ANTARCTIC BOTTOM WATER 109 



side of the ridge is in itself an indication that they are formed from the same source — 

 a current sinking from the surface. 



It is not unlikely that the region is a convergence between the current which flows 

 northwards out of the Weddell Sea and the drift towards the north-east out of the Drake 

 Passage ; a large number of icebergs often of enormous size are generally found there, 

 and although some may be aground in the shallow water found on the Scotia Arc it is 

 also possible that they are accumulated owing to the converging of the currents. The 

 Scotia Arc may play a large part in promoting intense vertical mixing, since the sudden 

 changes in depth are certain to give rise to turbulent movements in the deep currents. 

 The coldest bottom water at Sts. 637-9 na< ^ a potential temperature of —0-73° C, but 

 at St. 169 in 6o° 49' S, 51 00' W (Station List, 1929), where the bottom water is 

 separated from the water at the same depth to the north and south by shallow submarine 

 ridges (see Herdman, 1932, pi. xlv), the potential temperature was as low as — o-8o° C. 

 This temperature is, however, 0-22° C. higher than that of the bottom water in the south- 

 western part of the Weddell Sea, and the temperature distribution as far as it is known 

 at present (Wiist, 1933, pi. h) leaves no doubt that the region can only be regarded as 

 a secondary source of bottom water. 



There are sufficient data available to show that there is no other region in which 

 bottom water is formed farther east along the Scotia Arc. Six series of observations 1 

 made between 6i° 52' S, 42 23' W and 6o° 01' S, 32 22' W at the end of winter 

 (November 1932, see Appendix I) showed that there was a relatively large difference 

 of temperature, salinity and oxygen content between the surface water and the deep 

 water. The least difference was found in the shallow water south-east of the South 

 Orkney Islands. The water in the first 100 m. had a temperature of —1-41 10 — 1-56° C. 

 and a salinity of 34-29-34-37 °/ 00 , whilst the temperature and salinity of the deep water 

 between 400 and 700 m. rise to — o-oi° C. and 34-65 %o- Farther east the differences 

 were greater and showed that there was no likelihood of a uniform water column in 

 winter. Where the depth exceeded 1000 m. the deep water had an oxygen content as low 

 as 4-2-4-3 cc. per litre. 



The distribution of potential temperature in the bottom layer has now made it quite 

 clear that the main source of Antarctic bottom water lies in the south-west part of the 

 Weddell Sea and it is generally agreed that in this region the cold and highly saline shelf 

 water sinks down the continental slope, mixes to some extent with the warmer deep 

 water, and then spreads northwards. Mosby (1934, pp. 83-4) has shown from a con- 

 sideration of the temperature, salinity and oxygen data collected by Brennecke, that the 

 bottom water has the properties of a mixture of shelf water and warm deep water in 

 equal proportions. 



The region is usually inaccessible, and since the time of Brennecke no further observa- 

 tions have been made. Although there is no doubt that the cold water sinks from the 

 shelf there is no series of observations to show it actually doing so. Wiist 's section to 

 illustrate the sinking (1933, P- 45) is not quite fair, because the bottom water at the shelf 



1 At Sts. 1036, 1038, 1039, 1041, 1042 and 1044. 



