144 A TEXTBOOK OF OCEANOGRAPHY 



ooze dredged from the ocean floor in the tropics is so cold that 

 it cannot be handled without discomfort. The lowest deep- 

 sea temperatures are found in the oceans of the Southern 

 Hemisphere, and, broadly speaking, higher temperatures are 

 recorded as one recedes from Antarctic regions. 



One theory of the cause of such low deep-sea temperatures 

 is that they are due to submerged Polar currents, and that 

 there is, in fact, a vertical circulation of the following kind : 



The cold surface waters of the Polar regions sink to the 

 bottom on account of their greater density. This water layer 

 moves towards the Equator, increasing in temperature from 2 

 to 3 C. on the way. In tropical regions the heated surface 

 waters move towards the Poles. The facts which support the 

 theory of a bottom current from the Polar to tropical regions 

 may be grouped under six headings : 



1. All oceanic depths greater than 2,000 metres are filled 

 with water of the same density (with small variations) and a 

 temperature between o and 3 C. This points to a vertical 

 circulation. In this enormous volume of deep-sea water there 

 are only minute variations of temperature, and these would be 

 still smaller but for the fact that near the freezing-point there 

 are but slight variations in the density of sea-water for each 

 degree of change in the temperature. 



2. The lowest bottom temperatures are found where the 

 great oceans have both wide and deep connection with Polar 

 areas, and the bottom temperatures increase the farther one 

 recedes from the Poles and the nearer one approaches the 

 Equator. The depths of the North Polar seas are shut off from 

 neighbouring depths by slight ridges, so that the bulk of the 

 abyssal waters of the great oceans comes from the Antarctic, 

 and it follows that the southern oceans have the lowest bottom 

 temperatures. On the western side of the South Atlantic there 

 is a tongue of water of only 0-3 C., which reaches to 30 S. 

 Lat. (the Argentine Deep). This water is separated by a 

 comparatively narrow mixed layer from water of 2'8 C., 

 which is the average temperature of Atlantic water of that 



