Oxygen in the ocean

Oxygen is in a state of super-saturation in surface waters, mostly because of entrainment of bubbles from breaking waves (see §2.9.2). As these bubbles of air are carried below the surface, the increase in pressure forces gas into solution. Hence not only oxygen but most atmospheric gases are slightly supersaturated at the ocean surface. There is also biological production of oxygen in parts of the water column where there is sufficient light for photosynthesis

Fig. 3.8. Geographical distribution of anomalous total CO2 (in ^mol kg-1) in the surface waters of the Atlantic, based on measurements of salinity, phosphate, nitrate, and alkalinity during GEOSECS and TTO expeditions. The total CO2 has been corrected for fossil fuel emissions. [Data from Broecker and Peng, 1992.]

Fig. 3.8. Geographical distribution of anomalous total CO2 (in ^mol kg-1) in the surface waters of the Atlantic, based on measurements of salinity, phosphate, nitrate, and alkalinity during GEOSECS and TTO expeditions. The total CO2 has been corrected for fossil fuel emissions. [Data from Broecker and Peng, 1992.]

Fig. 3.9. Variation of oxygen concentration with depth at four locations during the GEOSECS cruises of 1972-3. Site (a) is in the Norwegian-Greenland Sea (74°55'N, 1°07'W); (b) is in the western sub-tropical Atlantic (31°48'N, 50°46'W);

(c) is in the tropical east Atlantic (10°59'N, 20°32'W);

(d) is in the central North Pacific (31°22'N, 150°02'W). The units are micromoles of oxygen per kg sea water.

Fig. 3.9. Variation of oxygen concentration with depth at four locations during the GEOSECS cruises of 1972-3. Site (a) is in the Norwegian-Greenland Sea (74°55'N, 1°07'W); (b) is in the western sub-tropical Atlantic (31°48'N, 50°46'W);

(c) is in the tropical east Atlantic (10°59'N, 20°32'W);

(d) is in the central North Pacific (31°22'N, 150°02'W). The units are micromoles of oxygen per kg sea water.

(see Chapter 4). This is almost balanced by oxygen consumed by respiration. As light levels decrease with depth through the ocean the balance swings further towards consumption of oxygen rather than production, and eventually there is only respiration. Oxygen therefore tends to reach a minimum at some depth below the surface. This depth dependence of the dissolved oxygen level is illustrated in Fig. 3.9.

This is only a summary of what occurs. In Chapter 4 we will investigate the biological processes in more detail, particularly the gaseous by-products of life which influence the climate through precipitation mechanisms and the greenhouse effect. In the remainder of this section we will examine some geographical anomalies to the simple pattern of oxygen concentration described in the last paragraph.

Fig. 3.10 shows a section of surface oxygen levels, compared to saturation, northwards through the tropical east Pacific. These levels are close to 5%

Fig. 3.10. Difference between the observed and saturation O2 contents for surface waters in the equatorial zone in the central Pacific. Two transects are shown, both of which reveal the usual super-saturation of surface waters in oxygen and the equatorial deficit due to upwelling of oxygen-poor water. [Fig. 3.7 of Broecker and Peng (1982). Reproduced with permission of W. S. Broecker.]

Fig. 3.10. Difference between the observed and saturation O2 contents for surface waters in the equatorial zone in the central Pacific. Two transects are shown, both of which reveal the usual super-saturation of surface waters in oxygen and the equatorial deficit due to upwelling of oxygen-poor water. [Fig. 3.7 of Broecker and Peng (1982). Reproduced with permission of W. S. Broecker.]

super-saturation, for the reasons discussed above, but within 100 km of the equator the oxygen level drops dramatically, becoming unsaturated. From our discussion of the tropical ocean circulation in §2.11.3, and the enhanced equatorial levels of surface pCO2 discussed in the last section, you can probably deduce the cause for this equatorial anomaly. The upwelling brings water from depth, which is not only rich in carbon dioxide, but depleted in oxygen through previous biological activity. Oxygen levels might therefore be expected to be depressed in most upwelling regions. The under-saturation in oxygen is not sufficient, however, to decrease the biological activity of these regions. Indeed, upwelling regions are usually very productive, because of the transport of nitrates and phosphates - the food supply for phytoplankton - to the surface. This increased productivity leads to pronounced oxygen minima at depth beneath upwelling regions. In a few areas of the global ocean - the Arabian Sea and the tropical eastern margins of the Pacific - almost all the oxygen is consumed at the thermocline. In §4.2.2 we will see that this has implications for the ultimate fate of a greenhouse gas, nitrous oxide, produced by biological activity.

Fig. 3.9 showed oxygen levels rising below the thermocline, as well as the general decline with depth in the upper ocean. This reversal occurs because of the transport of oxygen-rich water from the polar regions of the North and South Atlantic. When this deep water is formed it will be saturated with oxygen. Moreover, as this water is cold it will have oxygen levels about 60% higher than those in the tropics; Table 3.1 and Fig. 3.1 illustrate this temperature dependence of solubility. Some of this oxygen is naturally consumed by biological processes in the formation regions, but only 5-10%. Less is consumed in the North Atlantic because the timescale of deep water formation is rather faster there. As the deep water spreads to fill the world ocean slight decreases in this oxygen level occur, due to the respiration of the small amount of life in the deep ocean. This decrease is a major piece of evidence for the deep ocean circulation hypothesized in §1.3.2 and Fig. 1.14. There is a clear decrease in oxygen levels between the Atlantic and the Indian and Pacific Oceans, shown in Fig. 3.11. This deep circulation provides the longest timescales, of centuries, for the ocean's influence on climate.

Fig. 3.11. Distribution of Actual Oxygen Utilization (AOU) in the deep ocean, at a depth of 4000 m. AOU is the saturation oxygen content minus the measured oxygen content. Higher values indicate greater use of oxygen by biological organisms. Two sources of deep water are seen, from the North Atlantic and Weddell Seas. [Fig. 3.8 of Broecker and Peng (1982). Reproduced with permission of W. S. Broecker.l

Fig. 3.11. Distribution of Actual Oxygen Utilization (AOU) in the deep ocean, at a depth of 4000 m. AOU is the saturation oxygen content minus the measured oxygen content. Higher values indicate greater use of oxygen by biological organisms. Two sources of deep water are seen, from the North Atlantic and Weddell Seas. [Fig. 3.8 of Broecker and Peng (1982). Reproduced with permission of W. S. Broecker.l

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