Suboxic zone

Whereas total concentrations of solid Fe and As remained relatively constant over time (not shown), dissolved Fe and As in the porewaters have increased substantially since 1982 (Fig. 1). The concentrations in the suboxic zone have doubled or tripled. Similarly, HA-extractable Fe and As concentrations increased significantly at stations 23 and 24 since 1982 (Fig. 2). This increase is concomitant with an accumulation of AVS in the sediment over the last 10 years (G. Chaillou, unpublished). 

A consequence of the vertical stratification is that the surface layer (from 0 to 50-200 m) is well oxygenated while the deep layer (50-200 to 2000 m) is anoxic and contains high sulfide concentrations. At the boundary between the oxic surface and anoxic deep layers, there is a suboxic zone (at approximately 50 to 100 m depth) where the concentrations of oxygen are lower than the detection limit. 

The suboxic zone is defined as the region between where oxygen decreases to near zero (O2 < 10 xM) and where sulfide first appears (H2S > 1 iM) [16, 17]. Many important redox reactions involving Fe, Mn, N, and other intermediate redox elements occur in the suboxic zone. Similar redox reactions take place in sediments throughout the world s oceans, but they are easier to study in the Black Sea because they are spread out over a depth scale of tens of meters (rather than centimeter or millimeter scales as in sediments). The Black Sea suboxic layer hydrophysical structure is very stable compared with other ocean redox regions such as Cariaco Trench, which is influenced by mesoscale eddies, or the Baltic Sea that is influenced by inflows of the North Sea saline oxygenated waters in cold winters. 

Production of oxidized Mn in the form of Mn(III) has been observed for Mn(II)-oxidizing bacteria and in incubations with Black Sea suboxic zone water [59]. Dissolved Mn(III) has been directly observed in the suboxic zone in... 

Silicate has conservative characteristics in the suboxic zone. Its vertical distribution practically coincides with that of salinity and density because silicate is not involved in the processes connected with changes in redox conditions. Silicate is not consumed in the processes of chemosynthesis, and its distribution reflects the degradation of OM produced only in the euphotic zone. Silica concentrations are low in the surface and increase smoothly from 50 to 100 xM across the suboxic zone. 

Murray JW, Codispoti LA, Friederich GE (1995) Oxidation-reduction environments the suboxic zone in the Black Sea. In Huang CP, O Melia CR, Morgan JJ (eds) Aquatic chemistry interfacial and interspecies processes. Adv Chem Ser. ACS, Washington,... 

N2O profiles from oceanic regions with suboxic zones such as the Arabian Sea and the eastern tropical North Pacific Ocean, which are sites of intense denitrification activities, generally show a two-peak structure (Fig. 2.3) N2O maxima are found at the upper and lower boundaries of the oxygen minimum zone (OMZ), whereas in the core of the suboxic zone, N2O concentrations are considerably depleted (Bange et ah, 2001b Cohen and Gordon, 1978). In anoxic water masses such as found in the central Baltic Sea, the Cariaco Basin, and Saanich Inlet, N2O concentrations are close to the detection limit or not detectable (Brettar and Rheinheimer, 1991 Cohen, 1978 Hashimoto et ah, 1983 Ronner, 1983 Walter et ah, 2006b). 

Pacific Ocean revealed that N2O in the suboxic zone was enriched in relative to N2O in the atmosphere whereas N2O was slightly depleted in relative to atmospheric N2O (Yoshida et al., 1984). This in line with the argumentation that N2O in suboxic waters is consumed during denitrification and that N2O is formed during nitrification in oxic waters. Surprisingly, Yoshida et al. (1989), found N-enriched N2O in the oxic waters of the upper 2000 m of the western North Pacific and concluded that N2O is mainly produced by denitrification. A conclusion which seems to be counterintuitive, because denitrification in oxic waters should be negligible. 

N2 in the suboxic deep waters (70—440 m) of the western part of the central Baltic Sea was supersaturated up to 8%, whereas in the surface layer N2 was slightly undersaturated (Ronner and Sorensson, 1985). The accumulation of N2 resulted from denitrification. In the suboxic intermediate water masses (150—1000 m) of the central Arabian Sea, N2 concentrations were found to be significandy enhanced with a maximum in 250 m (Codispoti et al., 2001, 2005). This corresponds to a minimum of of dissolved N2 in the core of the suboxic zone, which drops from 0.6%o... 

In order to quantify the loss of fixed nitrogen via denitrification and anammox we definitely need more N2 concentration measurements and more formation rate measurements in suboxic zones such as found in the Arabian Sea and the eastern tropical Pacific Ocean. 

Much attention has recently been focused on interactions between various manganese and nitrogen species because solute profiles through the suboxic zones of anoxic basins such as of the Black Sea and marine sediments suggest that reactions involving these two species may be occurring (Murray et ai, 1995 Luther et ai, 1997, 1998). Luther et al. (1997, 1998) have proposed two reactions with manganese that result in denitrification ... 

In the open-ocean suboxic zone denitrification does not reach completion, but a minimum in NOs profile invariably occurs within the SNM (Figs. 14.3 and 14.8). 

German C. R., Holliday B. P., and Elderfield H. (1991) Redox cycling of rare earth elements in the suboxic zone of the Black Sea. Geochim. Cosmochim. Acta 55, 3553-3558. 

Selenium profiles in sediments from the northeast Atlantic Ocean indicate concentrations of —0.2-0.3 mg kg in the oxic zone, and typically 0.3-0.5 mg kg below the redox boundary reflecting immobilization under reduced conditions (Thomson et al., 2001). Similar increases for cadmium, uranium, and rhenium have also been observed in the suboxic zone. 

Oxidation of As(III) has also been observed in groundwater. Arsenite was injected into oxic [200-300 pM (micromoles per liter) O2] and suboxic (1-6 pM O2) zones of groundwater at a site on Cape Cod, Massachusetts, USA. Significant oxidation of As(III) to As(V) along a 2.2 meter flow path was observed in both the oxic and suboxic zones (Kent et al., 2001). Manganese oxides are present in the mixutre of oxides that coat these aquifer solids (Fuller et al., 1996) and are the most likely electron acceptor in both the low O2 suboxic zone and possibly the oxic zone. 

Sholkovitz (21) argued that it is difficult to separate the Mn and Fe redox cycles because dissolved Mn and Fe are well correlated in the suboxic zone. This theory is true to the extent that they both increase in the suboxic zone, but there is a clear difference in their distributions. The sharp increase in dissolved iron begins about 0.15 density units deeper than dissolved Mn (22, 23). 

The observation of the suboxic zone immediately leads to three questions ... 

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