Reduction sequence sediments

The rate of the biotic reduction of Fe oxides by a strain of Corynehacterium under 02-free conditions followed the order natural ferrihydrite > synthetic goethite > hematite (Fischer (1988) (Fig. 12.29) in accordance with the sequence in reducibility by Fe-reducing bacteria isolated from a eutrophic lake sediment (Jones et al., 1983). Iron from ferrihydrite reduced by Shewandla alga was found to be isotopically lighter than that of the ferrihydrite Fe by a 5 ( Fe/ " Fe) of 1.3 %o This difference may be used to trace the distribution of microorganisms in modern and ancient earth (Beard etal. 1999). 

The property of chemotropicity testifies to the balance of the redox layer system with respect to the vertical fluxes of the oxidants and reductants supplied. This should be the well-defined sequence of changes with depth of the favorability of the potential redox reactions [ 17,75] that can be realized by the bacterial community. The development of bacteria in this case should affect the distributions of nutrients. By modern estimation [79] the chemosynthetic production is comparable with photosynthetic production, and that should in the same manner affect the consumption of inorganic nutrients and production of their organic forms. Besides this the possible abiotic chemical reactions and the sedimentation of particulate matter of different densities should also play their roles in this mechanism. 

Arsenic in soilds has been fractionated by Jackson s T28) procedure for soil phosphorus (15. 27). In this laboratory, a modification of Jackson s procedure is being used on sediment solids. A series of 1 molar solutions of NH4CI, NH4OH, acid ammonium oxalate (29) and HCl are used in sequence. The chloride fraction, or exchangeable fraction, contains weakly adsorbed, coulombically bound arsenic. The hydroxide fraction, contains iron and aluminum arsenate precipitates and surface precipitates (i.e. adsorbed arsenic species having both chemical and coulombic bonding to oxide surfaces). The oxalate, or reductant soluble fraction, contains arsenic occluded in amorphous weathering products. The acid, or calcium, fraction contains arseno-apatites. 

Figure 13 Shown on the left is the sequence of events envisioned by Archer and Maier-Reimer (1994) for the transition from interglacial (I) to glacial (G) conditions. An increase in respiration CO2 release to the sediment pore waters enhances calcite dissolution, thereby unbalancing the CaCOs budget. This imbalance leads to a buildup in col ion concentration in the deep sea until it compensates for the extra respiration CO2. On the right is the sequence of events envisioned for the transition from G to I conditions. The input of excess respiration CO2 to the sediments ceases, thereby reducing the rate of calcite dissolution. This leads to an excess accumulation of CaC03 on the seafloor and hence to a reduction in carbonate ion concentration which continues until steady state is reestablished.

Details of sulfur isotope geochemistry are presented elsewhere in this volume (see Chapter 7.10) and are only highlighted here as related to paleo-environmental interpretations of finegrained siliciclastic sequences. Formation of sedimentary pyrite initiates with bacterial sulfate reduction (BSR) under conditions of anoxia within the water column or sediment pore fluids. The kinetic isotope effect associated with bacterial sulfate reduction results in hydrogen sulfide (and ultimately pyrite) that is depleted in relative to the ratios of residual sulfate (Goldhaber... 

The order of competing terminal electron accepting processes can vary with any number of factors that influence the thermodynamics of the system. One factor that must be considered in ecosystems with mineral sediments or soils is the composition of Fe(III) and Mn(IV) minerals. The typical sequence of Fe(III) reduction before SOl reduction can be reversed with a change in the abundance of labile Fe(III) minerals such as ferrihydrite (Postma and Jakobsen, 1996). This is one explanation for the common observation that the zones of Fe(III) reduction and SOl reduction overlap in marine sediments (Boesen and Postma, 1988 Canfield, 1989 Canfield et al., 1993b Goldhaber et al., 1977 Jakobsen and Postma, 1994). Postma and Jakobsen (1996) predicted that the overlap between Fe(III) reduction and SOl reduction should increase as Fe(III) oxide stability (or surface area) increases. 

Compared to the anoxic sediment, the oxic surface sediment is enriched in As (up to 345 nmol g-1). The enrichment is weak where the sedimentation rate is high, but enrichment becomes stronger with decreasing sedimentation rate. Phosphorus enrichment in the surface layer is evident only where the sedimentation rate is low. However, upon burial, both P and As are released from the solid phase to the pore water, irrespective of the sedimentation rate, and the distributions of both elements show subsurface maxima that correspond to the depth where the dissolution rates of solid phases of P and As are maximum. From the relative location of these maxima, it can be concluded that As is released to the pore water at shallower depths than at which phosphate is released. Mucci etal. (2000a) proposed that As is released upon the reduction of As(V) to soluble As(III), and that this takes place earlier than the reduction of Fe(III) to Fe(II) in the diagenetic sequence. In contrast, the... 

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