214 DISCOVERY REPORTS 



Referring to the absence or great scarcity of diatoms in the blue mud deposits of the Weddell Sea and 

 their immense abundance in its surface waters Pirie (19056) suggests tentatively that the sinking cells 

 may be swept away by the bottom current off to the north and east to be laid down as the great belt of 

 diatom ooze that in the South Atlantic (Murray and Philippi, 1908; Hough, 1956) is found principally 

 between 50° and 60° S. Drygalski (1928), somewhat more emphatically, expresses the same opinion. 



In more recent times we of the Discovery Investigations recall vividly the unusually loud and clear- 

 cut echoes we used to receive when sounding over the very region traversed by Ross and Bruce, fairly 

 conclusive evidence that the bottom there is hard, and hard perhaps because it is swept clean of 

 echo-deadening ooze by a current of considerable power. Moreover, since the bottom water can be 

 traced over an immense distance to the north, Wiist (1933) having shown that in the western Atlantic 

 basin it flows for nearly 7000 miles directly to 40° N, it seems reasonable to conclude that in the 

 high latitudes near its source it is moving with considerable velocity.^ In the Indian Ocean too it 

 travels a long way, Sewell (1948) suggesting that, as a carrier of certain Antarctic copepods, its influence 

 may extend as far as the Bay of Bengal. If it does in fact extend so far^ this would again imply rapid 

 transport, for copepods are short-lived and to survive carriage over such an immense distance they 

 would have to travel quickly. 



In his recent review of the principal Antarctic water movements and their climatological and 

 zoogeographical significance Deacon (1959) has written: 



We have a fair knowledge of the water masses and a qualitative idea of their movements but cannot get much farther 



without direct measurements of the currents and the water circulation at all depths By analogy with what we 



begin to know about the North Atlantic ocean the deep currents probably travel i or 2 miles a day, but there is some 

 evidence from the behaviour of sounding lines and dredges and from the movements of plankton that the bottom 

 current from the Weddell Sea may be stronger: and, judging from its effect on the bottom temperature in all the 

 deep channels leading from the Antarctic ocean, it must have a transport comparable with that of the Gulf Stream.^ 



Perhaps there are other phenomena contributing to violent bottom movements below the Weddell 

 Sea. It is becoming widely recognised (Heezen, 1959) that turbidity currents engendered on the 

 continental slope may now and then travel down the slope with tremendous force, their influence 

 extending far out over the abyssal plain. Below the great expanse of shelf water to the north of the 

 Ross Ice Barrier the bottom is covered by a deep glacial mud of such exceedingly fine texture that it 

 feels soft as velvet to the touch. A similar deposit (Wordie, 1921 a) is found at the head of the Weddell 

 Sea where, by virtue of the proximity of the vast Antarctic ice-sheet and its enormous capacity for 

 depositing terriginous material, it could be building up until its load became unstable and so from 

 time to time (Heezen, 1959) trigger off a turbidity current on the adjacent slope moving away at 

 enormous speed. Such a current was triggered by the earthquake which shook the continental 

 slope south of Newfoundland in November 1929, Heezen and Ewing (1952) calculating that it had 

 moved off down the slope at 50 knots and was still travelling at well over 12 knots far out over the 

 abyssal plain well over 450 miles away.* 



1 A recent re-examination of the data obtained by the 'Meteor' expedition reveals unexpectedly high velocities in the 

 bottom water as it flows along the foot of the continental slope off South America, Wust (1955) calculating that there it may 

 be reaching a maximum speed of 8 nautical miles a day or about a third of a knot. 



2 It seems unlikely in fact that it docs, Sverdrup (1954) pointing out that although the deep northward flow is particularly 

 conspicuous in the Atlantic, where it can be traced to at least 30° N, in other oceans it reaches only to about 20° S. 



^ Surface velocities of as much as 4^ and 5 knots have been recorded in the Gulf stream near Cape Hatteras (Malkus, 1953 ; 

 Stommel, 1958). 



* Bruun (19576) suggests that such currents, starting perhaps as slides on the slopes, may carry small animals in suspension 

 for long distances, while Laughton (1959), in a recent review of the forces at work in moulding the face of the deep-sea floor, 

 calls attention to their far-reaching influence as sediment carriers and to their enormous initial velocities. 



