30 

 I 



1000m 



2000m 



3000m- 



4000m 



PHOSPHATE AND SILICATE CYCLES 



40" 50° 



I I 



sub-tropical antarctic 



convergence: convergence 



sub-antarctic zone 



^ G- <T 



9 



SOUTH 



5UB-TR0PICAL WATER 



ANTARCTIC 

 INTERMEDIATE CURRENT 



^MIXED- 

 WATER ' 

 REGION 



ANTARCTIC 



SURFACE CURRENT 



WARM 

 DEEP CURRENT 



/ 



ANTARCTIC 

 BOTTOM CURRENT 



/ 



Fig. 6. The vertical circulation of water in the South Atlantic Ocean. 



THE PHOSPHATE AND SILICATE CYCLES IN THE 



SOUTHERN OCEAN 



It will be advisable to discuss at an early stage of the report the means by which 

 the phosphate and silicate contents of the Antarctic zone are maintained at such high 

 values. 



The vertical sections in the plates at the end of this report show that at the surface 

 phosphate varies from a value of about 130 mg. in winter at the ice-edge to zero in the 

 tropical zone. Similarly the silicate content diminishes from values of 3000-3500 mg. 

 at the surface at the ice-edge in winter to zero in the subtropical and tropical zones. 

 These nutrient salts are being continuously lost to the Antarctic zone in the form of 

 plankton or in the free state when the Antarctic surface water, which has a northerly 

 component of movement, sinks into sub-Antarctic water at the Antarctic convergence. 

 Similarly large quantities of both these nutrient salts are carried away northwards out 

 of the Antarctic zone by the Antarctic bottom water. Obviously a return process must 

 exist or the Antarctic zone would be denuded of these salts. 



In the western part of the South Atlantic Ocean (section 2) the phosphate cycle is 

 well illustrated by the vertical section in 30 W which is given in Plate IV. In the 



