File:Redox zonation.png

English: Redox zonation illustrated using reaction path modeling. The model simulates titration of a closed system with acetate. The system initially contains oxygen, nitrate, ferrihydrite (Fe(OH)3), and sulfate as potential electron acceptors for acetate oxidation. As acetate is added, electron acceptors are depleted in order of decreasing free energy yield. Once sulfate is depleted, acetate is consumed by acetoclastic methanogenesis. Dinitrogen, ferrous iron, and methane concentrations increase in response to nitrate reduction, iron reduction, and methanogenesis, respectively. In the iron-reducing zone, sulfate concentration decreases slightly in response to formation of an aqueous iron-sulfate complex (FeSO4(aq)). In the sulfate-reducing zone, mackinawite (FeS) precipitation lowers ferrous iron concentration and prevents significant accumulation of dissolved sulfide (not shown). Initial conditions in the model system are consistent with those listed in Table 8.1, except sulfate concentration is 150 µM, 400 µM of ferrihydrite are included, and dinitrogen is not present. The model was calculated with the React module of The Geochemist’s Workbench software. The calculation assumes that reactions reach equilibrium at each step along the reaction path.