Geoscience Reference
In-Depth Information
of carbonates in surface waters in 1766, 2007 and 2100 is shown in
Table 5.1.
The three main forms of calcium carbonate (CaCO
3
) in seawater
are aragonite, calcite and magnesian calcite. Aragonite is around 1.5
times more soluble than calcite, whereas the solubility of magnesian
calcite is less than that of calcite when the magnesium content is weak
(less than 4%), however, it is higher when the magnesium content is
above 12% [DIC 10]. The equilibrium of calcium is the following:
CaCO
Ca
2
+
(
aq
)
+
CO
2
3
−
(
aq
)
⇋
[5.1]
3
The equilibrium constant is defined by the solubility product of
calcite or aragonite:
*
2
+
2
3
−
K
=
[Ca
]
×
[CO
]
[5.2]
sp
sat
sat
The saturation of CaCO
3
is defined by the ratio of the ionic product
measured with the ionic product expected for a solution that is in
equilibrium with calcite or aragonite:
2
+
2
3
−
[Ca
]
×
[CO
]
Ω
=
[5.3]
K
*
sp
Seawater is in equilibrium with the considered mineral when Ω =
1, over-saturated when Ω > 1 (favors the precipitation of CaCO
3
) and
under-saturated when Ω < 1 (favors the dissolution of CaCO
3
).
CO
2
becomes a weak acid (carbonic acid) when it dissolves in
seawater. It increases the concentration of
HCO
−
and C
T
and reduces
3
3
−
and the saturation of calcium
CO
the pH, the concentration of
carbonate:
2
−
−
CO
+
CO
+
H O
→
2HCO
[5.4]
2
3
2
3
Many processes, for example, enzymatic activity [MAD 88], depend
on the pH. The two biological processes, which have been the most
