Oxic zone

Figure 5.2.4 shows the variation of the concentration of LAS and long-chain SPCs with depth, with reference to the wet sediment (sediment + interstitial water), versus the variation of NO3 and SO4-, in the interstitial water. A very sharp decrease in the concentration of LAS and low concentrations of SPC are observed in the oxic zone, the lower... 

In the case of Zn and Cd, a subsurface dissolved concentration maximum in the oxic zone suggests supply via remineralization of sinking detrital biogenic particles. As with Pb and Cu, the dissolved concentrations of Zn and Cd decline as the anoxic zone is... 

Departures from idealized redox zonation can also result from temporal shifts in sedimentation rates and in the depth of the oxic zone. Some examples are provided in the next section using iron and manganese as case studies. These elements are particularly useful as records of past changes in sedimentation rates because they respond to local redox conditions by undergoing postdepositional migration. 

Iron and manganese are initially supplied to the sediments as a component of the sinking flux of POM and particifiate oxyhydroxides. Remineralization of the POM releases iron and manganese to the pore waters. In the presence of O2, the solubilized metals are oxidized and precipitate as oxyhydroxides, thereby increasing the inorganic particifiate phase in the oxic layer. Continuing sedimentation eventually carries this particulate Mn and Fe below the oxic zone. 

In the anoxic zone, heterotrophic respiration of particulate Mn02 and Fc203 or FeOOH causes manganese and iron to be reduced to Mn (aq) and Fe (aq). As dissolved ions, these trace metals diffuse through the pore waters. The ions that diffuse upwards will reenter the oxic zone, where they react with O2 to reform the oxyhydroxides. This produces a metal-enriched layer that lies just above the redox... 

A proposed electron shuttle In which the oxidative power of O2 can be conveyed below the oxic zone In marine sediments. 

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... 

Not all of the remobilized phosphate is reprecipitated below the redox boundary. Some escapes by diffusing upward through the pore waters. Once this phosphate enters the oxic zone, it is readsorbed by Fe(ni)OOH along with any Fe that has similarly diffused upward. The Fe that diffuses downward into the sulfete-reducing zone precipitates sulfide to form pyrite (FeS). 

As the water table rises with increasing reservoir stage, these processes control the upward shift of the redox zonation anoxic reactions reestablish themselves in the once-oxic zone. Several main processes are important for mobilizing contaminants during this transition. 

These processes are catalyzed by bacteria and probably involve both inorganic and organic iron and manganese species (22). They may also be strongly controlled by microbial competition between Fe(III) and sulfate-reducing bacteria (27). Associated with these reduction reactions is the reduction of residual sulfate (produced in the oxic zone by bacterially catalyzed reactions) similar to eq 7 (21). 

The disappearance half-life of elemental phosphorus in water also depends on the physical state of phosphorus. For example, the disappearance half-life of collodial phosphorus was 80 hours at 30°C and 240 hours at 0°C at concentrations between 10-50 mg/L (Bullock and Newlands 1969), compared to a half-life of 2 hours in solution form at 10 °C (Zitko et al. 1970). The half-life of white phosphorus in solution increased from 2 to 20 hours when the phosphorus was present in the sorbed state in sediment (Zitko et al. 1970). In anoxic water, the estimated half-life of a solid chunk of white phosphorus that was protectively coated due to oxidation/hydrolysis at the oxic zone was 2.43 years (Spanggord et al. 1985). 

The vertical distribution of dissolved oxygen in the Black Sea reflects its specific features as the density stratified basin, which has a permanent H2S zone under the pycnocline [23]. The thickness of the oxic zone varies between 70 and 100 m in areas of central cyclonic gyres with elevated isopycnal surfaces, and between 120 and 200 m in peripheral areas. 

This psuedo-zero-order dependence is called Monod kinetics (Devol, 1978). These kinetic rate formulations become important if the rate constants for organic degradation differ for different electron acceptors. Oxic degradation, in particular, appears to be faster than sulfate degradation, but it is confined to the upper oxic zone of the sediment (Figure 5). 

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