ANTARCTIC BOTTOM WATER 97 



data from a good number of stations the observations from any one month are not 

 sufficient to enable charts to be drawn. Antarctic bottom water which was formed in 

 winter will have a greater nutrient salt content than that formed in summer. 



The phosphate content of the Antarctic bottom water is always large but with the ex- 

 ceptions mentioned on p. 81 the content is always less than that of the warm deep water. 



In the Drake Passage along section i (Plate III), the bottom water had a content of 

 the order of 135 mg. in April 1930. In the Scotia Sea values of 160 mg. were recorded 

 in April 1934, and of 140 mg. in September and October 1934 and in January 1935. 

 The bottom water around South Georgia in September 1934 had a content of about 

 150 mg. compared with 135 mg. in February 1935. 



In April 1931 in the Antarctic zone in the western part of the South Atlantic Ocean 

 (section 2, Plate IV), the bottom water had a phosphate content of between 125 mg. and 

 145 mg., the content being greater towards the south. In the sub-Antarctic zone the 

 content was found to be about 140-150 mg., whilst at 38 S a value of 156 mg. was 

 recorded at the lowest observation which was made in mixed warm deep and bottom 

 waters. North of 38 S the West Atlantic basin is narrowed by a westward extension 

 of the mid-Atlantic ridge. This ridge cuts off the direct passage of bottom water to the 

 north in our section, and the lowest observations on the actual ridge, made within 100- 

 200 m. of the bottom, show that the warm deep water at this depth has a low phosphate 

 content. North of this ridge where the depth of the ocean is about 5000 m. the general 

 content of the bottom water was about 85-95 m §- with one isolated value of 117 mg. 

 at a depth of 4900 m. at 21 13' S. 



In March-April 1933 along section 3 in the eastern half of the South Atlantic Ocean 

 (Plate V), the bottom water in the Antarctic zone had a phosphate content of about 

 100-105 m g-; there was little variation from the ice-edge in 69 20-8' S as far north as 

 41 ° S where the content at the deepest observation, some 240 m. from the bottom, was 

 136 mg. The position of this station was north of the Atlantic-Indian cross-ridge which 

 acts as a partial barrier in the east to the northward flow of Antarctic bottom water ; the 

 bottom water found north of this ridge has become mixed with warmer water which 

 usually has a lower content. The latitude of 41 S is the position in which maximum 

 phosphate was recorded throughout the section ; a value of 157 mg. was found at a depth 

 of 1000 m. in the Antarctic intermediate water. It is probable that the maximum 

 decomposition of the Antarctic plankton occurs here, and perhaps the debris which rains 

 down to the bottom undergoes further decomposition to enrich the bottom water in this 

 position. It is a striking fact that nowhere else throughout the entire section between 

 69 20-8' S and 12 29-8' S does the phosphate content of the bottom water approach a 

 value of 136 mg. North of this position the phosphate content decreased considerably. It 

 must not, however, be forgotten that the nutrient salt content of the bottom water varies 

 and the high value found at 41 ° S may be to the production at some previous time of 

 bottom water with a high content. Thus in the eastern part of the South Atlantic Ocean a 

 considerable variation in nutrient salt content occurs in the Antarctic zone. The following 

 observations are typical examples. 



D XIX x 3 



