deep-sea sediments, the clear-cut relationship between calcium carbonate content and water depth
indicates that carbonate dissolution plays the major
role in governing carbonate distribution patterns. To
a first approximation, the dissolution of carbonate
on the seafloor is a function of the corrosiveness or
saturation state of the overlying bottom waters.
The amount of calcium carbonate that will dissolve in sea water if thermodynamic equilibrium is
reached is governed by the following reaction:
CaCO 3 s
ð Þ2Ca
2þ aq
ð Þ þ CO 3
2À aq
ð Þ
At equilibrium, the rate of carbonate dissolution is
equal to the rate of its precipitation and the sea water
is said to be saturated with respect to the carbonate
phase. In the deep sea, the degree of calcium carbonate saturation (D) can be expressed as:
D ¼
Ca
2þ
Â
Ã
seawater
 CO 3
2À
Â
Ã
seawater
Ca
2þ
Â
Ã
saturation
 CO 3
2À
½
saturation
where [Ca
2þ ] seawater and [CO 3
2À ] seawater are the in
situ concentrations in the water mass of interest and
[Ca
2þ ] saturation and [CO 3
2À ] saturation are the concentrations of these ions at equilibrium, or saturation, at
the same conditions of pressure and temperature.
Since shell formation and dissolution cause the concentration of [Ca
2þ ] to vary by less than 1% in the
ocean, the degree of calcium carbonate saturation
(D) can be simplified and expressed in terms of the
concentration of the carbonate ions only:
D ¼
CO 3
2À
Â
Ã
seawater
CO 3
2À
½
saturation
D is thus a measure of the degree to which a seawater
sample is saturated with respect to calcite or aragonite, and so provides a measure of the strength of the
driving force for dissolution. Values of D41 indicate
oversaturation while values of Do1 indicate undersaturation and a tendency for calcium carbonate to
dissolve. Since the saturation carbonate ion concentration increases with increasing pressure and decreasing temperature, calcium carbonate is more
soluble in the deep sea than at the surface. At the
depth in the water column where D ¼ 1, the transition from oversaturated to undersaturated conditions
is reached. This depth is known as the saturation
horizon (Figure 3). Aragonite is always more soluble
than calcite, and its respective saturation horizon is
shallower, because the saturation carbonate ion
concentration for aragonite is always higher for the
same conditions of pressure and temperature.
Observations from studies of surface sediments
have allowed definition of regionally varying levels in
the ocean at which pronounced changes in the presence or preservation of calcium carbonate result from
the depth-dependent increase of dissolution on the
seafloor. The first such level to be identified was simply the depth boundary in the ocean separating carbonate-rich sediments above from carbonate-free
sediments below. This level is termed the calcite (or
carbonate) compensation depth (CCD) and represents
the depth at which the rate of carbonate dissolution
on the seafloor exactly balances the rate of carbonate
supply from the overlying surface waters. Because the
supply and dissolution rates of carbonate differ from
place to place in the ocean, the depth of the CCD is
variable. In the Pacific, the CCD is typically found at
depths between about 3500 and 4500 m. In the North
Atlantic and parts of the South Atlantic, it is found
60
20
80
60
20
60
20
20
60
20
60
20
80
20
80
40
20
Figure 2 Global distribution of calcium carbonate (weight-% CaCO 3 ) in surface sediments of the ocean. Data compilation from
Archer (1996); reproduced with permission from the American Geophysical Union.
338 CALCIUM CARBONATES
indicates that carbonate dissolution plays the major
role in governing carbonate distribution patterns. To
a first approximation, the dissolution of carbonate
on the seafloor is a function of the corrosiveness or
saturation state of the overlying bottom waters.
The amount of calcium carbonate that will dissolve in sea water if thermodynamic equilibrium is
reached is governed by the following reaction:
CaCO 3 s
ð Þ2Ca
2þ aq
ð Þ þ CO 3
2À aq
ð Þ
At equilibrium, the rate of carbonate dissolution is
equal to the rate of its precipitation and the sea water
is said to be saturated with respect to the carbonate
phase. In the deep sea, the degree of calcium carbonate saturation (D) can be expressed as:
D ¼
Ca
2þ
Â
Ã
seawater
 CO 3
2À
Â
Ã
seawater
Ca
2þ
Â
Ã
saturation
 CO 3
2À
½
saturation
where [Ca
2þ ] seawater and [CO 3
2À ] seawater are the in
situ concentrations in the water mass of interest and
[Ca
2þ ] saturation and [CO 3
2À ] saturation are the concentrations of these ions at equilibrium, or saturation, at
the same conditions of pressure and temperature.
Since shell formation and dissolution cause the concentration of [Ca
2þ ] to vary by less than 1% in the
ocean, the degree of calcium carbonate saturation
(D) can be simplified and expressed in terms of the
concentration of the carbonate ions only:
D ¼
CO 3
2À
Â
Ã
seawater
CO 3
2À
½
saturation
D is thus a measure of the degree to which a seawater
sample is saturated with respect to calcite or aragonite, and so provides a measure of the strength of the
driving force for dissolution. Values of D41 indicate
oversaturation while values of Do1 indicate undersaturation and a tendency for calcium carbonate to
dissolve. Since the saturation carbonate ion concentration increases with increasing pressure and decreasing temperature, calcium carbonate is more
soluble in the deep sea than at the surface. At the
depth in the water column where D ¼ 1, the transition from oversaturated to undersaturated conditions
is reached. This depth is known as the saturation
horizon (Figure 3). Aragonite is always more soluble
than calcite, and its respective saturation horizon is
shallower, because the saturation carbonate ion
concentration for aragonite is always higher for the
same conditions of pressure and temperature.
Observations from studies of surface sediments
have allowed definition of regionally varying levels in
the ocean at which pronounced changes in the presence or preservation of calcium carbonate result from
the depth-dependent increase of dissolution on the
seafloor. The first such level to be identified was simply the depth boundary in the ocean separating carbonate-rich sediments above from carbonate-free
sediments below. This level is termed the calcite (or
carbonate) compensation depth (CCD) and represents
the depth at which the rate of carbonate dissolution
on the seafloor exactly balances the rate of carbonate
supply from the overlying surface waters. Because the
supply and dissolution rates of carbonate differ from
place to place in the ocean, the depth of the CCD is
variable. In the Pacific, the CCD is typically found at
depths between about 3500 and 4500 m. In the North
Atlantic and parts of the South Atlantic, it is found
60
20
80
60
20
60
20
20
60
20
60
20
80
20
80
40
20
Figure 2 Global distribution of calcium carbonate (weight-% CaCO 3 ) in surface sediments of the ocean. Data compilation from
Archer (1996); reproduced with permission from the American Geophysical Union.
338 CALCIUM CARBONATES