828 KOCZY [chap. 30 



must be made about s{t) and i^{t) before this equation is applicable. These are 

 the assumptions discussed above. 



(1) i^ls{t) = c^{t) = constant (Piggott-Urry assumption). 



The concentration of ionium is constant during the past for the moment 

 when the sediment settles. The resulting radium distribution with depth is 

 then smooth and must be logarithmic. Three cores have demonstrated this to 

 be true, but they are exceptions. 



(2) i^{t) = constant (Pettersson assumption). 



This assumption alone does not allow age determinations and an additional 

 assumption has to be made. If it is assumed that .s-^' is constant, the same case 

 as (1) results and c^{t) would be constant. 



In order to exj)lain the irregularities of the concentration with depth, radium 

 diffusion and selective adsorption on some layers were assumed to blur the 

 picture. Picciotto (1960) showed that this explanation could apply in some 

 instances, but extremely high radium maxima in old layers also have a high 

 ionium content unsupported by uranium. The only possible conclusions are 

 that the rate of ionium precipitation and the rate of sedimentation are variable 

 with time, or that ionium is mobile. The latter is rather unlikely because of 

 geochemical reasons. Kroll (1953) designed a method giving the average rate 

 of sedimentation, but not the age of diiferent levels (see later discussion). 



5, Rate of Sedimentation 



The rate of sedimentation is estimated by dating of the sediment layers. In 

 general, the rate is given in length per time, which is rather inexact, as has been 

 discussed by Arrhenius (1947) and Koczy (1949). The appropriate unit, ex- 

 pressed as mass/time/area, is independent of compaction of sediment and water 

 content. The rate of sedimentation is easily obtained for a given period of time 

 from the difference in the ages of the upper and the lower boundary and the 

 amount of sediment in between. But other methods may be used. If it is known 

 that an element settles at a constant rate in the ocean and is added to the 

 independently settling sediment, the concentration of the element gives the rate 

 of sedimentation. One necessary condition is exact knowledge of the constant 

 rate of precipitation of this element, and another is that the element is not 

 contained in the sediment at the moment it settles. Thorium-230 and prot- 

 actinium-231 are hydrated in ocean water and are consequently unstable. 

 Both are produced by the rather soluble uranium isotopes. The amount pro- 

 duced seems to be much higher than the amount soluble in sea-water, Thorium- 

 230 and protactinium-231 are precipitated at a constant rate as long as the 

 uranium content in sea-water is constant. This situation is unique, and in no 

 other known instance is there an independent and rather constant rate of 

 precipitation of an element in the ocean. 



