592 CYCLES OF ORGANIC AND IXORGAXIC SUBSTANCES 



geological observations concerning their occurrences. Thus, 

 whereas manganese is in a reduced, dissolved state in sea water, in 

 the ferromanganese minerals it is in lioth the divalent and tetra- 

 valent forms, although mainly the latter. The only nonbiological 

 species in the ocean in sufficient amounts capable of oxidizing 

 manganese is dissoh'ed gaseous oxygen and the reaction 



20H- + Mn++ + i02 = MnO.2 + HoO 



has a free energy of —9 kcal at a pH of 8, a manganous ion con- 

 centration of 10-^ilf, and a partial pressure of oxygen of 0.25 

 atmosphere. All these concentrations are similar to those in near- 

 bottom sea waters. 



Since tetravalent manganese is not found in sea water, although 

 from the above data it is the thermodynamically stable form, a 

 reaction site or surface is probably necessary for the reaction to 

 proceed. The well-known catalytic properties of iron oxide surfaces 

 suggest the association of such material in the formation of the 

 nodules. Iron exists in sea water dominantly in particulate phases, 

 partly collodial. The accumulation of iron oxides on surfaces, 

 where either the bottom topography or the lack of sediment source 

 material does not allow any appreciable accumulation of solid 

 phases, would initiate formation of the ferromanganese minerals. 

 In areas of rapid deposition, where nodules or accretions are not 

 found, the burial of any iron oxide surface by other sediment 

 components minimizes any mineral formation. 



The catalytic oxidation of manganese on the oxide surfaces 

 leads directly to the formaticn of the ferrorranganese minerals. 

 Where an excess of iron is accommodated, the mineral goethite 

 appears. The scavenging of the reactive, high charge density ions 

 from sea water, those previously described as having relatively 

 short residence times, and their incorporation into the disordered 

 layer follow. The formation of the new mineral surface, allowing 

 sites for further oxidation of manganese, i.e., the reaction becomes 

 autocatalytic, completes the picture. 



The recent and significant observation by Graham (1959) that 

 these nodular materials contain appreciable amounts of organic 

 matter has resulted in the revival of the hypothesis of biological 

 origin. Although bacteria and other organisms have been sought 



