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7
The Carbonate System
7.1 Introduction
The major portion of carbon in the oceans occurs as part of the carbonate system. This
system involves the following equilibria:
CO 2 (g) = CO 2 (aq)
(7.1)
CO 2 (aq) + H 2 O = H + + HCO 3
–
(7.2)
HCO 3
– = H+ + CO 3
2–
(7.3)
Ca 2+ + CO 3
2– = CaCO 3 (s)
(7.4)
The carbonate system is very important since it regulates the pH of seawater and controls the circulation of CO 2 between the biosphere, the lithosphere, the atmosphere, and the
oceans. Recent interest in the carbonate system in the oceans has resulted from the “greenhouse effect” of CO 2 . As discussed in Chapter 5, the concentration of CO 2 in the atmosphere
has increased in the twentieth century (see Figure 5.19). Since CO 2 can absorb infrared (IR)
energy, this increase may cause the temperature of the earth to increase and could eventually melt the polar ice caps. The increase in CO 2 is related to the burning of fossil fuels
(coal, petroleum, and natural gas) and the production of cement (see Figure 7.1). Once the
CO 2 is in the atmosphere, it is available for primary productivity and weathering processes.
The CO 2 can enter the ocean by physical processes called the solubility pump (Figure 7.1)
and biological processes called the biological pump (Figure 7.2). Once the CO 2 enters the
oceans across the air–sea interface and participates in the equilibrium processes outlined
by Equation 7.1 to Equation 7.4, it also can be used by plants in primary productivity:
CO 2 + H 2 O → CH 2 O + O 2
(7.5)
These processes are far from simple since the rates of movement of CO 2 across the interface and from surface to deep waters vary with latitude, time, season, and biological processes. Diurnal and seasonal variations in the carbonate system are caused by the removal
of CO 2 by photosynthesis and solar heating (see Figures 5.21 and 5.22). The natural and
anthropogenic inputs of CO 2 also differ as a function of latitude (Figure 7.3). The uptake of
CO 2 of the oceans is slow because of physical and chemical factors. The exchange involves
the hydration of CO 2 , which is a slow process relative to ionization. As discussed further
7
The Carbonate System
7.1 Introduction
The major portion of carbon in the oceans occurs as part of the carbonate system. This
system involves the following equilibria:
CO 2 (g) = CO 2 (aq)
(7.1)
CO 2 (aq) + H 2 O = H + + HCO 3
–
(7.2)
HCO 3
– = H+ + CO 3
2–
(7.3)
Ca 2+ + CO 3
2– = CaCO 3 (s)
(7.4)
The carbonate system is very important since it regulates the pH of seawater and controls the circulation of CO 2 between the biosphere, the lithosphere, the atmosphere, and the
oceans. Recent interest in the carbonate system in the oceans has resulted from the “greenhouse effect” of CO 2 . As discussed in Chapter 5, the concentration of CO 2 in the atmosphere
has increased in the twentieth century (see Figure 5.19). Since CO 2 can absorb infrared (IR)
energy, this increase may cause the temperature of the earth to increase and could eventually melt the polar ice caps. The increase in CO 2 is related to the burning of fossil fuels
(coal, petroleum, and natural gas) and the production of cement (see Figure 7.1). Once the
CO 2 is in the atmosphere, it is available for primary productivity and weathering processes.
The CO 2 can enter the ocean by physical processes called the solubility pump (Figure 7.1)
and biological processes called the biological pump (Figure 7.2). Once the CO 2 enters the
oceans across the air–sea interface and participates in the equilibrium processes outlined
by Equation 7.1 to Equation 7.4, it also can be used by plants in primary productivity:
CO 2 + H 2 O → CH 2 O + O 2
(7.5)
These processes are far from simple since the rates of movement of CO 2 across the interface and from surface to deep waters vary with latitude, time, season, and biological processes. Diurnal and seasonal variations in the carbonate system are caused by the removal
of CO 2 by photosynthesis and solar heating (see Figures 5.21 and 5.22). The natural and
anthropogenic inputs of CO 2 also differ as a function of latitude (Figure 7.3). The uptake of
CO 2 of the oceans is slow because of physical and chemical factors. The exchange involves
the hydration of CO 2 , which is a slow process relative to ionization. As discussed further