i 4 DISCOVERY REPORTS 



known amount of silicate, as represented by the plankton, is also contained in the sinking 

 water which progresses northward as part of the Antarctic intermediate current. Even 

 at 7 N there remain over iooo mg. of silicate at the salinity minimum of this layer. 

 The lower boundary of the intermediate current is in contact with the upper layers of 

 the warm deep current which north of 20 S in this section is composed of North Indian 

 deep water. South of this latitude the warm deep water is mainly composed of water 

 of North Atlantic origin with traces of North Indian deep water appearing in the upper 

 layers. The silicate content of North Indian deep water is much greater than that of 

 North Atlantic deep water, as a comparison of sections 3 and 6 in Plates VI and XI 

 shows. Consequently in our section up the East African coast the warm deep water has 

 always a greater content than the Antarctic intermediate water, in contrast to the section 

 up the eastern side of the South Atlantic where the intermediate water has a greater 

 silicate content north of the subtropical convergence than have the upper layers of the 

 warm deep water. The sinking Antarctic surface water at the Antarctic convergence 

 flows northwards with an initial content of between 850 and 2500 mg. of silicate. By 

 the time this water has reached 7 N there are about 1000 mg. of silicate remaining at 

 the depth of minimum salinity of the layer. The difference between these quantities 

 has been lost to the northward flow of the intermediate water by mixing with the south- 

 ward flow of the upper layers of the warm deep water. 



The circulation of the bottom water between the Weddell Sea and 30 E is such that 

 the tendency of the cold water is to keep to the northern side of the Atlantic- Antarctic 

 basin, whilst a stream of warmer bottom water exists along the edge of the Antarctic 

 continent. A cyclonic circulation has been suggested for the bottom water in this region 

 with temperature evidence for a southward movement in the bottom water in the eastern 

 part of the basin. Sections across the Southern Ocean south of the Indian and Pacific 

 Oceans suggest that the eastward current of water from the Weddell Sea spreads across 

 these oceans without further additions of cold, poorly saline and highly oxygenated 

 water sinking from the continental shelf of Antarctica. East of Enderby Land (50 E) 

 the coldest bottom water is found near the continental shelf, and the temperatures suggest 

 that the westward movement has ceased. Our section (section 6, Plate XI) begins in 

 44I E, and it will be noticed that south of 56 S the deepest observations show that the 

 Antarctic bottom water has a silicate content greater than 7500 mg. at 4000 m. It is 

 uncertain, however, how far south of 61 f° S this huge amount of silicate is found owing 

 to the absence of deep observations at St. 1361 (64 37-6' S, 44 16-3' E). In addition 

 to an eastward movement in the bottom water in our section there must also be a 

 northerly component which carries away a large amount of silicate from the Antarctic 

 zone. This amount is, however, very much less in our section than it would be farther 

 towards the east, where a gap exists between the Atlantic- Indian cross-ridge and the 

 Kerguelen-Gaussberg ridge. At St. 1368, in 44 54-6' S, 42 30-1' E, the sea-bottom has 

 risen from depths greater than 4000 m. to a depth of 955 m., thus effectively cutting 

 off any possible northward flow of bottom water in this longitude. Actually at the 

 station on the southward side of the ridge, St. 1367 in 47 41-2' S, 44 427' E, the deep 



