PHOSPHATE AND SILICATE CYCLES 15 



observations show that at 3000 m. the water is mainly warm deep water which is already 

 carrying back silicate to the Antarctic zone. 



South-east of New Zealand two sections (12 and 13, Plates XVIII and XX) are 

 available. The section in Plate XVIII was made in September 1932 and the other, in 

 Plate XX, in January 1934. In these sections the greatest concentration of silicate is 

 always found at the deepest observations. As we have seen in the Southern Ocean south 

 of the Atlantic and Indian Oceans the major portion of the silicate which leaves the 

 Antarctic is returned in the lower layers of the warm deep water. In the Pacific sector 

 the position is more complicated owing to the more complex nature of the deep currents 

 in this part of the ocean, where the intermediate, warm deep and bottom currents are 

 not separated by any well-marked boundaries. Extensive mixing no doubt takes place 

 between these currents, and silicate exchanges undoubtedly occur, but it is quite plain 

 that east of New Zealand as much as 5300 mg. of silicate are found at 3500 m. in the 

 highly saline layer which is travelling towards the north. Deacon (1937, p. 103) has 

 stated that a large part of the current which flows southward between 1500 and 2500 m. 

 north of 50 S in the more central part of the ocean, sinks between 50° and 55 S to mix 

 with the bottom water ; this means that a large part of the southward movement does 

 not make the climb to the higher level of the current in the Antarctic zone but returns 

 directly towards the north with the bottom current, carrying with it large amounts of 

 silicate. On the other hand, the observations made across the sub-Antarctic zone in 

 the South Pacific Ocean suggest that the volume of Antarctic intermediate water carried 

 back to the south by the warm deep current is exceptionally large, although Deacon 

 (loc. cit. p. 106) states : " It is not safe to assume that all the water at the level of maximum 

 temperature of the deep water north of the Antarctic convergence finds its way into the 

 upper part of the warm deep layer in the Antarctic zone because it usually has too low 

 a salinity ; it appears to be part of a southward eddy which is returned to the north with 

 the surface water which sinks at the convergence." In view of the absence of data from 

 lower latitudes in the South Pacific Ocean any discussion of the return of silicate to the 

 Antarctic zone south of the South Pacific Ocean must be deferred. 



We have seen therefore that phosphate leaves the Antarctic in the surface and bottom 

 currents either in the free state or, in the surface current, in the form of plankton. 

 North of the Antarctic convergence, decomposition of the Antarctic plankton occurs 

 chiefly in the Antarctic intermediate current, and the phosphate so released is 

 transferred by mixing to the upper layers of the southward-going warm deep water. 



Silicate is chiefly lost to the Antarctic zone in the Antarctic bottom water, although 

 large quantities also travel northwards in the surface current. Unlike phosphate, silicate 

 is always at a maximum in the Antarctic zone, where the greatest mortality of phyto- 

 plankton occurs. The return of silicate to the Antarctic is effected by means of the warm 

 deep water, to which silicate is transferred by mixing at both the upper and lower boundary 

 surfaces with the intermediate and bottom currents respectively. Since the greater 

 amount is carried out of the Antarctic in the bottom water the major quantity of silicate 

 is returned in the lower portion of the warm deep water. 



