Preformed nutrient concentrations

The global efficiency of the biological pump can be evaluated in terms of a global mean preformed nutrient concentration for surface water that is folded into the ocean interior. In the calculation of this mean preformed nutrient concentration, each subsurface water formation... 

Within the polar oceans involved in deep water formation, certain regions are more important than others. The Antarctic Zone, the most polar region in the Southern Ocean, is involved in the formation of both deep and intermediate-depth waters, making this region important to the atmosphere/ocean CO2 balance. The quantitative effect of the Subantarctic Zone on atmospheric CO2 is less certain, depending on the degree to which the nutrient status of the Subantarctic surface influences the preformed nutrient concentration of newly formed subsurface water (Antarctic Intermediate Water and Subantarctic mode water), but its significance is probably much less than that of the Antarctic. 

The concept of nutrient and O2 change along a surface of constant density in the upper ocean is illustrated in Fig. 6.18. When waters subduct (surface waters flow along density horizons into the thermo-cline) it is assumed that they carry with them O2 concentrations near saturation equilibrium with the atmosphere and preformed nutrient concentrations. The assumption of saturation equihbrium is not exactly correct but in most cases this is probably not a serious error because surface oxygen measurements indicate near-saturation equilibrium everywhere except in the Southern Ocean south of the polar front, where concentrations can be up to 10% undersaturated. As a water parcel moves along a constant-density surface into the upper thermocline, respiration consumes the O2 concentration while creating nutrients and AOU in the water mass. At any point in the ocean interior, preformed nutrient concentrations can be calculated if one knows the temperature, salinity (for determining [02 ), nutrient and O2 concentrations. 

A useful apphcation of preformed nutrient concentrations is that they are intrinsic to different water masses and sometimes can be used as conservative tracers. For example, the main sources of deep water in the Pacific Ocean are North Atlantic Deep Water (NADW), Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW), all of which are at least partly homogenized in the Antarctic Circumpolar Water (AACW). It is not possible to determine how much of each of these sources contributes to Pacific deep water by using end member mixing of the conservative properties temperature and salinity because salinities of the end members are not sufficiently different. Since concentrations of DIP are well above detection limits in... 

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