Control of CO2 Content of the Atmosphere by the Oceans


  1. Control of CO2 content of atmosphere by the ocean

    1. Consider a hypothetical planet with a crust made of single mineral (Wallastonite) CaSiO3. We could use the composition of a feldspar just as well (Figure 1) but using this mineral simplifies the exercise because its chemical composition is simpler than that of feldspars. Carbon dioxide in the atmosphere combines with water to produce weakly acidic rain. This acidic rain reacts with the wallastonite to produce a set of ions and a weak acid.

      2CO2 + 3H2O + CaSiO3 --> Ca++ + 2HCO3- + H4SiO4

      These ions move through the soil to streams and eventually to the ocean. In the ocean;

      Ca++ + 2HCO3- --> CaCO3 + H2O + CO2

      H4SiO4 --> SiO2 . H2O + H2O

      These reactions release one carbon dioxide molecule back to the atmosphere and create two insoluble compounds, opaline silica and calcium carbonate.

      The insoluble biological products; SiO2 and CaCO3 settle to the ocean floor and are moved by plate tectonics to subduction zones where they are carried deep into the Earth and heated converting them back into Wallastonite and releasing carbon dioxide thus ending the cycle and returning the second CO2 molecule to the atmosphere and CaSiO3 to the continents.

      SiO2 + CaCO3 --> CaSiO3 + CO2.

    2. How does this cycle control the CO2 content of the atmosphere?

      1. The rate of tectonic plate motions set the rate at which CO2 is released from the Earth's interior to the atmosphere. If release from the earth's interior exceeds the rate at which CO2 is removed from the ocean by the formation of calcium carbonate shells by oceanic biological processes then carbon dioxide will accumulate in the atmosphere and visa versa.

        1. In the first case the planet, all else being the same, will become warmer (the greenhouse effect) and wetter (a warmer atmosphere causes greater evaporation, all else being equal).

        2. The carbon dioxide content of soils will rise causing an increase in the rate of weathering.

        3. An increase in moisture (rain) will, in areas where water is a limiting factor, cause increased vegetative growth. Root respiration by plants will cause an increase in the carbon dioxide concentration of soils.

      2. Processes (a) through (c) will increase weathering rates sending more Ca++ to the sea. This increased Ca++ will increase the rate of formation of CaCO3 shells by marine organisms. With time the CO2 withdrawn from the sea by organisms will balance the increased CO2 being added by volcanoes.

    3. Exchange of carbon dioxide between the atmosphere and the ocean (Figure 2).

      1. At a given temperature the exchange of carbon dioxide molecules between the atmosphere and the ocean reaches equilibrium. This equilibrium is controlled by the partial pressure (that pressure caused by the molecules of carbon dioxide alone) of carbon dioxide in the atmosphere and that at the surface of the sea. At these temperatures (all else being equal) the number of carbon dioxide molecules that escape from the sea surface is balanced by the number that enter the sea from the atmosphere.

      2. If the temperature of the ocean were to rise (all else staying the same) then the kinetic energy of the carbon dioxide molecules in the seawater will rise and more carbon dioxide molecules will leave the ocean than would enter the ocean. This will continue until the partial pressure of carbon dioxide in the atmosphere was increased to the point that it balanced that pressure at the sea surface.

      3. If the ocean were to cool then the reverse of the above would happen.

      4. Consequently carbon dioxide is more soluble in cold than in warm water.

      5. When the oceans exchange (“breath”) carbon dioxide with the atmosphere they take it up (“inhale”) in the cold surface waters of high latitudes and release (“exhale”) from the warm surface waters of low latitudes.

      6. Increasing the carbon dioxide concentration of the atmosphere will cause the ocean to take up (inhale) more carbon dioxide. Because the oceans surface layer mixes slowly with the deep ocean (hundreds of years) the increased carbon dioxide content of the surface ocean will be mixed very slowly into the large carbon reservoir of the deep ocean. The rate of our adding carbon dioxide to the atmosphere is too fast for the deep ocean to be a significant reservoir. So the carbon dioxide content of the atmosphere will rise as the concentration in the shallow ocean rises.

  2. Alkalinity

    Both positive and negative ions are added to the sea from rivers. To keep the oceans charge neutral an adjustment occurs in the HCO3- CO3= system. If more positive than negative ions are added to the sea then HCO3 changes to CO3=, CO3= having two negative rather tan one negative charge.

    Alkalinity is defined as A = HCO3- + 2CO3=

    The buffering capacity of the ocean is the total CO3= concentration in the ocean. If carbon dioxide is added to the ocean there is a change from CO3= to HCO3 - . If enough carbon dioxide is added to the ocean then it could exceed the buffering capacity and the oceans acidity would increase. This is a real problem associated with adding carbon dioxide to the atmosphere independent of global warming.

    The calcium carbonate deposits on the ocean floor are also a buffer for with added carbon dioxide the depth at which carbon dioxide dissolves (the carbonate compensation depth) will rise. If the buffering capacity of the ocean is exceeded and all the calcium carbonate on the ocean floor is dissolved as atmospheric concentrations of Carbon dioxide rise then the acidity of the ocean will rise.


Jim Hays, Spring 2004.
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