SECT. 3J AGE DETERMINATION IN SEDIMENTS BY NATURAL RADIOACTIVITY 823 



a period of 200,000 years. It should be evident that the variation of sea level 

 during the fluctuations of the Ice Age had a pronounced effect on the uranium 

 content of sea- water. During maximum glaciation, the area favorable for 

 uranium precipitation would be considerably smaller than during the inter- 

 glacial stages. In addition, the uranium in the layers where it was concentrated 

 would be oxidized and could consequently be transported in solution to the 

 ocean. 



C. Radium 



Radium is found in excess over ionium in sea-water (Koczy et al., 1957). 

 Further ionium determinations are necessary, however, in order to substantiate 

 this statement, since it is based on the determination of the thorium isotopes in 

 North Sea water and in one sample of Pacific deep-sea water (Sackett, Potratz 

 and Goldberg, 1958). The explanation for this high radium content in sea-water 

 was given by Koczy (1954). He assumed it is supplied from the ocean floor. In 

 favor of this explanation is the deficit of radium in the surface layer of sedi- 

 ments (Picciotto and Wilgain, 1954; Picciotto, 1960). An attempt was made to 

 determine the rate of diffusion of radium in sediments by Koczy and Bourret 

 (1958), They were able to establish the equation for the distribution of the 

 radium content in a sediment layer under the assumption that the rate of 

 sediment precipitation, the rate of ionium precipitation, and the rate of radium 

 diffusion are constant: 



c{x) = coe-^WD)+—-J^ — - [e-x^WD)-e~^^/% (13) 



{ixivyD — A 



where c{x) is the radium concentration at depth x, A is the half life, D is the 

 effective diffusion coefficient, /u, is the decay constant of ionium, v is the rate of 

 total sedimentation, and T is the ionium content. This formula was applied to 

 the radium distribution of a core measured by Piggot and Urry (1942) and 

 remeasured by Volchok and Kulp (1957). The upper part of the curve allows us 

 to determine the diffusion rate of radium, the lower part of the curve gives the 

 rate of sedimentation. In this particular core, the maximum rate of diffusion 

 was found to be of the order of 10~9 cm-/sec. Thereby, it was assumed that about 

 IQ-io g radium are given off to sea-water from the sediment per square meter 

 per year. The low value of the radium diffusion indicates that radium is rather 

 strongly adsorbed on clay. The ratio of the radium in solution in the interstitial 

 water to the radium adsorbed must be about 1 to 10,000. Consequently, the 

 transport of radium from sediments to sea-water must be rather dependent on 

 the adsorption rate. The rate of radium release from the ocean floor was deter- 

 mined from the radium distribution close to the water-sediment interface and 

 also from the total amount of radium that has to be sustained in sea-water. 

 The calculation made for six different stations during the Swedish Deep Sea 

 Expedition gave a value of the same order of magnitude, i.e. between 100 

 and 180xl0~i2 g radium/m- per year. Further studies of the geochemical 



