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Gayana (Concepción)

versión impresa ISSN 0717-652Xversión On-line ISSN 0717-6538

Gayana (Concepc.) v.70  supl.1 Concepción oct. 2006 

Suplemento Gayana 70: 59-61, 2006

Mechanisms controlling the suboxic layer of the black sea


Mecánismos que controlan la capa subóxica del Mar Negro

Temel Oguz

Institute of Marine Sciences, Middle East Technical University, P.O. Box 28, Erdemli 33731, Mersin, Turkey,


The Black Sea is a classic anoxic ocean basin resembling the earth's ancient oceanic conditions. Its present structure has evolved during the last 2.5 billion years, and is characterized by a well-defined sub-oxic zone at the interface between the oxic and sulfidic layers where many important bacterially-mediated redox reactions occur. The sub-oxic zone involves extremely low oxygen and sulfide concentrations with no perceptible vertical gradients. This is due to very strong density stratification along the sub-oxic and anoxic layers, which markedly restricts the downward flux of oxygen and upward flux of chemical properties and therefore maintains a stable structure with some inter-annual variations. The upper layer biogeochemical structure above the deep anoxic pool involves three distinct layers (Fig. 1). The biologically productive, oxic layer extends to the depth of nearly 50 m. About 90% of the sinking particles are remineralized inside this layer as well as in the subsequent 20-30 m deep "upper nitracline" zone. There, nitrate attains maximum concentrations around 8 µM, and is re-supplied to the surface layer to refuel the biological pump. Only a small fraction (~10%) of particulate matter sinks to the deeper anoxic part of the sea. This loss is however compensated by lateral nitrogen input mainly from the River Danube, by wet deposition and nitrogen fixation.

Keywords: Black Sea, ancient ocean conditions, sub-oxia, anoxia, nitracline.


El Mar Negro es una clásica cuenca oceánica anóxica que presenta condiciones parecidas a las de océanos primitivos de la Tierra. Su actual estructura evolucionó durante los últimos 2,5 mil millones de años y se caracteriza por una bien definida zona sub-óxica en la interfaz entre las capas óxica y sulfurosa donde ocurren muchas y muy importantes reacciones redox mediadas por bacterias. La zona sub-óxica implica concentraciones extremadamente bajas de oxígeno y sulfuro sin gradientes verticales perceptibles. Esto se debe a una fuerte estratificación por densidad a lo largo de las capas sub-óxica y anóxica, la que restringe fuertemente el flujo, hacia abajo del oxígeno y el flujo hacia arriba de las propiedades químicas, manteniendo por tanto una estructura estable con algunas variaciones inter-anuales. La estructura biogeoquímica de la capa por encima del pozo anóxico profundo involucra tres diferentes capas. La capa óxica, productiva biológicamente, se extiende hasta una profundidad de cerca de 50 m. Alrededor del 90% de las partículas que se hunden son mineralizadas dentro de esta capa así como en la subsiguiente zona de 20-30 m de espesor denominada "nitroclina superior". En esta zona, el nitrato alcanza las máximas concentraciones de alrededor de 8 µM, siendo este inyectado hacia la capa superficial para alimentar la bomba biológica. Sólo una pequeña fracción (~10%) del material particulado se hunde hacia las partes anóxicas más profundas de este mar. Esta pérdida es sin embargo compensada por deposición húmeda y fijación del nitrógeno del influjo lateral proveniente principalmente del Río Danubio.

Palabras Claves: Mar Negro, condiciones oceánicas primitivas, sub-oxia, anoxia, nitroclina.

The oxygen concentration in the surface layer undergoes pronounced seasonal variations from about 250 to 450 µM. Irrespective of the season, the oxygen concentration then decreases almost linearly to concentrations of about 100 µM at st ~15.3 kg m-3 and about 10 µM at st ~15.6 kg m-3 due to intense oxygen consumption during the decomposition process of organic matter. Oxygen concentrations vanish completely at st ~16.2 kg m-3, below which is characterized by gradually increasing hydrogen sulfide concentration (Fig. 1). The oxygen deficient (O2 <10 µM), non-sulfidic layer having a thickness of 10-to-40m coinciding with the lower nitracline zone is referred to as the "Suboxic Layer (SOL)".

Figure 1. Vertical biogeochemical structure of the Black Sea involving profiles of oxygen (O2), sulfide (H2 S), nitrate (NO3-), ammonium (NH4+), and nitrogen gas (N2), as well as the vertical zonation of the oxic, suboxic and anoxic layers. The boundaries between these layers are shown by dashed lines (after Konovalov et al. 2005).

The SOL was not detected until 1988 (Murray et al. 1989), mainly because of contamination of water samples with atmospheric oxygen in the earlier measurements.

