Nonskeletal Carbonates 85
3.7 Nonskeletal Carbonates
3.7.1 Carbonate Saturation. In the present ocean, carbonate precipitation is either
within organisms (shells, skeletons) or is associated with their metabolic activity
(algal crusts). This need not always have been so: a certain proportion of the limestones and associated carbonates in the geologic record may have been precipitated
inorganically.
Where would one look for such inorganic precipitation today? To guide us in the
search for likely places, we need to consider briefly the chemistry of carbonate
precipitation and dissolution.
Seawater which spontaneously precipitates a mineral - for example, aragonite,
CaC03 - is said to be supersaturated with this mineral phase. Seawater which dissolves the mineral is undersaturated. When precipitation just equals dissolution,
saturation obtains, that is, the solution is in equilibrium with the solid. The degree of
saturation is expressed as the ratio of the ionic product of reactants present to the
product necessary for saturation (brackets indicate concentrations):
Dsat = [Ca 2 +] [C03 2 -]observed/[Ca 2 +] [COl-]equilibrium.
(3.2)
Clearly, Dsat equals 1 for saturation, less than 1 for undersaturation, and is greater
than 1 for supersaturation.
Whenever Dsat ist greater than 1, we expect spontaneous precipitation. However,
even though Dsat is indeed greater than 1 in the surface waters of tropical oceans,
inorganic precipitation is negligible. It is commonly assumed that the presence of
magnesium interferes with the expected reaction. Thus we need to find an unusually
high Dsat, if we are to see inorganic precipitation. Additionally, the presence of
appropriate crystal nuclei should be favorable.
Hifh temperature and a low C02 content in the water increase the product [Ca 2 +]
[C03 -] by increasing the concentration of carbonate ion. The effect of adding or
subtracting C02 is readily seen from the following equations:
C02 + H20 = H+ + HC03- ,
HC03- = H+ + C03= .
(3.3)
(3.4)
Almost all the inorganic carbon in the ocean is in the form of bicarbonate, HC03-.
Removal of C02 drives the reaction of Eq. (3.3) to the left, subtracting hydrogen
ions. In tum, this drives the reaction of Eq. (3.4) to the right, opposing the change in
hydrogen ions. Increasing the temperature lowers the solubility of C02. Also, C02 is
taken up by algae during photosynthesis - hence the precipitation of CaC03 as crusts
on many shallow-water tropical algae.
The following equation summarizes the process surrounding carbonate precipitation and dissolution in a simplified manner:
CaC03 + H20 + C02 ~ Ca 2 + + 2 HC03 .
(3.5)
Dissolution proceeds from left to right, precipitation from right to left. Note that C02
is used up during dissolution of carbonate (to help make bicarbonate), and is released
3.7 Nonskeletal Carbonates
3.7.1 Carbonate Saturation. In the present ocean, carbonate precipitation is either
within organisms (shells, skeletons) or is associated with their metabolic activity
(algal crusts). This need not always have been so: a certain proportion of the limestones and associated carbonates in the geologic record may have been precipitated
inorganically.
Where would one look for such inorganic precipitation today? To guide us in the
search for likely places, we need to consider briefly the chemistry of carbonate
precipitation and dissolution.
Seawater which spontaneously precipitates a mineral - for example, aragonite,
CaC03 - is said to be supersaturated with this mineral phase. Seawater which dissolves the mineral is undersaturated. When precipitation just equals dissolution,
saturation obtains, that is, the solution is in equilibrium with the solid. The degree of
saturation is expressed as the ratio of the ionic product of reactants present to the
product necessary for saturation (brackets indicate concentrations):
Dsat = [Ca 2 +] [C03 2 -]observed/[Ca 2 +] [COl-]equilibrium.
(3.2)
Clearly, Dsat equals 1 for saturation, less than 1 for undersaturation, and is greater
than 1 for supersaturation.
Whenever Dsat ist greater than 1, we expect spontaneous precipitation. However,
even though Dsat is indeed greater than 1 in the surface waters of tropical oceans,
inorganic precipitation is negligible. It is commonly assumed that the presence of
magnesium interferes with the expected reaction. Thus we need to find an unusually
high Dsat, if we are to see inorganic precipitation. Additionally, the presence of
appropriate crystal nuclei should be favorable.
Hifh temperature and a low C02 content in the water increase the product [Ca 2 +]
[C03 -] by increasing the concentration of carbonate ion. The effect of adding or
subtracting C02 is readily seen from the following equations:
C02 + H20 = H+ + HC03- ,
HC03- = H+ + C03= .
(3.3)
(3.4)
Almost all the inorganic carbon in the ocean is in the form of bicarbonate, HC03-.
Removal of C02 drives the reaction of Eq. (3.3) to the left, subtracting hydrogen
ions. In tum, this drives the reaction of Eq. (3.4) to the right, opposing the change in
hydrogen ions. Increasing the temperature lowers the solubility of C02. Also, C02 is
taken up by algae during photosynthesis - hence the precipitation of CaC03 as crusts
on many shallow-water tropical algae.
The following equation summarizes the process surrounding carbonate precipitation and dissolution in a simplified manner:
CaC03 + H20 + C02 ~ Ca 2 + + 2 HC03 .
(3.5)
Dissolution proceeds from left to right, precipitation from right to left. Note that C02
is used up during dissolution of carbonate (to help make bicarbonate), and is released