Biogeochemical zonation

The fundamental work by Froelich et al. (1979) established a conceptual model for the organic matter respiration in marine sediments which has been modified, verified and extended in nnmerous aspects since then. Froelich and colleagnes found a snccession of electron acceptors nsed by dissimilatory bacteria according to their energy gain. Conseqnently, a biogeochemical zonation of the sediment resnlts where O, NO3, bioavailable Mn(IV) and Fe(III) and SO diminish sncces-sively with depth. Apart from the consnmption of electron acceptors the production of rednced species snch as Mn ", Fe, MS and CH ... 

Fig. 8.1 Schematic representation of the biogeochemical zonation in marine sediments. The names of the main zones were proposed by Froelich et al. (1979) and Berner (1981, in parenthesis). The depth scale is quasi-logarithmic die exact depths, however, vary strongly and increase from the shelf to the deep sea. The pore water chemistry shows relevant dissolved species. Peak heights and concentration scales are arbitrary. The chemical profiles reflect the depdi sequence of the dominant mineralization processes through which organic matter is oxidized to CO, (Modified from Froelich et al. 1979).

This sequence of reactions has led to the concept of biogeochemical zonation, with each zone named for the solute that serves as the principal electron acceptor or that is the principal product (Figure 1). The sequence of zones is determined by the chemical free energy yields released by possible reactants. The thickness of each zone is dependent on the rate of reaction consuming the electron acceptor, the rate of a reactant s transport through sediments, and the concentration of the acceptor in bottom waters or in solid phases. Zones may overlap, and tracer studies have shown that they need not be mutually exclusive. The existence of the deeper zones depends on the availability of sufficient reactive organic matter. 

Under each given set of circumstances, the process that allows for the maximal energy flow is selected. Many of the biogeochemical cycling reactions of individual elements are actually connected by factors such as redox potentials. Individual minerals are oxidized only at specific redox potentials. This leads to zonations in soil and aquatic environments where minerals accumulate in specific chemical forms and where specific microbial populations proliferate. 

Snyder, M., TaiUefert, M., and Ruppel, C. (2004). Redox zonation at the sahne-influenced boimdaries of a permeable surficial aquifer Effects of physical forcing on the biogeochemical cycling of iron and manganese. J. Hydrol. 296, 164—178. 

Figure 2 Schematic depth zonation of biogeochemical processes in marine sediments.

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