_ 4 DISCOVERY REPORTS 



Silicate is withdrawn in the Antarctic zone from the photosynthetic layer of the 

 surface water by phytoplankton and Radiolaria, although chiefly by the former. Part 

 of the phytoplankton is consumed by the zooplankton, and the silicate skeletons are 

 excreted immediately by the animal organisms. These excreted skeletons and other 

 dead diatoms sink to the bottom, and although a proportion pass into the southward- 

 flowing warm deep water, others pass into the bottom water causing a very high silicate 

 content in this layer. The existence of the belt of diatom ooze near the Antarctic con- 

 vergence is evidence of a tremendous mortality of diatoms in this position. Some of 

 the free silicate in the Antarctic surface water together with the zooplankton and un- 

 consumed phytoplankton travels north-eastwards to the Antarctic convergence and 

 sinks into sub-Antarctic water eventually forming part of the intermediate current. Thus 

 silicate leaves the Antarctic zone in two northward-going layers, the Antarctic inter- 

 mediate and bottom currents. Because of the low concentration of silicate in the sub- 

 Antarctic surface water the mixed water formed 100-200 miles north of the Antarctic 

 convergence does not have a large silicate content. On the other hand, the Antarctic 

 bottom current has a very large silicate concentration, between 5000 and 7000 mg. in 

 the Antarctic zone. Thus although silicate leaves the Antarctic in two ways, the greater 

 portion is carried by the Antarctic bottom water. 



Plate VI shows a vertical section of silicate content along section 3 in the eastern part 

 of the South Atlantic Ocean between 69 20-8' S and 3 46-2' N. As may be seen a moderate 

 amount of silicate leaves the Antarctic zone at the convergence in the sinking surface 

 water and a much larger concentration in the bottom water which also has a northerly 

 component of movement. The lower layers of the warm deep water are seen to be 

 returning this high concentration southwards again at quite a high latitude. The 

 silicate content at the depth of the minimum salinity of the intermediate current varies 

 between 500 mg. and 2 1 50 mg. , but the upper limit is usually of the order of 1 000 mg. with 

 very small variation north of 26 S. The distribution of silicate from south to north 

 at the depth of minimum salinity of the intermediate current also forms a series of 

 maxima and minima which, however, are not so pronounced as in the phosphate curves. 

 An enormous mortality of diatoms occurs in or immediately south of the vicinity of the 

 belt of diatom ooze, and the silicate skeletons of the diatoms are excreted rapidly after 

 consumption of the plant life by the zooplankton. Since sinking of dead and excreted 

 diatoms occurs it would not be expected that the curve of silicate distribution would 

 show as marked a variation as does the similar curve for phosphate distribution. The 

 variation that does occur, however, must be related to the seasonal variation in silicate 

 content of the constituent waters of the intermediate current. In the section under 

 review maximum silicate at the depth of minimum salinity of the current is found at 

 St. 1 165 in 41 S at 900 m. ; this was also the depth and position of maximum phosphate 

 content of the current. Since both phosphate and silicate are maximal for the inter- 

 mediate current at St. 1165 it is reasonable to assume that maximum regeneration in 

 the current takes place at this position. 



South of Cape Town the depth of the salinity minimum lies at about 1500 m. 



