SKCT. 3] AGK DETERMINATION IN SEDIMENTS BY NATUBAL RADIOACTIVITY 



821 



An improvement of this method, as proposed by Lai {in litt.), is obtained 

 when the ratio of two cosmic-produced isotopes, such as 26A1 to ^^Be, can be 

 employed. 



For the natural radio-isotopes, the ratio of ionium to thorium has also been 

 used. The use of this ratio is possible when it is assured that only thorium, 

 which is precipitated from sea-water together with ionium, is measured. 



I 



Fig. 5. Decay of radioactive substance. The ordinate gives tlie latio of the original amount 

 of the radio-nuchde to the amount left after a certain time. The abscissa gives the 

 non-dimensional units indicating the time as multiples of mean lifetime. 



3. Geochemical Considerations 



The basic condition for all age determinations is the elimination or addition 

 of radio-nuclides to a closed system which, after the completion of the processes, 

 does not exchange with the surroundings. Age determination in marine sedi- 

 ments is based on the same principle. Because the sediments are a mixture of 

 many components of different origins and different histories during the trans- 

 port to their final burying ground on the ocean floor, many difficulties arise. 

 In the following sections, some of the most important processes that have a 

 bearing on age determination are discussed. 



A. Thorium 



As early as 1908, Joly had found a high radium content in sediment samples 

 he had received from the Challenger Expedition. Hans Pettersson gave the 

 right explanation in 1937 by suggesting that ionium (230Th) is precipitated in 

 the ocean and added to the sediment. His hypothesis was confirmed by Koczy 

 et al. (1957) and later reconfirmed by Sackett, Potratz and Goldberg (1958). 

 The yield of separation of sso^h from sea-water was more than 98%. It is 

 believed that it is as high as 99.8%. If we assume that the uranium content of 

 ocean water is 3 x 10"^ g U/ml and that the produced ionium is evenly spread 

 over the ocean floor, about 1.9 x 10~i3 g saoTh is precipitated per square centi- 

 meter per year on the ocean floor. The first assumption, based on several 



