RADIOACTIVE ELEMENTS AND DEEP-SEA SEDIMENTS 383 



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Fig. 8. Photomicrograph of pelagic clay particles embedded in a nuclear 

 photographic emulsion. A highly radioactive debris of a fish skeleton emitting a 

 number of a-particle tracks (Picciotto and Arrhenius, unpublished). 



Finally, a number of communications submitted to this Congress 

 hold promise of noticeably enriching our knowledge in these fields. 



Possible Chronological Methods 



Every radioactive nuclide present in the sediments may, in 

 principle, serve as the basis for a particular method of age deter- 

 mination. We have, however, seen that the sole knowledge of the 

 concentration of a radionuclide in a sediment does not suffice, and 

 further information is, in fact, necessary. In the case of deep-sea 

 sediments, this supplementary information is practically missing, 

 and is generally replaced by more or less plausible hypotheses. 

 Each of these hypotheses in combination with the use of a par- 

 ticular radionuclide constitutes a so-called method. 



A brief review of the various methods in use, or suggested, up 

 to the present, are in order. 



