SECT. 3] AGE DETERMINATION IN SEDIMENTS BY NATURAL RADIOACTIVITY 826 



amount- has been in ionic solution in sea-water. Although it has been employed, 

 the ratio of loge to "Be is a very uncertain isotope ratio to be used for age 

 determination. 



4. Practical Methods of Age Determination 



The few methods available that allow us to determine the time period 

 elapsed since a component of a sediment layer reached the ocean floor are 

 not generally applicable. Some of them are limited to a single component, which 

 may be the carbonate or the clay; some do not work in shallow-water sediment; 

 and unproved assumptions are often made for others. The theoretical equations 

 applied and the geochemical processes involved are discussed above. In the 

 following section, the methods in practice are described. 



A. Cosmic- Ray -Produced Radio-nuclides 



a. Carbon- 14 



This method was first used by Arrhenius, Kjellberg and Libby (1951) for a 

 Pacific core taken during the Albatross Expedition. Further determinations on 

 deep-sea samples were carried out by Rubin and Suess (1955), Rubin (1956), 

 Ericson, Broecker, Kulp and Wollin (1956), and Broecker, Ewing and Heezen 

 (I960). It is assumed that the ^^Cj^^C ratio in the carbonates is constant at the 

 time of the carbonate formation, and the change of the ratio is used to determine 

 the age according to theoretical method Type 3 given on page 820. The method 

 is generally limited to about 30,000 years and may be extended, by using 

 extremely carefully developed techniques, to 65,000 years (Haring, De Vries 

 and De Vries, 1958). 



b. Other nuclides 



Potential methods are given by the use of beryllium- 10 (Peters, 1957; 

 Arnold, 1956), silicon-32 (Lai, Goldberg and Koide, 1959) and aluminum-26. 



c. Aluminuni-26/beryllium-lO ratio 



By using two cosmic-ray -produced radio-nuclides, a mixture of Types 2 and 3 

 is obtained and would allow absolute age determination with the only assump- 

 tion that both nuclides are produced in constant ratio. Lai {in litt.) proposed the 

 use of aluminum-26 in connection with bervlhum-10. The difficulties involved 

 in the use of this method are great and would entail the development of very 

 low background counters. It would allow age determination from 500,000 to 

 about 10,000,000 years. 



B. Natural Radio-nuclides 



a. Ionium/uranium or protactinium/uranium ratios 



When uranium is incorporated in a mineral without its following products, 

 the increase of the daughter elements ionium (230Th) and protactinium (23iPa) 



