588 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



thorium compared to sea waters which are isolated from land 

 drainage for times comparable to the residence time of thorium. 



The uranium-derived isotope of thorium, Th-''°, provides a con- 

 venient normalization for intraocean comparisons of thorium 

 contents. Uranium has a residence time of the order of a million 

 years, a time that is long with respect to oceanic mixing processes. 

 Hence, one would expect and one does find a uniform content of 

 uranium in sea water, a value around 3.5 yug liter. Thus, the rate 

 of production of its daughter ionium per unit volume of sea water 

 is constant in all oceans. The ionium input from land is less than 

 one-fourth of this oceanic production value and is unimportant in 

 our considerations. Since the aqueous chemistry of thorium in- 

 volves only one valence state, we can make the initial assumption 

 that both thorium and ionium will be present in the oceans in 

 similar chemical forms. Thus, we shall seek out the ratio of ionium 

 to thorium as a m^easure of possible thorium variations in the 

 ocean. 



Although the thorium content of sea water is so low that its 

 actual value so far has escaped detection by modern analytical 

 techniques, some surface minerals on the sea floor, which accumu- 

 late thorium in measurable amounts from the over-lying sea 

 waters, should provide a conx'enient index of the sea water con- 

 centrations. The thorium isotopic ratios given in Table II were 

 obtained from the authigenic minerals in the upper 2 or upper 4 

 cm of deep-sea sediments. The lowest values of thorium compared 

 to ionium are found in the South Pacific Ocean whereas the 

 highest numbers occur in the Atlantic Ocean. Intermediate values 

 are found in the North Pacific. 



These \'alues are reasonable when one looks at the relative 



Table II. X'alues of Ionium/Thorium Ratio in H\drogenous Minerals from 



Surface Sediment Samples 



lo/Th, Units of Disintegrations of Ionium/ 

 Area Disintegration of Thorium/Unit Time 



Atlantic 1 2 



North Pacific 25 



South Pacific 150 



