Gases may enter solution either by purely physical absorption 

 or chemical combination with the liquid. For the ocean-atmosphere 

 system, physical absorption occurs in the case of oxygen, nitrogen, 

 and rare gases. Carbon dioxide, on the other hand, reacts with water 

 and dissolved salts. In comparison with the oxygen-nitrogen ratio in 

 the atmosphere of 1:4, that of air dissolved in water is more nearly 

 1:2. Saturation quantities are a function of water temperature and 

 decrease by 50 percent from polar to equatorial regions. There is 

 ample air available at the sea surface, therefore, gas contents in the 

 upper layers are near saturation. As nitrogen is relatively inert, 

 only minor variations with depth are observed, since all water has 

 been in contact with the atmosphere at some time. The oxygen content 

 is markedly affected by biological factors, such as generation by 

 photosynthesis in the upper layers, which occasionally results in 

 super saturation • 



Oxygen consumption by animal respiration and bacterial oxidation 

 reduces the oxygen content and results in a pronounced minimum at 

 depths of 100 to 1,500 m in the middle and lower latitudes. Deeper 

 waters of polar origin are relatively rich in oxygen. In stagnant 

 water, where there is no photosynthesis and oxygen depletion occurs, 

 as in the deep portion of the Black Sea, hydrogen sulphide may be 

 formed. The partial pressure of C02 in the sea in contact with the 

 atmosphere tends toward equilibrium with that in the air. However, 

 areas exist where carbon dioxide-rich water is brought to the surface 

 by rising currents, while in other regions the CO2 partial pressure 

 is frequently low in spring and summer where plant plankton is 

 assimilating. Figure 4-3 is based on the condition that the volume of 

 the atmosphere is about 2.9 times that of the ocean. Figure 4-3, 

 comparing gas concentrations in air and seawater, shows that the ocean 

 is a sink for CO2. 



The pH is a measure of hydrogen ion concentration on a logarithmic 

 scale. The pH of pure water is 7. An acidic solution has a pH of less 

 than 7, whereas an alkaline solution has a pH greater than 7. Seawater 

 is normally alkaline with a pH between 7.5 and 8.4. 



b. Equation of State and Thermal Properties 



The equation of state represents the density of seawater as a 

 function of salinity, temperature, and pressure. This equation is of 

 particular importance in oceanography in that minor changes in density 

 affect stability, water mass transfer, and turbulent mixing. For 

 these purposes, the oceanographer must know the density to 1 part in 

 10 and must resort to indirect measurements of chlorinity, electrical 

 conductivity, refractive index, or the like for density determination, 

 with corrections for depth and temperature. The expression for density 

 is unwieldy and in practice it is customary to use the notation 

 fff = {P -1) X 103 where p is in g/cm3, and to use tables of (Tf as a 

 function of salinity, temperature, and pressure. 



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