86 DISCOVERY REPORTS 



in the subtropical zone the maximum was found in the intermediate current. The 

 content at the maximum in the Antarctic zone varied according to the position of the 

 stations— those on the Kerguelen-Gaussberg ridge being greater (140-160 mg.) than 

 those west of the ridge (1 15-130 mg.). North-east of the ridge in the sub-Antarctic 

 zone as much as 145-155 mg. was found as a maximum content in the south and 

 130-140 mg. in the north. South of Australia in the Antarctic zone the maximum 

 was 140-150 mg., the value falling towards the north. South-east of New Zealand 

 two sections in 1932 and 1934 gave conflicting results. In the Antarctic zone of the 

 central part of the South Pacific Ocean maximum phosphate appeared to be in the 

 upper part of the warm deep water and this maximum (150-160 mg.) was considerably 

 greater than that in a corresponding area in the South Indian Ocean and south of 

 Australia. 



In the ice-edge region of the South Pacific sector on two cruises which were separated 

 by a short interval of time considerable variation of phosphate content in the warm 

 deep water was found. 



Longitudinal sections through each of the three main oceans in the southern hemi- 

 sphere show that the return of silicate to the Antarctic zone is effected in the southward- 

 flowing warm deep water. The major part of the silicate is returned in the lower part 

 of the current, whilst a smaller proportion returns in the upper layers. The silicate 

 content of the warm deep water is of paramount importance to the life cycle of the 

 Antarctic plankton. In the Antarctic zone and parts of the sub-Antarctic zone we can 

 fix the depth of the upper layers of the warm deep water by noting the position of the 

 temperature inversion between the surface and warm deep currents. The maximum 

 temperature of the warm deep water is found just below the temperature inversion 

 referred to and represents the temperature of the upper part of the current. Another 

 important level in the warm deep water may be fixed by the depth of the salinity 

 maximum of the layer. If we discuss the silicate content at these two depths wherever 

 possible, a good knowledge of the amount of silicate in this important water mass will 

 be obtained. 



On three occasions in 1934-5 a line of observations was made northwards from the 

 South Orkney Islands to the Antarctic convergence. The silicate content at the depths 

 of maximum temperature and salinity of the warm deep water has been plotted and is 

 shown in Figs. 23 and 24. In April 1934 the most southerly station was situated in the 

 very deep area just north of the islands and the silicate content at the depth of maximum 

 temperature in the upper layers of the warm deep current fell from about 5500 mg. in 

 the south to 2700 mg. just north of the Antarctic convergence. Between the latitudes 

 of 53I and 55 S an intermediate rise in silicate content in the upper part of the current 

 occurred towards the north in all three sets of observations; this increase of silicate may 

 be a permanent feature connected with a rise in the sea-bottom in this locality which 

 causes upwelling. Very little conclusive evidence of seasonal variation at the depth of 

 maximum temperature is shown, particularly in the middle part of the line of stations. 



