RADIOACTIVE ELEMENTS AND DEEP-SEA SEDIMENTS 381 



lation of the clays is of the order of 1 cm per 1000 years the ratio 

 of activities of ionium to uranium lies between 10 to 20. In the 

 Central Pacific, where the rate of accumulation is of the order of 

 1 mm per 1000 years, this ratio is greater than 50. According to 

 some as yet unpublished observations (Picciotto, Rosholt, and 

 others) Protoactinium is also present in excess with respect to 

 ^235^ in the upper sediment layers. 



3. The radium produced in the sediment, by ionium, has been 

 clearly shown to migrate, in certain cases. Nevertheless, this 

 diffusion of radium does not explain all the irregularities in its 

 distribution. Figure 5 shows a case in which a secondary maximum 

 concentration of radium is supported by a real maximum ionium 

 concentration. 



4. The ionium concentration generally decreases with the depth 

 of the sediment, but this decrease is often found to be quite 

 irregular, and does not appear to correspond to any simple model 

 for the ionium distribution. 



5. The mechanisms of transport of ionium from the ocean to 

 the sediments are practically unknown. Recent observations 

 obtained mainly from radioautographs of a. particles (see Figs. 7-9) 

 indicate that these mechanisms are varied and complex. These 

 observations, moreover, stress the importance of certain authigenic 

 minerals, for example phillipsite (Arrhenius and Goldberg, 1955), 

 and the importance of biogenic constituents (Arrhenius et al., 

 1957), much as in all marine geochemical problems. 



6. The presence of beryllium-10 in Pacific clays has been shown 

 by Peter's group (Goel et al., 1957) and by Arnold (1956). 



Table II. Radioactive Nuclides in Pelagic Clays 



