Pore water redox zones

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... 

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). 

Preliminary work (10) on the transition from oxidized surface sediment to reduced subsurface sediment in Milltown Reservoir showed that the redox transition occurs in the upper few tens of centimeters. Strong chemical gradients occur across this boundary. Ferrous iron in sediment pore water (groundwater and vadose water) is commonly below detection in the oxidizing surface zone and increases with depth. Arsenic is also low in pore water of the oxidized zone, but increases across the redox boundary, with As(III) as the dominant oxidation state in the reduced zone. Copper and zinc show the opposite trend, with relatively high concentrations in pore water of the oxidized surface sediment decreasing across the redox boundary. 

Figure 4 Idealized depth dependence of pore-water and solid-phase sulfur constituents and their sulfur isotopic values. The zones labeled I, II, III, and IV are classified according to processes of transport of sulfur and redox...

Pore-water concentration profiles of redox-sensitive ions (nitrate, Mn, Fe, sulphate and sulphide) and nutrients (ammonium and phosphate) demonstrate the effects of degradation of OM. In freshwater sediments, the redox zones generally occur on a millimetre to centimetre scale due to the high input of reactive OM and the relatively low availability of external oxidators, especially nitrate and sulphate, compared to marine systems. A typical feature for organic-rich freshwater sediments deposited in aerobic surface waters, is the presence of anaerobic conditions close to the sediment-water interface (SWI). This is indicated by the absence of dissolved oxygen and the presence of reduced solutes (e.g. Mn, Fe and sulphides) in the pore water. Secondary redox reactions, like oxidation of reduced pore-water and solid-phase constituents, and other postdepositional processes, like precipitation-dissolution... 

The concentrations of Cu, Zn, Fig, Cd, and other metals in sediment pore water are controlled by the solubility of metal sulfides in the reduced zone where sulfate reduction and sulfide formation is dominant. Change in redox condition upon burial results in a system where the growth of diagenetic copper, zinc, and arsenic sulfides control the distribution and partitioning of metals and arsenic in the sediment. In polluted sediments, sulfate reduction plays a key role in the formation and retention of sedimentary S as metal sulfides. The majority of the iron and manganese in coastal lake sediments is associated with sulfidic forms, especially in saline area high in available sulfate that is reduced to sulfide. 

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