38 ECHO-SOUNDING AND HYDROGRAPHICAL STUDIES 



is greater than that of either of the other oceans, and it gets deep 

 and bottom water from the north as well as from east and west. 

 Consequently, it is in the Atlantic that we find the greatest renewal of 

 the psychrospheric water masses and the most active horizontal inter- 

 change of water in the abyss. The North x^tlantic abyssal current is sup- 

 plied by the sinking of cold water in the north-west and of highly saline 

 water from the Mediterranean. It has a temperature of 2 — 3° C, a salinity 

 of about 34.9 per thousand, and an oxygen content exceeding that of 

 any other abyssal current. It is the greatest of all oceanic currents, moving 

 southward at depths between 1,500 and 4,500 metres to about latitude 

 40° S, where it rises above the cold water that sinks near the shores of 

 the Antarctic continent and penetrates to all the deepest parts of the 

 Atlantic along the bottom. The North Atlantic current carries some of 

 the water from the Antarctic bottom current southward again, along 

 with water from the overlying layer; and, after mixing with these water 

 masses and attaining a temperature of 0.5 — 2.0° C, a salinity of about 

 34.75 per thousand, and a persistently high oxygen content, reaches the 

 Antarctic regions, where it forms the main constituent of the circumpolar 

 current which supplies the abysses of both the Indian Ocean and the Pacific 

 Ocean with large volumes of water rich in oxygen and plant nutrients. 



In the Indian Ocean also there is a sinking of cold bottom water along 

 the xA.ntarctic coast, but here a southward-flowing current like that in 

 the Atlantic is absent or nearly absent. It is true that a current of highly 

 saline water poor in oxygen comes in from the Red Sea, but, owing to 

 the small volume of water involved, its effect south of the Equator is 

 slight. The great bulk of the deep water in this ocean is influenced by a 

 flow from the x^tlantic. 



While current systems in the upper layers of the Pacific are more 

 complex than they are in either of the other oceans, the characteristics 

 of the deep layers are great horizontal and vertical uniformity and very 

 slow movement. Here, of course, there is no significant influx of new 

 water masses except for those coming in with the Antarctic circumpolar 

 current. The oxygen content and salinity of the deep water layers reveal 

 a very slow, circular, and clockwise movement; that is to say, northward 

 in the western part and southward in the eastern part of the ocean. But 

 though these water masses will take decades and even centuries to reach 

 the various regions of the Pacific, we shall find at all the deep levels 

 water which can be traced back to its origin in the southern Atlantic, in 

 spite of the volumes which have flowed in on the way. 



On the Galathea Expedition we took about 270 stations: 200 in connec- 



