SILICATE CYCLE 13 



amount held by the plankton ; these amounts of silicate travel northwards in the Antarctic 

 intermediate water. As the Antarctic intermediate current progresses northwards it 

 steadily loses silicate by mixing with the upper layers of the warm deep water, which in 

 this section have a lower silicate content, north of the subtropical convergence, than the 

 intermediate water above them. The course of the northward flow of the Antarctic 

 intermediate water may be traced from salinity measurements, and even as far north as 

 4 N there still remained about 1000 mg. of silicate at the salinity minimum of this water. 

 Farther north than our section extends more and more silicate must be lost to the inter- 

 mediate water and transferred to the southward-flowing warm deep layer. 



The second loss of silicate from the Antarctic zone occurs in the Antarctic bottom 

 water, which as Plate VI shows had a content greater than 5000 mg. in March 1933 

 south of the eastern part of the South Atlantic Ocean. In our section some of this loss 

 is prevented by the rise of the sea-bottom from an ocean depth greater than 4000 m. 

 to one of 2633 m. at St. 11 60, which is situated on the Atlantic-Indian cross-ridge in 

 52 41-5' S, 14 30-4' E. North of this cross-ridge the lower layers of the warm deep 

 water are seen to be returning silicate southwards, and this silicate is carried towards 

 the surface in this current which rises rapidly near the Antarctic convergence. The 

 majority of the silicate present in the Antarctic surface water is undoubtedly returned 

 in the lower levels of the warm deep water which itself does not have a large silicate 

 content but to which silicate is added by mixing at the boundary surface with the 

 Antarctic bottom water. 



Between Cape Town and the ice-edge north of Enderby Land in 66|° S, 42J E, 

 section 4, Plate VIII, passes through a possible gap in the Atlantic-Indian cross-ridge 

 in about 50 S, 30 E and the sea-bottom does not rise above 4000 m. anywhere in our 

 section. Consequently a greater amount of silicate is seen in the bottom observations 

 in this section than was evident in the previous section (3, Plate VI), when the sea-bottom 

 rose to a depth of 2633 m. in 52 41-5' S, 14 30-4' E. Silicate may be seen to leave the 

 Antarctic zone in the usual manner, i.e. in the sinking Antarctic surface water, and in 

 larger quantities, in the Antarctic bottom water. Mixing at the upper and lower boundary 

 surfaces of the warm deep water transfers silicate to the southward-flowing warm deep 

 water which returns it to the Antarctic zone. Some of the silicate in the Antarctic 

 intermediate current, which at 38 S contains as much as 2750 mg. at the position of 

 minimum salinity in this current, must be utilized by the plankton in the water which 

 upwells on the continental shelf of South Africa and is thus lost to the silicate cycle in 

 the Antarctic zone. 



Plate XI shows a section from the ice-edge in 64 37-6' S, 44 16-3' E to n° 32-3' N, 

 52-03' E, which is composed of observations made in May 1934 and April-May 1935 ; 

 a break in the iso-lines at about 39 S indicates the time interval between the two halves 

 of the section. This plate shows the distribution of silicate in the western part of the 

 South Indian Ocean and the Southern Ocean to the south of it (section 6). At the 

 Antarctic convergence the Antarctic surface water sinks below the surface carrying with 

 it between about 850 and 2000 mg. of silicate in its upper and lower surfaces ; an un- 



