Anoxic condition interfaces

Estuaries exhibit physical and chemical characteristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1 to 10 years for lakes and 10 years for oceans), redox and salinity gradients are often transient, and diurnal variations in nutrient concentrations can be significant. The biological productivity of estuaries is high and this, coupled with accumulation of organic debris within estuary boundaries, often produces anoxic conditions at the sediment-water interface. Thus, in contrast to the relatively constant chemical composition of the... 

The occurrence of anoxic conditions causes cycling of iron and manganese at the oxic-anoxic interface (6-10). In lakes with a significant seasonal cycle, iron and manganese oxides are reduced during anoxia, and Fe(II) and Mn(II) are released into solution. The Fe(II) and Mn(II) species are reoxidized, and Fe(III) and Mn(III,IV) precipitate as oxides during lake overturn, when the reduced species come into contact with oxygen. 

Anoxic or almost anoxic conditions are found in stagnant basins such as the Black Sea or in basins with sill depths in the oxygen minimum layer like the Santa Barbara or San Pedro basins off California (16). Under such conditions anoxic conditions may develop in the water under the sill depth or in the sediment just below the sediment-water interface. 

Between 1999 and 2005, a negative trend of -8.6%/year (-0.01 nmol/(kg year)) was calculated for Cddiss in waters Above Halocline. It was assumed that this phenomenon was due to the stabilization of anoxic conditions in the deep water, which led to the increase in the export of Cddiss from surface waters by time lag. To support this idea, the exchange of dissolved metals by vertical turbulent mixing at the oxic-anoxic interface has been calculated, as described later. 

Figure 6 shows schematically the aquatic redox cycle of iron. Under the conditions usually encountered in natural aquatic systems, the reduction of iron(III) is accompanied by dissolution and the oxidation of iron(II) by precipitation. Reductive dissolution of iron(III) hydroxides occurs primarily at the sediment-water interface under anoxic conditions in the presence of reduct-ants, such as products of the decomposition of biological material or exudates of organisms. Reductive dissolution of iron(III) hydroxides, however, can also occur in the photic zone in the presence of compounds that are metastable with respect to iron(III), that is, compounds that do not undergo redox reactions with iron(III) unless catalyzed by light. The direct biological mediation of redox processes may also influence the redox cycles of iron (Arnold et al., 1986 Price and Morel, Chapter 8, this volume). Dissolved oxygen is usually the oxidant of... 

In summary, preservation of a seawater redox signal is favored by rapid mineralization and stabilization at the sediment/water interface and uptake of REE from pore waters (rather than from seawater) is likely to reduce or eliminate any inherited cerium anomaly. Negative cerium anomalies in ancient marine biogenic apatite therefore suggest oxic conditions in the water column and possibly in the upper pore waters, but the lack of a negative cerium anomaly in biogenic apatite does not necessarily indicate sub-oxic or anoxic conditions in the water column (Kemp and Trueman in press). 

Iodine in sediments is supplied from dead organisms and decayed compounds. When detritus in the sediments is decomposed, iodide is liberated under anoxic conditions and diffused in pore water (the concentration is very high). The iodide is partly diffused in seawater, thus higher concentration of iodide was observed in and near the bottom water and the concentration decreased with the distance from the water-sediment interface. The iodide produced on the bottom is very slowly oxidized to iodate in the deep... 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Few examples of studies on cycling of trace elements (other than iron and manganese) at oxic-anoxic interfaces are found in the literature (11-17). Trace element cycling in the water column of a eutrophic lake (Figure 1) is affected by a number of processes related to the redox conditions. 

There has been a long controversy in the literature concerning the lowest O2 concentrations where denitrification can occur (Robertson and Kuenen, 1984). Some believe it can occur at aerobic-anaerobic interfaces (Christensen and Tiedje, 1988 Bonin et al., 1989), while others believe that low O2 conditions inhibit synthesis of the essential enzymes for denitrification (Payne, 1976 Kapralek et al., 1982). It has also been speculated for some time that anoxic microsites may allow for the occurrence of denitrification in aerobic environments (Jannasch, 1960). Laboratory studies indicate that there are differential effects of O2 on the different steps of denitrification, whereby the NO3- reduction step is less sensitive than N( >7 or N2O reduction (Bonin and Raymond, 1990). 

---