18 GOLDBERG L CHAP - 1 



terrestrial minerals, coupled with their widespread distribution on the sea-floor, 

 affirms their authigenic character. Further support comes from the previously- 

 cited lead-isotopic analyses of Chow and Patterson (1962), which indicate a 

 geographic dependence of the relative amounts of radiogenic lead isotopes in 

 the minerals. This isotopic composition of the lead in a given body of water, 

 which furnishes these atoms to the iron-manganese accretions, is related to 

 the input of the element from a specific land-mass ; different continental areas 

 yield different isotopic assays of lead based upon their rock composition. 



The overall chemistry for the oxidation of manganese in the marine environ- 

 ment has been discussed in a previous section. Of significance is the hypothesis 

 that such a reaction requires a surface, i.e. the ferromanganese mineral forma- 

 tion occurs at the liquid-sediment interface rather than in the water itself or 

 in a micro-environment. Whether the oxidation of manganous ions taken up 

 originally by the weathering waters from crustal rocks occurs in shallow-water 

 deposits (Manheim, 1961), pelagic sediments or even on desert rocks 1 , the 

 necessity of an available surface free of even moderate accumulations of other 

 sedimentary phases is evident. 



Certain chemical characteristics of this deposition may well lend themselves 

 as sensitive indicators of the redox potential of the environment. Cobalt and 

 nickel, like manganese, occur in crustal rocks in the divalent state and are 

 probably transported to the oceans in this form. Cobalt exists in sea-water at 

 a concentration of about one-seventh that of nickel (Taivo Laevestu, in litt.) 

 on the basis of recent refined analyses. Although cobalt and nickel show a strong 

 geochemical coherence in behavior during the major sedimentary cycle, the 

 ferromanganese phases contain nearly as much cobalt as nickel on the average 

 (Table V). Further, cobalt shows a wider spectrum of concentrations in nodules 

 than does nickel. 



An explanation for these abundances may well be found in the greater ease 

 of oxidation of cobalt from the divalent state to the insoluble trivalent form 

 CoOOH and a subsequent accommodation of the CoOOH as a solid solution in 

 the FeOOH of the ferromanganese minerals. Goldberg (1954) has pointed out 

 a co-variance of cobalt with iron in manganese nodules, whereas nickel 

 apparently follows the manganese concentrations. 



This concept may be seen more advantageously from the equation of 

 Sillen (1961) on the reduction of cobaltous ion in sea-water at 25°C : 



CoOOH(s) + 3H+ + e- = Co 2 + + 2H 2 log K = 29.3. 



Utilizing log [Co 2+ ] = — 8 and an activity coefficient for the cobaltous ion such 

 that log y Co = - 0.8, then log [Co 2 +] + log y Co - 3 log [H+] - log e~ = 28 and the 

 reaction has a tendency to proceed to the left. However, assuming a relatively 

 constant cobalt and hydrogen ion concentration in the oceans, the redox 

 potential could conceivably determine the amount of cobalt amassed by a 



1 The so-called desert varnish, described by Engel and Sharp (1958), has a markedly 

 similar composition to that of the ferromanganese minerals and probably forms by a 

 similar chemistry. 