The SOL is characterized by pronounced changes in nitrogen concentrations. Organic matter decomposition, via denitrification, results in a sharp decrease of nitrate concentrations at a thickness of about 30-40 m from their peak values to their trace values around 100 m depth or st ~16.0 kg m-3 isopycnal surface. Nitrate consumption due to oxidation of reduced manganese and ammonium may also contribute to reduction of nitrate concentrations within the lower part of the suboxic zone (Murray et al. 1995). Importance of the Anammox type reactions (NO2-+NH4+ gN2) in the Black Sea was recently shown by Kuypers et al. (2003). In these processes, bacteria utilize nitrate and nitrite ions to oxidize organic matter, reduced manganese and ammonium. The nitrate is then reduced to nitrogen gas with nitrite as an intermediate product. A nitrite peak with concentrations up to 0.5 µM is usually observed at st ~15.85±0.05 kg m-3 located approximately 10 m (or, equivalently, st ~0.1 kg m-3) above the zone of nitrate depletion (Fig. 1) and the onset of hydrogen sulfide.

The boundary between the suboxic and anoxic layers involve a series of complicated bacterially-mediated redox processes. As dissolved oxygen and nitrate decrease towards zero concentrations at the suboxic-anoxic interface, dissolved manganese, ammonium and hydrogen sulfide begin to increase at the interface. Marked gradients of particulate manganese around this transition zone near st ~16.0 kg m-3 reflect the role of manganese cycling (Konovalov et al. 2005). The deep ammonium, sulfide and manganese pools have accumulated as a result of organic matter decomposition within the last 5000 years, after the Black Sea was converted into a two-layer stratified system. The gradient of ammonium profiles in the vicinity of the suboxic-anoxic interface, however, implies that no ammonium is supplied to the photic zone from the anoxic region.

The anaerobic sulfide oxidation and nitrogen transformations coupled to the manganese and iron cycles have been considered one of the mechanisms to maintain stability of the interface structure between the suboxic and anoxic layers (Murray et al. 1995, Oguz et al. 2001, Konovalov et al. 2005). The upward fluxes of sulfide and ammonium are oxidized by Mn(III, IV) and Fe(III) species, which are generated by Mn(II) and Fe(II) oxidation by reactions with nitrate. The upward flux of ammonium is also consumed by NO3-/NO2- via Anammox reaction (Kuypers et al. 2003). These oxidation-reduction reactions are microbially catalyzed, but dissolved chemical reduction may also play a role in Mn(IV) reduction with sulfide. Modeling studies (Oguz et al. 2001, Konovalov et al. 2005) demonstrate that the latter mechanism alone could provide the observed redox structure. Anaerobic photosynthesis is considered an additional mechanism contributing to the stability of suboxic-anoxic interface zone. The reduced chemical species (HS-, Mn2+, Fe2+) are oxidized by anaerobic phototrophic bacteria in association with phototrophic reduction of CO2 to form organic matter. This mechanism was supported by the discovery of large quantities of bacteriochlorophyll pigments near the suboxic-anoxic boundary (Repeta et al. 1989, Jørgensen et al. 1991). A particular bacterium is capable of growth using reduced S (H2S or S0) at very low light levels (<<0.1% of the incident radiation at the surface).

It is difficult to quantify the anoxygenic photosynthesis as an efficient mechanism of basin-wide relevance. Its contribution must be limited to cyclonic regions where the anoxic interface zone is shallow enough to be able to receive sufficient light to maintain bacterial photosynthetic activity. Presence of a persistent suboxic zone structure with its lower boundary located at depths of around 160-180 m within the quasi-permanent anticyclonic gyre of the eastern basin therefore suggests that anaerobic sulfide oxidation should control first-order dynamics of the redox structure in the Black Sea. Anaerobic photosynthesis is expected to have an additional contribution to the dissolved oxygen-hydrogen sulfide separation, and thus to formation of somewhat thicker SOL at shallower depths within cyclonic areas of the basin.

The third mechanism is oxidation of H2S by oxygen and particulate manganese injected horizontally into the anoxic layer (Tebo et al. 1991). Because the sulfide layer is located only about 100 m below the surface in most parts of the Black Sea, the vertical extent of horizontal ventilation of the upper layer of the water column is a critically important issue. This process is most effective around the periphery of the basin and particularly in the vicinity of the Bosphorus Strait. The contribution of oxygen supplied by the Mediterranean waters of Bosphorus origin to sulfide oxidation are observed in the vertical profiles of T, S, O2 and H2S in the southwestern region of the Black Sea (Konovalov et al. 2003). On the other hand, no data exist to support the ventilation of suboxic and anoxic layers through atmospheric oxygen supply. The strong stability of the water column appears to hardly allow any seasonal variation of oxygen below the oxycline due to atmospheric ventilation. According to model simulations (Oguz 2002), episodic strong cooling events under realistic ranges of heat fluxes could only increase the density of the mixed layer up to st ~15.0 kg m-3 which corresponds to deepening of the mixed layer to about 75m within the cyclonic regions.


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