Minimum oxygen

Pederson, T.F., G.B. Shimmield, and N.B. Price. 1992. Lack of enhanced preservation of organic matter in sediments under the oxygen minimum of the Oman Margin. Geochimica et Cosmochimica Acta 56 545-551... 

The balance between relative rates of aerobic respiration and water movement were considered in Section 4.3.4. We saw that a subsurfece concentration minimum, the oxygen minimum zone (OMZ), is a common characteristic of vertical profiles of dissolved oxygen and is produced by in situ respiration. Waters with O2 concentrations less than 2.0 ppm are termed hypoxic The term anoxic is applied to conditions when O2 is absent. (Some oceanographers use the term suboxic to refer to conditions where O2 concentrations fall below 0.2 ppm but are still detectable.) As illustrated by Figure 4.21b, this water column is hypoxic in the OMZ. The dissolved oxygen concentrations are presented as % saturations in Figure 4.21c. With the exception of the mixed layer, the water column is undersaturated with respect to dissolved oxygen with the most intense undersaturations present in mid-depths. Surface supersaturations are the result of O2 input from photosynthesis and bubble injection. 

Oxygen minimum zones (gray) where intermediate and deep water have concentrations <1 ml O2/L. Source-. From Levin, L. A. (2002). American Scientist 90, 436-444. 

Denitrification in continental shelves and OMZs (oxygen minimum zones). Source From Seitzinger, S et al. (2006). Ecological Applications 16, 2064-2090. (See companion website for color version.)... 

Oxygen minimum zone (OMZ) Subsurface undersaturations in dissolved oxygen gas found within... 

Secondary nitrite maximum Subsurface concentration maximum in nitrite found in the oxygen minimum zone. Mainly caused by denitrification. 

Ward, B. B., and Zafiriou, O. C. (1988). Nitrification and nitric oxide in the oxygen minimum of the eastern tropical north pacific. Deep Sea Res. 35, 1127-1142. 

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. 

An estuary provides a particularly varied chemical environment which is characterized by strong ionic strength and pH gradients ranging from river to sea end members values. In addition one can usually observe a slight pH and dissolved oxygen minimum in the estuary itself which is enhanced in polluted estuaries. 

Douglas, E.L., Friede, W.A. and Pickwell, G.V. (1976). Fishes in oxygen minimum zones blood oxygenation characteristics. Science, New York 191,957-959. 

Typical vertical saturation profiles for the North Atlantic, North Pacific, and Central Indian oceans are presented in Figure 4.10. The profiles in the Atlantic and Indian oceans are similar in shape, but Indian Ocean waters at these GEOSECS sites are definitely more undersaturated than the Atlantic Ocean. The saturation profile in the Pacific Ocean is complex. The water column between 1 and 4 km depth is close to equilibrium with calcite. This finding is primarily the result of a broad oxygen minimum-C02 maximum in mid-water and makes choosing the saturation depth (SD) where Oc = 1 difficult (the saturation depth is also often referred to as the saturation level SL). 

At 20 °C, K = 10 - and so water of pH=8.1 in equilibrium with atmospheric O2 (p02 — 0.21 atm) has pe = 12.5. This conforms to surface conditions, but the pe decreases as the O2 content diminishes with depth. The oxygen minimum is particularly well developed beneath the highly productive surface waters of the eastern tropical Pacific Ocean, where there is a large flux of organic material to depth and subsequently considerable oxidation. The O2 becomes sufficiently depleted i.e., hypoxia) that the resulting low redox conditions causes NOs to be reduced to N02 - Aeolian transport of nitrate to Chesapeake Bay can lead to low O2 conditions. Similarly, intermittent hypoxia develops in parts of the Gulf of Mexico due to the riverine transport of nutrients derived from agricultural uses in the Mississippi catchment. 

C02 and a fall in pH. Thus, the depth profiles of pH would mimic that of O2 but the X C02 would exhibit a maximum at the oxygen minimum. 

PROP Iodine number maximum of 6, oxirane oxygen minimum of 6.0 percent. SAFETY PROFILE When heated to decomposition it emits acrid smoke and irritating fumes. 

Five depth profiles of NO and the corresponding NO production rates have been measured in the ETNP (Ward and Zafiriou, 1988) NO concentrations were in the range from 0 up to 65 pmol At four stations located in the open ocean, maximum NO concentrations were observed at the upper boundary of the oxygen minimum zone (OMZ, O2<10 pmol L ), whereas one coastal station showed an increase of NO from Opmol at the surface to about 20 pmol at the bottom in about 250m. Maximum NO production rates were found at the upper boundary of the OMZ at the open ocean stations. However, Ward and Zafiriou (1988) could not unambiguously identify the NO formation process because NO production rates and nitrification rates (i.e., NH oxidation rates) were not correlated. NO accumulation appeared when O2 concentrations were lower than 100 pmol L , whereas in the core of the OMZ with O2 concentrations close to 0 pmol denitrification seemed to cause a rapid turnover of NO. Highest ever-reported concentrations of dissolved NO were found off Peru ranging from 0 up to 400 pmol (Zafiriou, personal communication in Ward and Zafiriou (1988)). 

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). 

Castro-Gonzalez, M., and Fraias, L. (2004). N2O cycling at the core of the oxygen minimum zone off northern Chile. Mar. Ecol. Progr. Set. 280, 1—11. 

Cohen, Y., and Gordon, L. I. (1978). Nitrous oxide in the oxygen minimum of the eastern tropical North Pacific Evidence for its consumption during denitrification and possible mechanisms for its... 

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