372 THE DEEP SEA 



ticular phenomenon, the method employed must be based on a 

 radionuclide which has a half time T of the same order as the age t 

 to be determined. If / » T, the number of atoms of R remaining in 

 the system becomes too small to be detected. If / « T, the decrease 

 in the number of atoms of R will be very small, and will lie within 

 the limits of the experimental error. 



The ages which present an interest in this held lie between a 

 few thousand and a few million years. Radioactive nuclides with 

 half lives of this order, short with respect to the age of the earth, 

 are found in nature, and may be classified in two groups, according 

 to their origins: (1) the members of the three radioactive families, 

 and (2) radioactive nuclides formed by the interaction of cosmic 

 rays with constituents of the atmosphere or of the lithospherc. 



Among the first group we notice three nuclides with half lives 

 lying between 10^ and 10*^ years, namely (1) in the U-''^ family, 

 radium-226 {T = 1600 years), (2) ionium (Th^^o) {T = 80.000 

 years), in the U-^^ family, (3) protactinium-231 {T = 34,000 

 years) . 



The longest half life among the members of the thorium family 

 is that of mesothorium-1 (Ra--**) which is 6.7 years. 



The second group contains a large number of short-lived 

 nuclides. The only two which have a half life longer than 10^ years 

 and exist in measurable amounts in the ocean or its sediments, are 

 carbon-14 {T = 5500 years) and beryllium-10 {T = 2.5 X 10^ 

 years). 



Let us briefly recall what we know of the geochemistry of these 

 radioisotopes in the ocean and in its sediments. 



Natural Radioactive Nuclides in the Ocean 



Table I summarizes our present-day knowledge of the concen- 

 trations of radioactive nuclides in the ocean. It is noteworthy that 

 by far the largest part of the ocean's radioactivity is due to the 

 presence of K^". 



The nuclides tritium, carbon-14, and beryllium 10, formed by 

 cosmic ray action, are discussed in recent reviews by Begemann 

 and Libby (1957), Giletti et al. (1958), Broecker (p. 301), Merrill 

 et al. (1958), and Peters (1957). The other radioactive nuclides are 



