826 KoczY [chap. 30 



can be used for age determination. First it must be established that the separa- 

 tion of uranium from its following product is complete or at least the degree of 

 separation is constant and known. This is established for corals (Barnes, Lang 

 and Potratz, 1956) and for oolites (Tatsumoto and Goldberg, 1959). The 

 formula (7) apphed is given above under Type 1 (b) (p. 818). The upper limit 

 is about four half lives, i.e. 300,000 years by using ionium (230Th) and 100,000 

 using protactinium (23iPa). The use of the ratio of protactinium to ionium makes 

 the determination of uranium unnecessary. 



b. Protactinium/ionium ratio 



The age of one level in a Pacific core was determined by Sackett (1960), and 

 the affes of several levels in cores from the Caribbean and the Atlantic are 

 reported by Rosholt et al. (1961). This method is based on the assumptions that 

 ( 1 ) protactinium and thorium are precipitated to the same extent in sea-water, 

 i.e. with a yield of more than 98% ; (2) the isotope ratio or uranium-235 to 

 uranium-238 is constant in sea-water ; and (3) the uranium in the sediment is in 

 equilibrium with its following product. In order to correct for the uranium 

 content, the equation (10) of Type 2 is changed to 



f = 8.66 In 



230Th-W 



2.33 



104 yr, (14) 



23iPa-wJ 



where the bars indicate uranium equivalents. The limit of age determination 

 is about 175,000 years. Of the three given assumptions, the first two seem to 

 be valid as deduced from the chemistry of protactinium and thorium and the 

 results of isotope measurement by Senftle et al. (1957). The assumption that 

 uranium is in equilibrium with the daughter product may not be true as outlined 

 in geochemical considerations, but the error introduced is rather small. This 

 method gives the age of the material to which the ])rotactinium and ionium 

 are attached during precipitation. The determined age is not necessarily the 

 same for other components of the sediment because of the reworking of a part 

 of the sediment. 



c. Ionium/thorium ratio 



Picciotto and Wilgain (1954) proposed the use of the ratio of the two thorium 

 isotopes with atomic weights 230 and 232. The method was used by Baranov 

 and Kuzmina (1958) and by Goldberg and Koide (1958). The validity of this 

 method is doubtful. It assumes that the total thorium-232 content extracted 

 from the sediment is precipitated from sea-water. A further assumption is the 

 constant ratio of uranium to thorium in ocean water, which seems unlikely. 

 The development of suitable extraction methods yielding only the adsorbed 

 thorium-232 may improve the method. The limit of age determination would 

 be in the order of 400,000 years. 



d. Ionium or protactinium 



This method is based on the following assumptions: (1) The uranium content 

 of sea- water is constant, and (2) the precipitation of thorium isotopes is con- 



