Redox gradient

A redox gradient is the biogeochemical sorting of reductants and oxidants according to redox potential, with the most reducing conditions at depth, having its origin in the depletion of oxygen and the successive depletion of reactants with depth. They form in stratified environments where oxygen does not penetrate deeper than the immediate surface environment. Examples include waterlogged soils, marine pelagic and hemipelagic sediments, and, most notably, the Black Sea.
Redox gradients in marine sediments can limit the depth at which burrowing animals can dwell, as the anoxic environment of deeper sediment is unsuitable for animals to survive in.