 433 



the vertical gradients along the tropics are similar. According to the 

 "Smithsonian Physical Tables/' the observed difference in salinity of 

 surface and underlying water is equivalent to a difference of .003 in 

 density, where the density of fresh water is taken as 1.000. 



Since the decrease in density produced by warming water from tlie 

 temperature of its greatest density (i degrees centigrade) to the highest 

 temperatures which ever prevail in the ocean (30 degrees centigrade or 

 86 degrees Fahrenheit) is only .004, it is evident that the more saline 

 surface waters of the dry tropics now are only a trifle less dense than the 

 less saline but colder waters beneath the surface. During the days of 

 especially great evaporation the most saline portions of the surface waters 

 in the dry tropics are denser than the underlying waters, and therefore 

 sink and produce a temporary local stagnation in the general circulation. 

 Such a sinking of the warm surface waters is reported l)y Kriimmel as 

 observable by the sudden rise of temperature it produces at considerable 

 depths. If such a hindrance to the circulation did not exist, the velocity 

 of the deep-sea movements certainly would be greater than at present. 



If in earlier times such more rapid circulation occurred, low latitudes 

 were cooled more than now by the rise of cold waters, and at the same 

 time high latitudes were warmed by a greater flow of warm water from 

 tropical regions. In that case there was less contrast in climate between 

 the different latitudes. Hence, in so far as the rate of the deep-sea cir- 

 culation depends on salinity, there apparently has been an increase in the 

 contrasts from latitude to latitude. Thus increased salinity during geo- 

 logic history helps a little to explain the well established fact that during 

 most of the past the climatic contrast between tropical and subpolar lati- 

 tudes has been less than at present. 



Though the process of adding salts to the ocean has presumably gone 

 on at all time, two sorts of conditions may have temporarily hastened it, 

 and thus intensified whatever slight climatic effect increased salinity may 

 have produced. At times of continental glaciation, the average salinity 

 of the ocean is increased to the extent that the ocean level is lowered to 

 supply the water necessary to form the glaciers. It has been calculated 

 that the accumulation of the Pleistocene ice-caps lowered the ocean more 

 tliaii 200 feet; perhaps as much as 300 feet. The salts in this amount of 

 water would be added to the remaining ocean water and would increase 

 its salinity by about 3 per cent (from 3.5 per cent to 3.5? per cent), and 

 to that extent would tend to slow up the deep-sea circulation. 



An emergence of the continents would be accompanied by rapid ero- 

 sion, by a lowering of the level of ground-water, and by the transfer to 

 the ocean of part of the vast quantities of salt and other soluble mineral 



