MARINE BOTTOM DEPOSITS 



37 



this factor can be got only by a study of the rate 

 of deposition. The most promising method for ac- 

 complishing this is one based upon radioactive 

 considerations. Therefore, a study of ocean bottom 

 sediments from this point of view is of the greatest 

 importance in securing fundamental information 

 about these vast deposits. 



The radium content of the granitic rocks of the 

 earth varies from about 1-3 X lO"'- grams of radium 

 per gram of rock ; and of the basaltic rocks about 1 

 on the same scale. The sedimentary rocks average 

 less than the basalts, whereas the ocean bottom 

 sediments are found to contain several times as 

 much as even the granites. The average for Joly's 

 twelve determinations is 17.8 X 10""'- grams Ra 

 per gram of sample, which is considered to be 

 rather high. Pettersson's twenty-eight determina- 

 tions average 10.96 X 10~'- grams/gram, with a 

 maximum of 49.5 X 10~'^ grams/gram. Piggot's 

 re.sults average 6.52 X 10~'- grams/gram with 

 21.40 X 10~" grams /gram as the greatest. 



These high concentrations of radium are the more 

 remarkable when one considers that the uranium 

 represented by this radium must come originally 

 from the igneous rocks. Apparently it did not 

 concentrate to any great extent in the sedimentary 

 rocks at the time of their formation presumably in 

 shallow seas but has concentrated to a considerable 

 extent in those sediments which are now accumulat- 

 ing slowly in the deeper and more remote portions 

 of the ocean. 



Usually the red clays contain a higher concentra- 

 tion of radium than do the other deposits. Of the 

 samples examined by Petters.son and Piggot whose 

 characters are definitely known, 27 red clays average 

 12.1 X 10~'- grams Ra per gram, and 13 Globigerina 

 oozes average 4.1 X 10~'- grams Ra per gram of 

 sample. 



Joly suggested that the minute organisms of the 

 sea abstract uranium, more or less selectively, from 

 the water and when they die their skeletons carry 

 it to the bottom with them. However, the higher 



radium concentrations are not found associated with 

 any of the various skeletal deposits. 



Pettersson found high radium concentration 

 as-sociated with evidences of volcanic activity and he 

 suggests that the unusual concentrations are brought 

 about by submarine volcanism. Pettersson's ex- 

 planation seems rather specialized to be of general 

 application. 



Piggot points out that the oxides of uranium like 

 those of iron and manganese are among the least 

 soluble of its compounds and that it is in those 

 portions of the ocean bottom, in general, where the 

 oxides of manganese and iron are separated, as re- 

 vealed by the nodules of these elements, that the 

 uranium concentration as revealed by the radium 

 content is the higher. This accords with the 

 observations of the oxygen content with depth made 

 by the Carnegie, which revealed that though the 

 oxygen content fell off very rapidly down to about 

 1000 fathoms, it increa.sed from then on and soon 

 attained a magnitude about two-thirds of that at 

 the surface. Therefore the deep, undisturbed 

 areas, far from land and detrital debris, furnish an 

 oxidizing environment where the uranium separates 

 out, and appears in the highest concentrations at 

 those places of slowest sedimentation. 



The geophysical significance of this highly radio- 

 active material depends upon its thickness and its 

 history subsequent to being formed. If it be of 

 great thickness or have served to take such concen- 

 trations of radium into the structure of the earth's 

 crust, its influence must be considerable, either as 

 insulating the flow of heat into the ocean, as re- 

 quired by Joly's thermal cycles, or as providing 

 sources of intense energy for any part of the earth's 

 crust within which it may become incorporated. 



Obviously the elucidation of such questions awaits 

 considerable further research and more particularly 

 the development of some device which will provide 

 core samples, from a study of which some knowledge 

 of the character and rate of deposition may be 

 obtained. 



