Redox boundary

POi that is incorporated into the authigenic apatite is thought to be supplied primarily via the decomposition of organic materials at the sea floor. A variety of additional processes, such as cycling at redox boundaries or incorporation by microbial communities, may act to elevate pore water concentrations, promoting... 

Davison, W. (1985), "Conceptual Models for Transport at a Redox Boundary", in W. Stumm, Ed., Chemical Processes in Lakes, Wiley-Interscience, New York, pp. 31-53. 

Subsurface depth of the irorn redox boundary versus organic carbon flux to the ocean floor. Source From Bleil, U. (2000). Marine Geochemistry, Springer Verlag, p. 80. See Bliel (2000) for data sources. 

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... 

Suboxic Diagenesis Metals remobilized from reducing sediments. Upward diffusive transport through pore waters supplies metals to nodule bottoms. Accretion is episodic, occurring only when the depth of the redox boundary rises close to the sediment-water interface. 100-200 Todorokite/Birnessite (low Cu and Ni content) 48% 20-70 60-200... 

Three mechanisms have been proposed to explain how particulate metals could be transported within such sediments so as to support the growth of Fe-Mn nodules (1) anoxic microzones, (2) bioturbation, and (3) shifts in the depth of the redox boundary over time. Anoxic microzones are present within fecal pellets and the interiors of radiolarian shells where detrital POM is still present. Metals mobilized within these microzones should be able to diffuse through the sediments for substantial distances... 

Finally, nodules may not be growing under oxic conditions. The nodules found in oxic sediments may have formed at some earlier time when the redox boundary was closer to the sediment-water interfece. Changes in the position of the redox boundary are a consequence of changes in the flux of POM and bottom-water O2 concentrations. 

If temporal variations in the depth of the redox boundary are important, climatological events, such as ice ages and even seasons, should affect nodule growth rates. 

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). 

In coastal sediments where organic carbon concentrations are high, the redox boundary is at or near the sediment-water interfece. Under these conditions, denitrification acts as a sink for nitrate. In some settings, the rate of sedimentary denitrification is fast enough to drive a diffusive flux of nitrate from the bottom waters into the sediments. Remineralization of organic matter imder suboxic and anoxic conditions releases... 

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. 

Bacterially produced elemental sulfur can also react with hydrogen sulfide form polysulnde ions. Thus, polysulfide ions should constitute a significant fraction of sulfur nudeophiles in reducing sediments especially where sulfide oxidation is incomplete, such as in intertidal and salt marsh sediments (31321. The polysulfide ions should also be important at redox boundaries (anoxic/ suboxic) in the water column of marine anoxic basins, such as the Black Sea. 

With redox control largely responsible for phosphorus mobility in sediments, what might the consequences of oxygen depletion in the hypolimnion be If conditions in the surface sediments are not sufficiently oxidizing to precipitate iron (hydr)oxides and thereby adsorb the phosphate i.e. the redox boundary for iron may be in the overlying... 

The most likely setting of symbiosis is a microbial mat community, in which a complex community of cells is clustered across a redox boundary, cycling and recycling redox power (Nisbet and Fowler, 1999 Nisbet, 2002). The aerobic top of the mat would include photosynthetic cyanobacteria, above photosynthesizing purple bacteria. There would be a very sharply focused redox boundary. Below would be the green photosynthetic bacteria, and at the base the methanogens and the hydrogen producers. 

For bacteria living close to a redox boundary, sulfur is a marvellous reservoir. Should conditions become reducing, they can tap it and make H2S. Conversely, if conditions become strongly oxidizing, they can make SO species. Thus, the bacteria can sequester sulfur rather as in a piggy bank, saved for a needful day it becomes a redox currency. Even better, sulfur-bearing chemical species are common components of hydrothermal fluids—readily available ... 

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