RADIOACTIVE ELEMENTS AND DEEP-SEA SEDIMENTS 375 



respect to the nuclides of the three radioactive families present in 

 the ocean : 



1. The members of the three families are present in a very low 

 concentration in the ocean, much lower than in the earth's crust. 



2. The ratio of the concentration of Th/U is exceptionally low. 

 This ratio is less than 0.01 in the ocean, while it is of the order of 

 3 in rocks. This is a geochemical characteristic of the ocean, as 

 was emphasized by Pettersson (1937). 



3. The radioactive equilibria of the three families are completely 

 dislocated. If we call Ra/b the ratio of the specific activities of two 

 members A and B of the same family, we find the following 

 situations existent in the ocean (it must be remembered that if 

 A and B are in equilibrium, Ra/b = 1): 



-Klo/238 < 0.02 i^Pa/U235 < 0.1 



-^Ra/Io '^ 6 -KjVIsTh/Th ^ 4 



The ionium and the protactinium produced by the disintegration 

 of U^^^ and of U^^^ have practically disappeared. This indicates 

 that they have precipitated to the floor of the ocean in a time which 

 is short compared to their half life. On the other hand, the two 

 isotopes Ra and MsTh are in excess with respect to their respective 

 parents lo and Th. We have interpreted this as due to the redis- 

 solution into the ocean, of Ra and MsTh formed in the sediments 

 (Koczy et al., 1957). 



This situation, if confirmed, would present interesting possi- 

 bilities for studies of oceanic circulation, complementing those 

 opened by tritium and carbon- 14. Radium possesses two advan- 

 tages over C^^ in that its half life of 1600 years is more adaptable 

 to the scale of time of oceanic circulation and also in that it 

 (radium) is liberated from the ocean floor, whereas C^^ is intro- 

 duced at the surface. 



Thus, mesothorium, having a short half life (7 years) would 

 practically disappear not far from the bottom. Its detection could 

 be utilized to identify bodies of water that had been recently in 

 contact with the sediments on the floor, or to measure horizontal 

 currents leaving the continental shelf or passing in the neighbor- 

 hood of suboceanic ridges. 



