RADIOACTIVE ELEMENTS AND DEEP-SEA SEDIMENTS 385 



Methods Based on Decay of Ionium or of Protactinium 



Ionium and protoactinium, separated from their long-lived 

 parent elements in the sediment, should, in principle, be useful to 

 measure intervals of time of the order of 300,000 and 150,000 

 years, respectively. 



Ionium. The ionium method has become the most frequently 

 used since the first radium measurements carried out by Piggot 

 and Urry. Here again, a simple knowledge of the distribution of 

 the element alone in the depths of the cores does not suffice, and 

 two alternate hypotheses have been used in addition: (1) the 

 initial concentration of ionium in the sediment is constant, at any 

 given place (Piggot and Urry) ; (2) the initial concentration of 

 ionium is not constant, but the amount of ionium precipitated in 

 unit time, is constant, and, moreover, is independent of the rate 

 of accumulation of the sediment (Pettersson). Each of these 

 hypotheses finds an application in particular cases, but neither is 

 of general value. 



Protactinium. Any method of chronology based on the decay 

 of this element will suffer the same restrictions as in the ionium 

 method. Both these methods are based on the assumption that 

 the concentration of the uranium in the ocean remains constant 

 with respect to time. 



Ionium-Thorium. This method, proposed by Picciotto and 

 Wilgain (1954) and developed by Goldberg and Koide (1958), is 

 based on the assumption that the nuclides ionium and thorium, 

 being isotopes, are precipitated in the same ratio. In this case, the 

 ratio of ionium to thorium in the sediment is a function only of 

 age, and is independent both of the rate of sedimentation and of 

 the rate of precipitation of the ionium. This basic assumption 

 remains to be proved, as, even though they are isotopes, ionium 

 and thorium may exist in different chemical forms in the ocean, 

 and may not precipitate in a constant ratio. Furthermore, the 

 method assumes that the ratio of uranium to thorium in the ocean 

 has remained constant in time. 



Protactinium- Ionium. This method (Sackett and Potratz, 

 1958; Rosholt, 1959) assumes that lo and Pa are precipitated in 

 a constant ratio. This basic assLimption appears to be quite 



