RADIUM CONTENT OF OCEAN-BOTTOM SEDIMENTS 



Many determinations of the radium content of various 

 rocks from many localities on the continental surfaces 

 of the earth have been made, but very few such measure- 

 ments have been made on the materials comprising the 

 ocean-bottom sediments. 



The reason is obvious, but when the vast area of 

 these sediments is considered, and especially their high 

 radium content, it is most desirable that more such 

 measurements should be made. Joly (1908) has reported 

 twelve such determinations on samples taken by the 

 Challenger and Albatross : Hans Pettersson (1930) 

 twenty-eight determinations on samples, some of which 

 were from the Challenger collection and others from the 

 collection of the Prince of Monaco which were taken by 

 his yacht, the Princess Alice H. 



Two of Pettersson's analyses were of the same 

 Challenger sample as used by Joly, and in each case 

 Pettersson's figure is lower than Joly's. 



This accords with Joly's own opinion that his early 

 figures were too high and he has subsequently, in some 

 cases, revised them downward. 



To these meager data the writer is able to add 

 twenty-eight other determinations made on samples se- 

 cured by the Carnegie on her last cruise. 1 Of the many 

 bottom samples taken by the Carnegie, only those which 

 were of sufficient bulk to permit other studies as well 

 were used for radium determinations, since these tests, 

 of necessity, destroy the material used. 



Thedataof Joly and Pettersson are repeated in tables 

 1 and 2; those of the Carnegie samples, which with one 

 exception (station 6) were taken in the Pacific Ocean, are 

 given in table 3. 



All these determinations are shown on the accom- 

 panying map (fig. 1). This map also indicates the char- 

 acter of the ocean-bottom sediment as taken from vari- 

 ous publications on oceanography. The diameters of the 

 circles are proportional to the radium content as re- 

 ported. 



The paucity of these data, when considered in relation 

 to the vast areas covered by ocean-bottom sediments, 

 and, furthermore, these sediments being of an unknown 

 thickness, renders futile any general conclusions regard- 

 ing their geophysical significance. 



The most striking fact is the extraordinarily high 

 concentration of radium in these sediments compared 

 with that in ordinary rocks of the earth's structure. 

 Whereas the average concentration in granites might be 

 placed at about 2.5 x 10-12 grams Ra per gram of rock, 

 and for basalts at about 1.0 x 10-12 grams per gram, the 

 general average for all these sixty-eight determinations 

 is the astonishingly high figure of 11.76 x 10-12 grams 

 per gram of dry material. Some samples contain con- 

 siderably more than this, such samples usually coming 

 from the deeper parts of the bottom and those parts 

 more remote from land areas delivering detrital mate- 

 rial into the sea. 



This high radioactivity is the more astonishing when 

 it is remembered that the sedimentary materials of the 

 continents, even the clays, shales, and limestones, con- 



iThe samples were made available for study through 

 the kindness of Dr. J. A. Fleming, Director of the Depart- 

 ment of Terrestrial Magnetism, of the Carnegie Institution 

 of Washington. 



tain less radium than do the igneous rocks (Strutt, 1907; 

 Joly, 1912; Fletcher, 1912). 



The geophysical significance of this highly radioac- 

 tive material depends on the thickness of these sedi- 

 ments and their history subsequent to being formed. If 

 they are of relatively shallow depth and have not become 

 incorporated into crustal or subcrustal materials, their 

 radium content is of little geophysical significance. If, 

 on the other hand, they are of great thickness and hive 

 served to take such concentrations of radium into the 

 structure of the earth's crust, their influence must be 

 considerable either as blanketing or insulating the flow 

 of heat into the oceans, as is necessary for Joly's ther- 

 mal cycles, or as providing sources of intense energy 

 for any part of the earth's crust within which they may 

 become incorporated. 



Obviously the elucidation of such questions awaits 

 considerable further research, and the development of 

 some technique which will give some idea of either the 

 thickness of these sediments or the rate of their deposi- 

 tion. 



The sample from Station 153 was separated into 

 three parts by suspension in distilled water. That which 

 settled quickly consisted of relatively coarse skeleton 

 remains, small clay balls resembling excreta, skeletons 

 packed with clay, and small manganese nodules in the 

 form of black specks resembling finely ground pepper. 

 All this first fraction was of so small amount as to be 

 insufficient for a test run. 



A second fraction was of intermediate size and re- 

 mained suspended in the distilled water longer. It con- 

 sisted of the bulk of the clay intermixed with broken 

 skeleton fragments, and contained 10.92 x 10-12 grams 

 Ra per gram of material. The third fraction remained 

 suspended more than twelve hours and consisted of very 

 fine clay material. It contained 10.56 x 10-12 grams Ra 

 per gram of dry material. Apparently the treatment 

 with distilled water removed some of the radium. 



After drying, the fractions were kept for thirty days 

 before being tested. 



These tests would indicate that the uranium is fairly 

 evenly distributed throughout the material constituting 

 the sample. Probably the fraction containing the manga- 

 nese would show a somewhat greater radium content, as 

 these nodules are reported to be high in radioactivity 

 (Joly, 1908). This is substantiated in the sample from 

 Station 149. This sample contained no coarse material 

 of the nature of skeleton remains or other debris. It 

 consisted almost entirely of fine clay in which were a 

 great many very small crystals. It contained enough 

 manganese dust to concentrate and remove as a sepa- 

 rate fraction. Though not entirely pure, this fraction 

 was predominantly manganese nodules of very small 

 size. It contained 11.11 x 10-12 grams of Ra per gram 

 of dry material, which is considerably greater than the 

 6.04 X 10-12 gram/gram of the material as a whole. 



The sample from Station 151 (plate I) contained a 

 large manganese nodule weighing 62 grams and of 65 x 

 35 X 40-mm dimensions, which appeared to have formed 

 by concentric concretions, the 40-mm dimension being 

 the vertical one. On being sawn in two along the long 

 dimension, the structure revealed no nucleus or starting 

 point and reminded one very much of the appearance of 

 tree rings. One of these "rings" was a thin layer of 



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