ANTARCTIC BOTTOM WATER 101 



In 1935 an average value in the same area was 140 mg. It is evident that the very con- 

 siderable seasonal and annual variation in phosphate content which occur render the 

 charting of phosphate content at any given position in the bottom water impossible 

 until more data is at hand to make allowances for these changes. 



Across the Southern Ocean between Enderby Land and western Australia the content 

 was no mg., increasing on the Kerguelen-Gaussberg ridge to 125 mg. 



South of Australia the bottom water in the Antarctic zone had a content of 130-135 mg. 

 which decreased north of the convergence, whilst in the deep stations south of the 

 Tasman Sea, near Macquarie Island, the bottom water had been subjected to mixing 

 and had a content of 110-121 mg. 



South-east of New Zealand in the deep basin in 50-60° S the bottom water had a 

 low content of 71-79 mg. in the north and 123 mg. in the south, whilst on the Cape 

 Adare-Easter Island ridge the content was as much as 161 mg. In the central part of 

 the South Pacific, in the neighbourhood of the Cape Adare-Easter Island ridge, the 

 content was 135-145 mg., this was a general content throughout the remainder of the 

 Antarctic zone as far as 94° W where the value was 120 mg. In the sub-Antarctic zone 

 between 8o° and 90° W the content increased northwards from 122 mg. to 136 mg. in 

 55i° S. 



A number of observations in 1934 in the pack-ice region of the South Pacific showed 

 evidence of seasonal variation in the content of the bottom water at 4000 m. A westerly 

 cruise gave an average of 136 mg., whilst an easterly one gave an average value of 152 mg. 

 some five weeks later. Similarly in 8o° W in the same year a seasonal variation was 

 evident. 



The silicate content of Antarctic bottom water is always large, particularly in the 

 Antarctic zone. At a depth of 4000 m. a usual content of the bottom water in the 

 Antarctic zone is about 6000-7000 mg., but values greater than 7000 mg. have been 

 found, notably in December 1933 when as much as 8600 mg. was recorded in the 

 Scotia Sea and over 8000 mg. at the ice-edge in 78° W, both at a depth of 3500 m. 

 When the rain of silica skeletons from the very large concentrations of diatoms which 

 flourish in the Antarctic surface water is considered, it is clear that the Antarctic bottom 

 water must be the seat of a large amount of silicate ; the diatom skeletons which form 

 the diatom ooze are in contact with the bottom water and re-solution must occur con- 

 tinuously, whilst fresh supplies of dead diatoms are deposited after the time of phyto- 

 plankton efflorescence in the spring and summer months. 



In general the Antarctic bottom water in the Southern Ocean flows east and north. 

 Wiist (1933) showed that the Antarctic bottom water could be traced as far north as 

 40° N in the western Atlantic. As a result of the northward movement large quantities 

 of silicate are carried away from the Antarctic zone but they are replenished by the 

 southward movement of the warm deep water. Mixing between the bottom and deep 

 currents transfers silicate to the latter current. 



In the Antarctic zone the silicate content is not always a maximum at the bottom. 



