SOUNDINGS TAKEN DURING THE DISCOVERY INVESTIGATIONS, 1932-1939 Si 



but soft ooze will often fail to return more than a faint whisper, which only an operator with much 

 experience can distinguish from other noises. The strength of the signal returning from a hard or rock 

 bottom has often been sufficient to cause re-echoes, and many have been recorded from quite 

 considerable depths. In one instance, on 8 March 1938, at St. 2277 (65° 19-6' S, 81' 42' W) 

 a clear re-echo was heard at twice the observed depth of 2428 fm. (4440 m.). If the soundwave is of 

 sufficient strength to echo again between the hull of the ship and the bottom at this depth, and then 

 provide a clearly audible signal on its second return, it would appear that there should be ample 

 reserve of power to provide an echo even from a moderately soft bottom. Immediately south of the 

 Convergence, however, the deposit is almost pure diatom ooze and it apparently lies on the bottom as 

 a very soft covering, the upper part of which is flocculent and comparable to a very thick soup. 

 At St. 2519, in 51° 57-8' S, 19° 32' E, the bottom reversing water bottle of the deep hoist apparently 

 hit bottom and brought up an excellent sample of this liquid ooze from 2865 m. (1567 tm.). 



Fig. 6. 0° Line, February-March 1939. Vertical distribution of density in terms of tr,. 

 Antarctic Convergence in, approximately, latitude 49° 33' S. 



Generally, it may now be said that a belt of diatom ooze surrounds the globe in the southern hemisphere 

 and that its northern limit approximates closely to the northern limit of the Antarctic Convergence 

 (see Hart, 1934, pp. 185-6 and Deacon, 1945, pp. 11-20). We thus find in the neighbourhood of the 

 Antarctic'convergence that there may be two important factors which will cause weak echoes, i.e. the 

 horizontal trend of the discontinuity layers north of the line of the Convergence, and the bottom 

 deposit of diatom ooze to the south. For a short distance north of the Convergence, when that line 

 is displaced to its southern limit, it is therefore possible to have both factors, and under these 

 circumstances sonic soundings have been proved to be almost impossible to obtain. It seems probable, 

 however, that supersonic methods might be more satisfactory, as it is possible to obtain a better 

 concentration of the beam of soundwaves. The belt of diatom ooze eventually fades out to the south 

 into a belt of glacial mud from which it is possible again to hear a clear echo. 



An area with faint or no echoes was also found by the second Byrd Antarctic Expedition, when their 

 ship the 'Bear of Oakland' was on passage between New Zealand and the Ross Sea; though the 

 latitude in which they were puzzled by the faintness of the echoes was south of any position in which 

 we found the echo regularly to disappear or become very faint. Roos (1937, p. 582), in commentmg 

 on this occurrence, suggests that it might be due to a very soft bottom or to heavy rolhng which, by 



