SECT. 3] AGE DETERMINATION IN SEDIMENTS BY NATURAL RADIOACTIVITY 829 



If it is assumed that the uranium content of ocean water is 3 x 10 » g/ml and 

 the decay constant js 1.52 x lO^i" yr~^, 



230 



^ X 1.52 X 10-10 X 3 X 10-9 g lo = 4.45 x lO-^ g Iq 



are produced per year per millihter. When this amount is precipitated in a 

 4000-m deep ocean, 1.78 x lO^^ g of ionium settle per square meter per year. 

 The ionium content of the surface sample then directly gives the rate of sedi- 

 mentation: 



Biglm^ yr) = l''^l''^^~' . (18) 



^' concentration oi ionium 



The ionium content must be corrected for the ionium in equilibrium with the 

 uranium in the original sediment. It is not necessarily true that the original 

 ionium content of the sediment corresponds to the uranium content, but it is 

 the only possible assumption at the moment. It may be, as mentioned before, 

 that uranium is leached from the sediment, leaving a higher amount of ionium 

 than the amount in equilibrium with the final uranium content. It is believed 

 that this correction for slowly settHng deep-sea sediments is very small and may 

 be of the order of 1%, assuming that 25% of the original uranium content has 

 been lost during the residence time of the sediment in the ocean. 



Urry (1950) assumed a constant concentration of ionium in the settling 

 sediment. With this assumption, the ionium content must decrease monoton- 

 ously with depth in the sediment. The decrease would allow us to determine 

 age and hence the rate of sedimentation would be obtained. His assumption has 

 been disproved, since only a certain amount of ionium is produced per time 

 unit in ocean water. Accounting for a constant rate of ionium precipitation as 

 proposed by Pettersson (1937) and in order to obtain sedimentation rates and 

 ages, Kroll (1953) reasoned that the total ionium unsupported by uranium in 

 the sediment must be in equilibrium with the total uranium in sea-water. If it 

 is further assumed that the rate of sedimentation is constant, the distribution 

 of ionium in the sediment should show a logarithmic decrease with depth in the 

 sediment. All deviations from this theoretical curve must then be connected 

 with changes in the rate of sedimentation. Kroll could not use the above- 

 mentioned rate of ionium precipitation, as he did not know the correct value of 

 the uranium content of sea-water. He assumed that, at a level where the ionium 

 content was twice as large as the equilibrium amount with uranium, the average 

 rate of sedimentation and of ionium precipitation for the whole core could be 

 applied. In that way, he was able to construct a normalized curve for radium 

 distribution. By this method, he found that the uranium content of sea-water 

 had to be larger than it was assumed to be at the time he carried out the 

 investigation. 



Another method, giving only the average rate of sedimentation, is obtained 

 when it is assumed that each of two equally long subsequent parts of a sediment 

 core covers an equally long period of time and that the ionium precipitation 



