HYDROLOGY OF THE SOUTH ATLANTIC 233 



formed in this way it is difficult to distinguish between them, and it is perhaps best 

 to describe all the water which spreads northwards below the warm deep water as 

 Antarctic bottom water. If not, in a region such as the Drake Passage, where the 

 bottom water is not so cold as it is in the Weddell Sea, it must be called Antarctic 

 deep water. It is worth while making an exception of the cold deep current flowing 

 out of the Weddell Sea, because it can still be clearly recognized that it is the remains 

 of the warm deep current which flows westwards into the sea south of 66° S. 



In all probability the formation of Antarctic bottom water in the Weddell Sea is 

 largely the result of the warm deep current which flows into the southern edge of the 

 sea and is turned northwards off the east coast of Graham Land. 



TEMPERATURE, SALINITY AND OXYGEN CONTENT OF THE 

 ANTARCTIC BOTTOM WATER 



Because of the very gradual changes in temperature and salinity with depth between 

 the warm deep and Antarctic bottom layers, it is impossible to give a definite boundary 

 to the layer. The lowest temperature that we have found in Antarctic bottom water in 

 the open sea north of 70° S is -0-55° C. and the lowest salinity 34-65700; salinities of 

 34-63 7oo have, however, been found by the German Atlantic Expedition. Above this 

 water there are mixtures with varying amounts of warm deep water. The difference in 

 the properties of the Antarctic bottom layer and the warm deep layer is greater farther 

 north. Wiist (1933, pp. 71 et seq.) shows that water which has its origin in the Antarctic 

 bottom and deep layers can be traced as far as 40° N on the western side and as far as 

 35° N on the eastern side of the South Atlantic. There is, however, only a sharp dis- 

 tinction between the two layers south of the Equator in the west and south of the Walfisch 

 ridge in the east. North of the Antarctic Zone the small discontinuity in the density 

 gradient which shows the boundary between the two layers is found when the tempera- 

 ture is about 1° C. and the salinity 34-70700- 



VERTICAL DISTRIBUTION OF PHOSPHATE AND NITRATE 

 IN THE DEEP WATERS OF THE SOUTH ATLANTIC OCEAN 



The vertical distribution of phosphate along the meridian of 30° W is shown in 

 Plate IX. North of the Rio Grande ridge the greatest phosphate content is found in 

 the Antarctic intermediate layer, and the warm deep water contains less phosphate. 

 The warm deep water is not therefore the source of the high phosphate concentrations 

 which are found in the Antarctic Zone. Phosphate is added to the warm deep current 

 south of the Rio Grande ridge, and the source of the phosphate is probably in the rich 

 plankton of the Antarctic Zone which decomposes as it is carried downwards at the 

 Antarctic convergence into the bottom of sub-Antarctic water. The decomposition is 

 probably greatest in the region of greatest phosphate content and least oxygen content, 

 that is to say, between the north-going sub-Antarctic and Antarctic intermediate waters, 

 and the south-going warm deep water, between 43 and 38° S. The hydrogen-ion con- 

 centration is also greatest between these currents. 



