SECT. 1] THE OCEANS AS A CHEMICAL SYSTEM 15 



substituting for calcium in the lattice while the uranyl ions are apparently 

 taken up by adsorption on the surfaces of the apatite crystallites (Sheldon, 

 1959). Further evidence for a reducing environment comes from the finding of 

 a wooden log, mineralized by phosphorite intrusions, dredged from a 410 m 

 terrace in the Gulf of Tehuantepec (Goldberg and Parker, 1960). The sea-water 

 in contact with the mineralizing log was depleted in oxygen and contained 

 maximal values of phosphate ions in the sea-water column. This area is quite 

 active biologically and the deficiency in oxygen in the waters results from the 

 combustion of the rather large amounts of organic matter falling from the 

 euphotic zone. 



It is important to ascertain precisely the site of the phosphorite formation — 

 the sediment-water interface or an environment within the sediments them- 

 selves. Do these minerals receive their components directly from the overlying 

 marine waters or from the substances in the deposits? Recent work on the 

 state of uranium in Black Sea waters bears to these points. Kolyadin et al. 

 (1960) sought the mode of occurrence of uranium in the deep reducing 

 waters where the hydrogen sulfide contents approach 8 ml/1, and the oxygen 

 concentrations are below the limits of analytical sensitivity. They found the 

 uranium to be ionically dispersed and in the hexavalent state. Such an observa- 

 tion corresponds with physico-chemical calculations, as the authors point out 

 that the reaction 



U(OH) 4 (s) + 3C0 3 2 " = U0 2 (C0 3 )3 4 - + 2H 2 + 2<? 



requires a redox potential of 0.4-0.5 V for the uranium and carbonate con- 

 centrations of sea-waters. However, the redox potential in the Black Sea does 

 not exceed 0.2 V, a value higher than that met with in the superficial waters 

 above the phosphorite deposits. 



Thus, we are led to the hypothesis that the reduction of uranium must take 

 place within the reducing atmosphere of the sedimentary environment 

 where high redox potentials can be attained and/or high concentrations of 

 hexavalent uranium can be amassed. In either case our concern is with reac- 

 tions that take place within the solid phases. The high uranium concentrations 

 found by Manheim (1961) in the zones adjacent to the most stagnant basins of 

 the Baltic Sea, as well as the unusually high uranium concentrations in phos- 

 phorites, which are usually of the order of several hundred parts per million, 

 emphasize the above duality. 



These observations coincide with the geological evidence on the deposition 

 of phosphorite (McKelvey et al., 1953). Marine apatites form in coastal deposits 

 on the east sides of continents where upwelled waters give rise to the abundant 

 production of organic matter. The rapid accumulation in the sediments of the 

 organic phases results in a depletion of oxygen, as well as furnishing a source 

 of phosphate. The sedimentary environments in which the decomposition of the 

 organic phases takes place must also have the necessary redox potential for 

 the reduction of hexavalent uranium as well as those solid phases which can 

 pick up this uranium from sea-water. 



