REDOX ZONATION - EQUILIBRIUM CONSTRAINTS ON THE FE(III) SO4-REDUCTIONINTERFACE/

The concept of redox zonation during degradation of organic matter, wh ich is usually explained by the overall energy yield of different reac tions, has been reevaluated. At least for reduction of Mn(IV), Fe(III) , sulfate, and methanogenesis, the sequential occurrence of these proc esses is much easier explained by a partial equilibrium approach where the fermentive step is overall rate limiting, while the electron acce pting processes are considered to be close to equilibrium. Using the p artial equilibrium approach, an explanation is sought for the simultan eous occurrence of Fe(III) and sulfate reduction, observed in several field studies. Calculations of conditions for equilibrium between Fe(I II) and sulfate reduction indicate that, depending on the stability of the iron oxide, simultaneous reduction of Fe(III) and sulfate is ther modynamically possible under a wide range of sedimentary conditions an d sulfate reduction may even occur before Fe(III) reduction. In Fe2+-r ich environments, the pH of the porewater has in addition a strong inf luence on whether Fe(III) or sulfate reduction is favored. In natural sediments, the presence of a wide range of iron oxide stabilities is l ikely to cause considerable overlap between zones of Fe(III) and sulfa te reduction, while a better confined stability range of iron oxides s hould cause more distinct zones of Fe(III) and sulfate reduction.