124 THE OCEAN FLOOR 



have another origin. The only acceptable explanation 

 so far advanced is that they are ionium supported. 

 However, no definite proof of this hypothesis had been 

 produced until in the summer of 1952 Picciotto and his 

 co-workers in Brussels developed the photographic 

 method already mentioned for direct measurements of 

 the ionium present in sediment samples. 



It is of interest to note that the amount of ionium- 

 supported radium in the upper levels of a core is of the 

 same order as the quantity of potential radium to be 

 expected from the disintegration of the uranium present 

 in the superposed column of sea water, although some- 

 what in excess of the latter quantity by a factor varying, 

 according to Kroll's measurements, from 1.3 to 2.8. 

 From this fact he infers that the content of uranium 

 and/or ionium in ocean water has been higher during 

 the latter half of the Pleistocene than it is at present, a 

 conclusion supported also by calculations made by 

 Koczy.* 



Reverting to the question about the cause of the 

 complicated shape of the curves showing radium dis- 

 tribution in the sediment cores studied by Kroll, one 

 may note that he has calculated the variations in the 

 rate of sedimentation required to explain their shape 



'•' Recent measurements by Nakanishi, Smith-Grimaldis, and 

 others indicate a higher uranium value, although at present the 

 most plausible average would seem to be 2=i=lXlO-<' 

 gr U L. With this higher value, IlvoU's factor is reduced to an 

 average value near unity. 



