Oxic-anoxic interface

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). 

As with HgT, concentration profiles for MeHg in sediment often show dramatic changes with depth and considerable spatial variability. Typically, maximum concentrations ate observed at or near the oxic/anoxic interface, which is generally near... 

F.J. Stewart, I.L.G. Newton, and C.M. Cavanaugh, Chemosynthetic endosymbioses adaptations to oxic-anoxic interfaces. TRENDS Microbiol. 13, 439-448 (2005). 

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. 

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. 

Buffle, J., R. R. De Vitre, D. Perret, and G. G. Leppard. 1989. Physico-chemical characteristics of a colloidal iron phosphate species formed at an oxic-anoxic interface of a eutrophic lake. Geochimica et Cosmochimica Acta 53 399-408. 

Recently, workers (2) have been examining the equilibrium and kinetic factors that are important at the oxic-anoxic interface. The kinetic behavior is difficult to characterize completely due to varying rates of oxidation and absomtion above the interface and varying rates of reduction, precipitation and dissolution below the interface (2.51. Bacterial catalysis may also complicate the system (1). Although one can question the importance of abiotic thermodynamic and kinetic processes at this interface, we feel it is useful to use simple inorganic models to approximate the real system. Recently, the thermodynamics and kinetics of the H2S system in natural waters has been reviewed (0. From this review it became apparent that large discrepancies existed in rates of oxidation of H2S and the thermodynamic data was limited to dilute solution. In the last few years we have made a number of thermodynamic (7.81 and kinetic (9 101 measurements on the H2S system in natural waters. In the present paper we will review these recent studies. The results will be summarized by equations valid for most natural waters. 

The flux of DS across the sediment-water interface can in some cases be strongly influenced by the presence of chemoautotrophic bacterial mats in estuaries. These sulfur oxidizing bacteria occur at the oxic-anoxic interface and can occur as colorless or as pigmented forms (GSB and PSB). 

In oxic conditions nitrate is produced by respiration and reaches a maximum (3-10 iM) at ae = 15.30-15.50 kgm 3. Below this depth the concentrations of nitrate decrease sharply with vertical gradients 0.2-0.5 p,Mm4. After oxygen, nitrate is the second most abundant oxidizing agent in the oxic-anoxic interface. Nitrate disappears in the vicinity of ae = 15.90-16.00 kg m 3. 

Bezborodov AA, Eremeev VN (1993) Chernoe more. Zona vzaimodeistviya aerobnikh I anaerobnikh vod (Black Sea. The oxic/anoxic interface). MHI NASU, Sevastopol (in Russian)... 

Pimenov NG, Neretin LN (2006) Composition and activities of microbial communities involved in carbon, sulfur, nitrogen and manganese cycling in the oxic/anoxic interface of the Black Sea. In Neretin LN (ed) Past and present water column anoxia. NATO Sciences Series. Springer, Dordrecht, p 501... 

The average concentrations of reduced inorganic sulfur species in the anoxic zone of the Black Sea measured using a new colorimetric method developed by Volkov [61,62] are summarized in Table 3. Presented elemental sulfur data refer to the stun of elemental sulfur allotropes (zero-valent sulfur) and the zero-valent sulfur derived from some fraction (n - 1) of the original polysulfide S 2. Thiosulfate data in the table represent the total amount of thiosulfate, sulfite, and polythionates. At some stations in the Black Sea, Volkov [61] observed a concentration maximum of elemental sulfur at the oxic/anoxic interface associated with sulfide oxidation by dissolved oxygen and/or Mn oxyhydroxides. Increasing with depth, elemental sulfur concentrations are probably explained by the ongoing process of polysulfide formation... 

The sulfur budget for the Black Sea has been considered in several papers [23, 24,74-77]. Sulfide sources are sulfide production in sediments, sulfide flux at the sediment/water interface, and sulfide production in the water column. Sulfide sinks are sulfide oxidation at the oxic/anoxic interface and in the basin interior by dissolved oxygen of the modified Mediterranean water and iron sulfide formation in the water column. 

Integration of the measured H235S oxidation rates in the Black Sea chemo-cline yielded values between 53 and 125 Tgyear-1 [33,86,87]. Rate measurements and modeling data gave median sulfide oxidation rates at the oxic/anoxic interface in the range 20-50 Tgyear-1 [75]. 

The annual Bosporus flux into the Black Sea is estimated to be 120-312 km3 [88]. The role of dissolved oxygen intrusions below the oxic/anoxic interface is a controversial issue since the magnitude and physio chemical... 

Redox processes are important for elements which can exist in more than one oxidation state in natural waters, e.g. Fe and Fe, Mn, and Mn. These are termed redox-sensitive elements. The redox conditions in natural waters often affect the mobility of these elements since the inherent solubility of different oxidation states of an element may vary considerably. For example, Mn is soluble whereas Mn is highly insoluble. In oxic systems, Mn is precipitated in the form of oxyhydr-oxides. In anoxic systems, Mn predominates and is able to diffuse along concentration gradients both upwards and downwards in a water column. This behaviour gives rise to the classic concentration profiles observed for Mn (and Fe) at oxic-anoxic interfaces as illustrated in Figure 2. 

Example 3.2 In oxic waters, Mn2+ is normally oxidized to MnCfys)- Under certain circumstances, Mn3+ is also produced. In order to better understand manganese cycling at the oxic/anoxic interface, thermodynamics offers its predictive power. From this point of view, and assuming standard conditions and concentrations,... 

Brettar, I., and Rheinheimer, G. (1991). Denitrification in the central Baltic Evidence for H2S oxidation as motor of denitrification at the oxic-anoxic interface. Mar. Ecol. Prog. Ser. 77, 157—169. 

Kristensen, E. (2000). Organic matter diagenesis at the oxic/anoxic interface in coastal marine sediments, with emphasis on the role of burrowing animals. Hydrobiologia 426, 1—24. 

Figure 23.1 Seagrass influences on N cycling processes. The red line symbolizes the oxic—anoxic interface oxic microzones around seagrass roots not shown in the figure are also important.

Overall, the Late Ordovician extinction appears to be the result of purely terrestrial phenomena. High sea-level stands of the early Paleozoic allowed for animal diversification on shallow-water, epicontinental carbonate platforms that proved, however, to be highly sensitive to glacio-eustatic effects on ecospace availability and lateral shifts in the oxic-anoxic interface. Tectonic activity facilitated the establishment of Gondwanan ice sheets which robbed the shallow seas of water, leading to extinction, establishment of recovery ecosystems, and then the destruction of these as the ice sheets melted, perhaps catastrophically. 

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