Sediment oxic-anoxic boundary

At the oxic-anoxic boundaries a rapid turnover of iron takes place. This oxic-anoxic boundary may occur in deeper layers of the water column of fresh and marine waters, at the sediment-water interface or within the sediments. 

Transformation of Fe(II,III) at an oxic-anoxic boundary in the water or sediment column (Modified from Davison, 1983)... 

The iron cycle shown in Fig. 10.14 illustrates some redox processes typically observed in soils, sediments and waters, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(lll) hydr)oxides by organic ligands, which may also be photocatalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III)(hydr)-oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr) oxides is accompanied by the release of these substances into the water column. 

The freshly precipitated manganese and iron oxides which precipitate at an oxic-anoxic boundary within the lake water column or in the top layers of the sediment, form small particles with high surface area they cause an additional scavenging at... 

Dissimilatory reduction by anaerobic bacteria occurs in the anoxic hy-polimnion of stratified lakes and in sediments just below the oxic-anoxic boundary. It produces H2S,... 

Figure 2 Oxic-anoxic boundary in the water or sediment columf ...

At the oxic-anoxic boundary in the deeper portions of a stagnant water body or a sediment, a redox cycling of sulfur compounds of different oxidation states may occur (Figure 8.20) because reduced S may become reoxidized. 

Cowie, G. L and J. I. Hedges (1991) Organic carbon and nitrogen geochemistry of Black Sea surface sediments from stations spanning the oxic anoxic boundary. In Black Sea Oceanography (ed. E. Izdar and J.W. Murray), pp. 343-9. Boston, MA Kluwer. 

Figure 19. Transformations of Fe(II, III) at an oxic anoxic boundary in the water or sediment column (modified from Davidson, 1985). Peaks in the concentration of solid Fe(III) (hydr)oxides and of dissolved Fe II) are observed at locations of maximum Fe(III) and Fe(II) production, respectively. The combination of ligands and Fe(ll) produced in underlying anoxic regions are most efficient in dissolving Fe(III) (hydr)oxides. Redox reactions of iron—oxidation accompanied by precipitation, reduction accompanied by dissolution—constitute an important cycle at the oxic-anoxic boundary which is often coupled with transformations (adsorption and desorption) or reactive elements such as heavy metals, metalloids, and phosphates.

The tendency for a redox reaction to occur is reflected by the tendency for electrons to flow and produce an electrical current, which can be measured as a voltage, the redox potential (Eh). At the oxic/anoxic boundary in sediments Eh = 0, and conditions become more oxidizing (i.e. organic matter is more vigorously attacked by oxygen) as Eh becomes... 

Figure 4. Oxic (left) and anoxic (right) depositional envitonments generally result in poor and good preservation of deposited organic matter, respectively (after Demaison and Moore, 1980 0. The solid horizontal line separates oxic (above) fiom anoxic (below). In oxic settings, bottom dwelling metazoa bioturbate the sediments and oxidize most organic matter. In anoxic settings, especially where the oxic-anoxic boundary occurs in the water column, bottomdwelling metazoa are absent and sediments ate not bioturbated.

Oxic Diagenesis Metals remobilized from sediments lying in the oxic zone. Remobilization likely occurs in anoxic microzones adjacent to nodules. Bioturbation is an important metal transport agent. Some nodules now found in oxic sediments were likely formed during times when the redox boundary was closer to the seafloor. 10-50 Todorokite (high Cu and Ni content) 32% 5-10 15-20... 

Because reductants are present in most water-saturated soils or sediments and oxygen is relatively unavailable, the Eh declines as the water moves into the subsurface. The decline may be from oxic to anoxic sulfidic or nonsulfidic levels (Table 11.5). The rate and extent of Eh decline with distance from the surface depends on the availability and reactivity of sediment organic matter and other reductants. In the sediments of flooded rice paddies, wetlands, estuaries, and shallow lakes, which may be especially rich in fresh organic matter, the redox front or intetface (also termed a redox barrier or boundary by some), which is the zone of abruptly changing Eh values, may be only a few millimeters or centimeters thick. 

---