RADIOACTIVE ELEMENTS AND DEEP-SEA SEDIMENTS 377 



Radioactive Nuclides Present in Sediments 



We shall confine ourselves to the question of recent pelagic 

 sediments covering the floor of ocean deeps where the rate of 

 sedimentation is extremely slow. Offshore epicontinental sedi- 

 ments, being essentially formed of detrital materials, normally 

 have a composition which simply reflects that of the continental 

 rocks from which they originate. In the case of pelagic sediments 

 we would expect to find a situation complementary to that of the 

 ocean. In other words, we would expect to find in excess over the 

 radioactive equilibrium, the nuclides of the three radioactive 

 families, which have a lower than equilibrium concentration in 

 the ocean. 



The earliest measurements of radium were carried out by Joly 

 in 1908 on pelagic clays brought back by the Challenger expedition. 

 They indicated a very high radium content, between ten and fifty 

 times that of ordinary granitic rocks. These measurements have 

 since been confirmed by Pettersson and by other authors. The 

 origin of this high content was not, however, as easily agreed upon. 

 Pettersson, in 1937, suggested that this excess radium was due to 

 the presence of ionium, coprecipitated with ferric hydroxide. 

 Piggot and Urry (1942a,b) showed that the distribution of radium 

 in the depths of the sediment cores corresponded to that postulated 

 by Pettersson's hypothesis. They based a method of chronology 

 of the sediments on this, by indirectly determining the ionium by 

 way of the radium present. The basic assumption of their method 

 was that the radium, produced in sediments by disintegration of 

 ionium, remained where it was, and did not migrate. This assump- 

 tion appeared to be of rather doubtful value when diff^erences in 

 the chemical properties of radium and ionium are taken into 

 account. 



The numerous measurements carried out by Kroll (1953-1955) 

 on the sediment cores brought back by the Swedish expedition on 

 the Albatross, led by Pettersson, complicated the question even 

 further. Kroll observed very irregular distributions of radium, 

 which appeared to be incompatible with the simple model proposed 

 by Piggot and Urry. A possible explanation for these irregularities 



