SECT. 3 I THE SOUTHERN OCEAN 298 



current as well as the depth, the Antarctic bottom water being a very effective 

 solvent for calcareous material. 



The boundary between the diatom ooze and Globigerina ooze lies just north 

 of the Antarctic convergence and approximately parallel to it, but in certain 

 areas from which cores of the bottom sediments have been obtained, particu- 

 larly in the neighbourhood of the Crozet Islands, diatom ooze typical of the 

 Antarctic zone is found underlying the Globigerina ooze typical of warmer 

 water. It is believed that the superposition of Globigerina ooze occurred during 

 the retreat of the convergence from an advanced position held during the 

 Glacial Period. If the convergence was farther north it should be argued that 

 there must have been simultaneously a greater spreading of the Antarctic 

 bottom current, and this conclusion is supported by the occurrence of Globi- 

 gerina ooze superposed on red clay. In addition, the rinding of diatom ooze 

 superposed on glacial sediments in the northern part of the Antarctic zone 

 points to a simultaneous decrease in the amount of glacial material carried into 

 the sea from the Antarctic continent. 



Like the Antarctic convergence, the subtropical convergence has been found 

 to limit the distribution of some species of drifting plants and animals, and of 

 some fishes and bottom-living animals. Certain species are typical of sub- 

 Antarctic water and some of subtropical water. The two halves of the sub- 

 Antarctic zone, the northern stratified water with a marked highly saline 

 sub-surface layer, and the southern well mixed part, also appear to favour some 

 different species, for example the Polychaeta (Tebble, 1960, p. 237). 



The convergences are not boundaries but probably mark the normal limits 

 of circulatory systems that are favourable to the breeding and growth of species 

 that have become adapted to them. Most species, if not all, have circumpolar 

 distributions, but there are variations in abundance from sector to sector. 

 Marr (1956), dealing with Eu-phausia superba, the food of the Antarctic whales, 

 emphasizes the effect of the cold current from the Weddell Sea in extending 

 the distribution and increasing the abundance of the species in the Atlantic 

 Ocean. 



6. Nutrient Salts 



The work of Clowes (1938) and others shows that the amounts of phosphate, 

 nitrate and silicate in the Antarctic surface water rarely fall below the winter 

 maxima of temperate regions, so that Hart (1934) attributes the falling off of 

 phytoplankton after mid season to other factors. The surface water of the sub- 

 Antarctic zone has less nutrients, but at a deeper level, between the northward 

 movement in the Antarctic intermediate layer and the southward movement in 

 the more saline deep water, phosphate and nitrate appear to be regenerated, 

 possibly because of a large mortality of sinking phytoplankton as indicated by 

 abundant deposits of diatom ooze below this region. Oxygen consumption 

 appears to be highest in the same boundary layer between the sinking north- 

 ward and rising southward flows. The decomposition enriches the south-going 



