584 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



and Si^- from argon. The beryllium isotope with a half-life of 2.7 

 million years was simultaneously found by groups led by Dr. James 

 Arnold (1956) of Princeton and Dr. B. Peters (Goel et al., 1957) of 

 Bombay in the deep-sea sediments of the Pacific Ocean. Dr. Lai of 

 Bombay and I (1959) have sought and observed Si^- in siliceous 

 sponges taken from waters of the mixed layers. 



In both cases the marine realm provides the unique sites for the 

 accumulation of these radioactive species in amounts that are 

 readily extractable for quantitative assay. These cosmic ray- 

 produced isotopes come down to the land and sea surfaces in 

 atmospheric precipitation but are heavily diluted in the relatively 

 large amounts of stable silicon and beryllium contained in the 

 continental surfaces, whereas in the oceans, the quantities of 

 stable beryllium and silicon, at least in surface waters, are small 

 and allow the specific activity, the disintegrations of the isotope 

 per weight of its stable form per unit time, to attain reasonably 

 high values. 



The beryllium concentration in sea water (Merrill el al., 1960) is 

 of the order of 6 X 10"^ of a part per billion by weight. Hence, a 

 4000-m water column, representing the average oceanic depth, 

 contains per square centimeter about the same amount of beryllium 

 as a millimeter depth of average earth's crust which has for all 

 practical purposes a uniform beryllium content of a few parts per 

 million. Thus, two effects tend to reduce continental specific 

 activities of Be'" to values lower than those of the marine environ- 

 ment: (a) mixing of the radioisotope with stable beryllium to 

 levels on land greater than a millimeter; and (b) the runoff into 

 the oceans of any of the isotopes taken up by the rivers. Hence, 

 the authigenic minerals on the sea floor, solid phases that derive 

 their components from the chemical species in sea water, readily 

 revealed measurable quantities of Be^°. 



Similarly, the low concentrations of silicon in surface sea waters, 

 resulting from the assimilation of monomeric silicic acid and /or 

 its ions by the photosynthesizing diatoms, suggest a promising 

 medium for the attainment of high specific activities of Si^^ If we 

 assume the depth of the mixed layer to be 100 m and to contain 

 15 micromoles of silicon per liter, a 1-cm'^ column of such waters 

 contains about the same amount of inert silicon as 0.01 cm of 



