DEEP CURRENTS: ATLANTIC OCEAN 95 



Although the salinity of the warm water in the upper part of the deep current is much 

 greater than that of the Antarctic surface water, it is about 0-1-0-3 %o ' ess tnan tnat °f 

 the main body of the southward deep current — the North Atlantic deep current in the 

 Atlantic Ocean — from which it is derived, and it must therefore contain some Antarctic 

 water. According to my interpretation of the data the dilution of the highly saline deep 

 water takes place chiefly in the southern part of the subtropical regions, and in the sub- 

 Antarctic Zone, but it is also brought about gradually throughout the whole length of 

 the southern Atlantic, Indian and Pacific Oceans as the Antarctic intermediate currents 

 flow northwards above the more saline southward currents in the deep layer. In the 

 Atlantic Ocean (Deacon, 1933, pis. vii and viii) the less saline stratum of the deep layer 

 is formed by the mixing between the Antarctic intermediate current and the North 

 Atlantic deep current ; the exact boundary between the two currents is not yet known, 

 but it must lie in the discontinuity stratum between the two layers, and, even in the 

 tropical and subtropical regions, some of the water whose salinity is o- 1-0-3 °L l ess tnan 

 that of the main body of the Atlantic current must have a southward movement. The 

 spreading of the isohalines between the intermediate and deep currents in the Argentine 

 basin south of 35 S points, however, to the existence of a region of intense vertical 

 mixing between them and indicates that the warmer and poorly saline stratum of the 

 warm deep water is formed chiefly in this region. 



The return of Antarctic water towards the south in the deep current must be 

 largely responsible for the maintenance of the concentrations of nutrient salts at such a 

 high level in the Antarctic Zone. The north Atlantic deep current is itself not as rich in 

 phosphate as the warm deep water in the Antarctic Zone, and for the greater part it has 

 not more than 80 mg. P 2 5 per m. 3 (see Deacon, 1933, pi. viii). The water containing 

 most phosphate (140-160 mg. P,0 5 per m. 3 ) is found between the Antarctic inter- 

 mediate current and the warm deep current south of 35 S, principally in 38-43 S. 

 The presence of so much phosphate in this locality, which is also a region of low oxygen 

 content, suggests that it is liberated by the decomposition of plankton carried north- 

 wards in the Antarctic current. The mixing between the northward and southward 

 currents and probably a continuation of the decomposition in the southward current, 

 causes a large proportion of the phosphate which leaves the Antarctic regions to be 

 carried back again. It is not yet clear why the whole water column in the Antarctic 

 regions is so rich in phosphate compared with the deep and bottom waters in other parts 

 of the world, but such a cycle as that described above, and the extensive zonal move- 

 ments in the surface, deep, and bottom layers, are likely to play a large part in main- 

 taining the high concentration. The nitrate distribution in 30 W (Deacon, 1933, pi. ix) 

 points to the existence of a similar cycle of nitrogen content. 



In the Antarctic Zone, there is not a great depth interval between the levels of 

 maximum temperature and salinity in the warm deep layer, and the movements in the 

 upper part of the layer are probably very similar to those in the lower and more saline 

 part. The temperature distribution at the level of maximum temperature supports this 

 conclusion ; there is plainly the same cyclonic circulation in the Atlantic-Antarctic basin, 



