H2o

o h2co3

In addition, some CO2 undergoes simple reaction with water, for example, co2 + h2o o h+ + hco3-

which releases 2 mole-equivalents of Total Alkalinity and 1 mol of Dissolved Inorganic Carbon for each mole of CaCO3 dissolved. Increasing TAlk more than DIC leads to a decrease in the partial pressure of CO2 as seen in Figure 6.5. Because most Dissolved Inorganic Carbon is in the form of HCO3-, the main effect of dissolving CaCO3 in surface waters is (see Kheshgi, 1995)

Total dissolved inorganic carbon (DIC) is the sum of carbon contained in H2CO3, HCO3-, and CO32-. The atmospheric concentration of CO2 in equilibrium with surface water can be calculated from well-known chemical equilibria that depend on ocean total dissolved inorganic carbon, alkalinity, temperature and salinity (Zeebe and Wolf-Gladrow, 2001). The partial pressure of CO2 in the ocean mixed layer equilibrates with the atmosphere on a time scale of about one year.

The ocean is a highly buffered system, that is the concentration of the chemical species whose equilibrium controls pH is significantly higher than the concentrations of H+ or OH-. The pH of sea water is the base-10 log of activity of H+. Total Alkalinity (TAlk) is the excess of alkaline components, and is defined as the amount of strong acid required to bring sea water to the 'equivalence point' at which the HCO3- and H2CO3 contributions are equal (Dickson, 1981).

The principal effect of adding CO2 to sea water is to form bicarbonate ion, for example, thereby shifting CO2 from the atmosphere to the oceans in equilibrium, neutralizing the effect of CO2 on pH.

Ocean surface waters are super-saturated with respect to CaCO3, allowing the growth of corals and other organisms that produce shells or skeletons of carbonate minerals. In contrast, the deepest ocean waters have lower pH and lower CO32- concentrations, and are thus undersaturated with respect to CaCO3. Marine organisms produce calcium carbonate particles in the surface ocean that settle and dissolve in undersaturated regions of the deep oceans.

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