Environmental Engineering Reference
In-Depth Information
theory is that the energy differences associated with the changes in orbit
are too small to explain the large climate changes [3.2]. Recently, evi-
dence has been mounting that this periodicity is connected to fl ow pat-
terns in the deep ocean instead [3.8]. How is the connection made
between CO
2
concentrations in the atmosphere and those in the ocean?
Carbon chemistry in the oceans
The atmosphere is in direct contact only with the ocean surface, so
atmospheric CO
2
exchanges only with the surface layer. We will see that
the CO
2
in the surface layer gets slowly transported into the deeper lay-
ers of the ocean.
The interaction of CO
2
with the surface layer can be described as a
set of chemical reactions in which dissolved CO
2
reacts with water to
form
bicarbonate
(HCO
3
−
), and
carbonate
(CO
3
2
−
):
CO
2
(g)
CO
2
(aq)
HCO
3
−
H
+
CO
2
(aq)
+
H
2
O
+
HCO
3
−
H
+
CO
3
2
−
2H
+
+
+
The dissolved CO
2
(aq), bicarbonate, and carbonate anions are col-
lectively called Dissolved Inorganic Carbon (DIC). We see from these
equations that the CO
2
levels in the atmosphere are coupled to the acid-
ity of the oceans: higher alkalinity (higher pH, more basic) causes the
chemical equilibria in the bottom two equations to shift to the right, thus
yielding higher carbonate (CO
3
2
−
) in the ocean. Because CO
2
(aq) is con-
sumed by these shifts, the fi rst equilibrium equation tells us that higher
alkalinity increases the ability of surface oceans to take up atmospheric
CO
2
. A more acidic ocean will have the opposite effect: a decrease in the
ability of surface waters to take up CO
2
. The solubility of CO
2
in water
also depends on temperature: the lower the temperature, the higher the
solubility. At a typical surface sea water pH of 8.2, the concentrations of
the various components [CO
2
], [HCO
3
−
], and [CO
3
2
−
] are 0.5%, 89% and
10.5%, respectively [3.9].
The chemical equilibria affecting carbon in the ocean are further
complicated by the many minerals containing carbon that can dissolve in
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