380 



THE DEEP SEA 





to 



'o 10 

 5 



15- 



20 



30 



^0 



— r- 

 50 



50 



5 ^0 

 o 



30 - 



20 - 



10 



50 cm 



Core 86 B 



Fig. 6. Measurement on radioactive nuclides from Pacific clay core 86 B of the 

 Swedish deep-sea expedition, 1947-1948. Graph shows activities of the following 

 nuclides plotted against depth in core: radium (Kroll, 1955); uranium (Hahofer 

 and Hecht, 1954); ionium (Picciotto and Poulaert, unpublished). At top is shown 

 the thorium content of selected layers in the core (Picciotto and Wilgain, 1954). 



to construct a complete picture of the radioactivity of the pelagic 

 sediments. Meanwhile, the results of the aforementioned series of 

 research may be summed up as follows (Table II): 



1. The uranium content (1 to 4 parts per million) and the 

 thorium content (5 to 15 ppm) of pelagic clays are similar to those 

 of granitic rocks, and moreover, remain relatively constant. 



2, In the younger upper layers, ionium is present in definite 

 excess of its radioactive equilibrium concentration with uranium. 

 This excess becomes markedly greater in cases where the rate of 

 sedimentation is lower. In the Atlantic, where the rate of accumu- 



