SECT. 3] AGE DETERMINATION IN SEDIMENTS BY NATURAL RADIOACTIVITY 



817 



measured. Variations in the concentration of these elements are caused by the 

 decay as well as the differences in the rate of sedimentation. This difficulty is 

 serious when the addition of the radioactive elements to sediments is governed 

 by a process independent of sedimentation. 



86 



87 



88 



89 



90(Th) 91(Po) 92(U) 







a 



81(TI) 



82(Pb)-- 83(8i 



90 



91 



92 



Fig. 1 . Decay series of 238u, 235u and 232i'h^ showing half h ves and branching ratios of 

 the isotopes. The old symbols shown in the circles, squares and octagons in the figure 

 are giving way to the modem terminology shown on the top and bottom; i.e. 227^}^^ 

 not RdAc. (Data from Hollander, Perlman and Seaborg, 1953.) 



2. Theoretical Considerations 



In principle, three basic methods of age determination in sediments can be 

 delineated. Some variations of them are noted, but each of these can be reduced 

 to one of three basic tjrpes. In all cases, it must be assumed that the separation 

 of the radio-nuclides used for the determination of age has been achieved to a 

 known degree by a geochemical process in nature. 



A. Theoretical Method of Age Determination, Type 1 



In this method, the equilibrium between two radioactive nuclides is used to 

 obtain an estimate of an age. The parameters known, and sometimes available 

 for measurements, are the concentration of both radio-nuclides and their decay 

 constants. The equation is the following: 



iV^2 = iv^Qi 



A2 — A] 



(g-Ai«_g-A2«)_ 



(3) 



