SECT. 1] THE OCEANS AS A CHEMICAL SYSTEM 19 



given ferromanganese mineral. Where the redox potential of the environment 

 is relatively low, the cobalt/nickel ratios of sea-water would be expected in the 

 minerals, as only the divalent ions are involved. High values for the ratio would 

 be indicative of more strongly oxidizing conditions. 



If such an approach is valid, the sensitivity of delimiting the physico- 

 chemical conditions of the depositional environment may be enhanced by 

 investigating other metals which are derived from crustal rocks in a reduced 

 state and are amenable to oxidation. Lead and cerium, which enter the oceans 

 most probably as plumbous and cerous ions, have the higher oxidation state 

 solids Pb02 and CeO'2. Where previous considerations have indicated PbC>2 is 

 the stable particulate phase to be expected in equilibrium with plumbous ions 

 in the oceans, the oxidation of cerium is not evident from thermodynamic 

 considerations. However, the reaction may proceed by a combination of the 

 cerous and manganous oxidations to form a solid solution of the eerie oxides 

 with the manganese dioxides giving a thermodynamically possible reaction. 



The cerium concentrations in these minerals are especially valuable as they 

 can be normalized to those of its periodic-table neighbor lanthanum, whose 

 trivalent state chemistry would follow that of cerous ion in the major sedi- 

 mentary cycle. Present evidence suggests that an oxidation of cerium does 

 occur. In ferromanganese minerals, recovered from the sea-floor, the cerium/ 

 lanthanum ratio has a value around six, whereas in crustal rocks of the earth's 

 surface, the ratio is nearer two. An extreme case has been noted in a Triassic 

 ferromanganese nodule from Timor where the ratio was an order of magnitude 

 higher than in the more recent accretions. 



Hence, one would expect high cobalt/nickel ratios to be accompanied by 

 high lead concentrations and for especially strong oxidizing conditions at the 

 reaction site high cerium/lanthanum ratios. 



As a final example in this section, the inorganic precipitation of calcium 

 carbonate appears to characterize an oceanic chemical system, limited to 

 coastal waters in tropical or semi-tropical environments, in which the solubility 

 product of this substance is exceeded. The most studied case in point involves 

 the oolitic aragonitic sands of the Great Bahama bank, which are widely 

 distributed over a 100,000-square-mile area of the continental shelf between 

 Florida and Hispanolia. These oolites are readily distinguishable as resulting 

 from an inorganic precipitation process by their distinctive chemical and iso- 

 topic compositions. Their strontium and uranium concentrations are markedly 

 higher than most biogenous carbonates (2.4% SrC03 and 3.0 ppm of uranium ; 

 Tatsumoto and Goldberg, 1959). The isotopic ratios 13 C/ 12 C and ^O/^O in the 

 crystals are unique and extend over a more limited range than biologically 

 precipitated materials (Lowenstam and Epstein, 1957). 



The oolites precipitate from fairly shallow depths — most of the area of the 

 Great Bahama Bank is less than 5 m deep. Newell, Purdie and Imbrie (1960) 

 point out that oolite formation is most extensive in and just below the inter- 

 tidal zone. The factors encouraging the precipitation include not only the 

 topography and the warm temperatures of the waters but also the intense 



