SECT. 1] THE OCEANS AS A CHEMICAL SYSTEM 7 



Manganous ion is readily seen to be thermodynamically unstable in sea- 

 water from the reaction 



20H- + Mn2++|0 2 = Mn0 2 + H 2 0, 



which has a free energy of — 9 kcal at 25°C at a pH of 8, a manganous ion 

 concentration of 10~ 9 molar and a partial pressure of oxygen of 0.25 atm. 

 These concentrations are similar to those in near-bottom sea-water. Although 

 the bottom temperatures are near 0°C, this difference does not seriously affect 

 the free energy value cited above. Manganese occurs in crustal rocks primarily 

 in the divalent state and most probably enters the marine environment as a 

 dissolved species at this oxidation level. In order for manganese to reach its 

 thermodynamically stable form, a reaction surface is undoubtedly necessary. 

 The widespread occurrence of tetravalent manganese as y-manganese oxides 

 (Hans Wedepohl, in litt.) in the ferromanganese minerals of pelagic sediments, 

 which form on the sea floor in areas characterized by having oxygenated water 

 layers above them and by low rates of accumulation of other components in the 

 deposits, suggests that the associated iron oxides provide the necessary 

 catalytic sites. Iron occurs in sea-water principally as solid oxide phases which 

 accumulate on the sea floor. The subsequent formation of the mineral surface 

 provides then further surfaces for the oxidation of the manganese ions. 

 In the case of iodine we have 



I0 3 - + 6H+ + 6e- = I~ + 3H 2 



with a log K of 110.1. By inserting the previously cited values of pH and pE, 

 it follows that 



IO3-/I- = 1013.5 



and at equilibrium iodate would be the most abundant species of this element. 

 However, iodide is ubiquitously present in sea-waters, an observation initially 

 made by Winkler (1916) and more recently confirmed by Barkley and Thompson 

 (1960). It will be quite rewarding to seek out those factors that give rise to the 

 existence and maintainance of the reduced form of iodine. The marine bio- 

 sphere must be intimately involved in the iodine cycle. Plants and animals of 

 the sea contain reduced iodine in the forms of iodo tyrosines and thyroxine. 

 The forms of iodine assimilated from sea-water, as well as those regenerated 

 upon the destruction of the organic phases, provide the bases for future sig- 

 nificant investigations. 



These simple thermodynamic considerations are of further import in the 

 understanding of inorganic precipitation processes in sea-water. For example, 

 although divalent lead in the forms of Pb 2+ , PbOH + and PbSC>4 are the main 

 species in solution (Sillen, op. cit.), Pb0 2 would be the stable solid phase in 

 equilibrium in oxidizing environments, especially if it forms solid solutions with 

 some other phase or phases. Such a concept is suggested by the marked con- 

 centrations of lead in the ferromanganese minerals, where it attains values of 



