PHOSPHATE CYCLE u 



lost to the Antarctic in the sinking surface water and the bottom water mixes in at both 

 the upper and lower boundaries of the warm deep water, and this, because of its 

 southerly movement, returns the phosphate to the Antarctic zone. The various stations 

 south of the Antarctic convergence show that the major portion returns in the upper 

 layers of the warm deep water. 



In the western part of the South Indian Ocean and the Southern Ocean to the south 

 of this area, it has been possible to construct a section between 64!° S and ni°N 

 (section 6). This has been done by linking up a series of stations taken in May 1934 from 

 the ice-edge north of Enderby Land in 64^° S, 44^° E as far as St. 1367 in 47 41' S, 

 44 43' E and then across the subtropical convergence in the direction of Durban, with 

 a line of stations made in April-May 1935 from north of Marion Island (46 49' S, 

 37 49' E) through the Mozambique Channel and northwards to near Cape Guardafui 

 (ii° 50' N, 51 20' E). The complete section is shown in Plate X, and the time interval 

 of nearly a year between the two halves of the section is indicated by a break in the 

 iso-lines between Sts. 1370 and 1567. South of 50 S the average depth of the sea-bottom 

 is between 4000 and 5000 m. ; at 45° S the sea-bottom has risen to a depth of less than 

 1000 m. It is this rise to the Atlantic-Indian cross-ridge, on which stands the island 

 group of the Crozets, which in some degree prevents the loss of phosphate from the 

 Antarctic zone in the bottom water. East of the Crozets and west of Kerguelen, 

 however, there is a large gap between the cross-ridge and the Kerguelen-Gaussberg 

 ridge which must allow the passage of bottom water northwards. In our section (Plate 

 X) it is obvious that the cross-ridge has prevented a considerable loss of phosphate 

 towards the north, since in the far south the bottom water has a content of 150-160 mg., 

 whilst north of the cross-ridge in 45 S it is about 135 mg. 



Phosphate leaves the Antarctic zone in the sinking Antarctic surface water, which 

 just south of the Antarctic convergence has a content of 122 mg. compared with 160 mg. 

 in the upper layers of the warm deep water in the same position. As far north as about 

 the subtropical convergence in 40J S the warm deep water has always a greater content 

 than the sinking Antarctic surface water. Between about 40J S and about 29 S the 

 Antarctic intermediate water has a greater content than the warm deep water ; north of 

 29 S the influence of the North Indian Ocean deep water with its high phosphate 

 content (> 160 mg. at 800-1000 m. between 2\° and 7 N) causes the south-going warm 

 deep water to have a greater content than the north-going Antarctic intermediate water. 

 The course of the Antarctic intermediate current may be traced in the closely separated 

 iso-lines in the upper part of the section. Thus in the western part of the South Indian 

 Ocean the phosphate cycle is more complicated than in the South Atlantic Ocean. This 

 is because in the former the North Indian deep water has a very high phosphate content, 

 particularly in its upper part, whilst in the western part of the South Atlantic Ocean 

 for example, the North Atlantic deep water has a much lower phosphate content north 

 of the Rio Grande ridge than has the Antarctic intermediate water. In the western part 

 of the South Indian Ocean there appear to be two localities where decomposition of 

 plankton must be very extensive, since apart from the influence of the North Indian 



