822 . KOCZY [chap. 30 



measurements by Rona et al. (1956), Stewart and Bentley (1954) and Hecht 

 et al. (1956), gives values of 2.5 to 3.3 by 10~^ g U/ml. The second assumption is 

 a crude approximation and valid only when a constant depth of the ocean is 

 assumed. 



The ionium precipitation is constant as long as the uranium content in ocean 

 water is constant and the pH value of ocean water is about the same as at 

 present. But it is unlikely that the uranium concentration is constant in the 

 ocean, since the amount of uranium transported to the ocean is rather high 

 (8 X 10~^ g/cm^ per year) as compared with the amount of the uranium contained 

 in a water column of 1 cm^ base (12.6 x lO"* g). (See later discussion of uranium.) 



The mechanism involved in the precipitation of thorium is not very clear. 

 Pettersson (1937) assumed co-precipitation with iron. Holland and Kulp (1954) 

 assumed adsorption on clay. Arrhenius et al. (1957) also mentioned incorporation 

 into bone structures. Thorium-232 is a thorium isotope found in greatest abun- 

 dance in sea- water. It has not yet been possible to obtain exact values, but 

 only an upper limit of the concentration. According to the measurements by 

 Koczy et al. (1957), this is 2 x IQ-ii g Th/ml. The thorium content of fresh 

 water is so small that it has not yet been possible to determine it. It is believed, 

 therefore, that the thorium content found in sediments must be transported while 

 incorporated in the minerals which settle on the ocean floor, as the following cal- 

 culation will substantiate. The thorium content in the deep-sea sediment is about 

 5 to 10 x 10-6 g Th/g. If we assume a rate of sedimentation of about 1 g sediment 

 per square meter per year, about 5 to 10 x 10-^ g Th must be transported per 

 square meter per year to the sea. This would make necessary the conclusion that 

 the thorium content in fresh water is one order of magnitude higher than the 

 upper hmit estimated. Further thorium measurements in river water are 

 necessary in order to substantiate this conclusion. 



B. Uranium 



Selective enrichment of uranium is known to occur by the formation of corals 

 and oohtes (Barnes, Lang and Potratz, 1956; Tatsumoto and Goldberg, 1959). 

 In these calcareous organic remains the growth of thorium-230 and protactin- 

 ium-231 from the parent elements uranium-235 and -238 and the age of the 

 formation of the carbonate can be determined by the formulas given above. 

 Uranium is also enriched in many of the sediments formed under reducing 

 conditions. 



The uranium content of deep-sea sediment is generally lower than in sedi- 

 ments found on the continent or in the rocks where it originated. On the other 

 hand, black shales and sediments in land-locked water show a rather high 

 uranium content (Strom, 1948; Koczy, Tomic and Hecht, 1957). It may, 

 therefore, be concluded that uranium is leached from the inorganic material 

 formed in the deep-sea sediment and enriched in some shelf sediments. This 

 process makes the uranium content in sea-water dependent on the distribution 

 and the character of shelf sediments. Variation by a factor of 2 may occur during 



