9
Marine Carbonates: Their Formation and Destruction
318
the fact, that for instance only part of the measured calcium is present in the form of Ca
2+
-ions,
whereas the rest forms complexes or ion pairs
(CaSO 4
0
aq
, CaHCO 3
+
aq
, or others). This is true for
seawater, since the activity coefficients of these
complexes or ion pairs with sufficient accuracy
exert a constant influence.
The other approach employs a geochemical
computer model, such as PHREEQC (Parkhurst
1995; also Chap. 15) with an input of a complete
seawater analysis. Such a model will then calculate the activity coefficients and the species distribution of the solution according to the complete
analysis and the constants of the thermodynamic
database used. These constants are well known
with an accuracy which is usually better than the
accuracy of most of our analyses at least for the
major aquatic species. Together with the 'real' constant of the solubility product a reliable saturation
index (SI = log Ω) is then calculated. The
constants of solubility products are not accurately known for some minerals, but for calcite,
and also for most other carbonates, these constants and their dependence on temperature and
pressure are very well documented.
Both approaches used in the analysis of
seawater lead to identical results, thus leaving it
undetermined which method should be preferred
for seawater analysis. However, for marine pore water the first approach is not valid, since quite
different concentrations of complexes or ion pairs
are likely to occur, for instance, sulfate reduction
and/or an increase of alkalinity. Therefore we rely
on the second approach in our examples and
strongly recommend it for the marine geochemistry.
9.3.1
Primary Reactions of the CalciteCarbonate-Equilibrium with
Atmospheric Contact in Infinitely
Diluted Solutions
Upon making contact with the atmosphere, the
partial pressure (pCO 2 ) of carbon dioxide (CO 2,g )
determines the concentration of carbonic acid
(H 2 CO
0
3,aq
) in solution. The equilibrium reaction is
written as:
CO 2,g + H 2 O ⇔ H 2 CO 3
0
aq
(9.1)
The equilibrium is determined by the equation:
K CO2 = [H 2 CO 3
0
aq ] / {pCO 2 · [H 2 O]} = 3.38E-2
(25 ° C, 1 atm)
(9.2)
In this form the reaction combines two
different steps: One leads to the formation of
CO 2,aq in aqueous solution, and a second leads
from CO 2,aq to the formation of H 2 CO 3
0
aq
. The
brackets [...] denote activities, whereas parentheses embrace molar concentrations. The
activity of water [H 2 O] is equal to 1.0 in infinitely diluted solutions, whereas it is slightly
less in seawater (0.981). The value of the constant K CO2 – as for all other constants in the
following – is determined at a temperature 25
°
C
and under a pressure of 1 atm. The temperature
dependency of the constants was empirically
investigated by Plummer et al. (1978) for the
range between 5
°
C and 60
°
C. The computer
program PHREEQC (Parkhurst 1995) also
accounts for this temperature dependence. The
pressure dependence, which is important in
great water depths, is considered only in the
model SOLMINEQ (Kharaka et al. 1988). The
constants belonging to this program, corrected
for specific water depths, can be extracted and
explicitly entered into the database of the
generally more versatile model PHREEQC.
The aqueous complex H 2 CO 3
0
aq
dissociates into
protons and bicarbonate ions:
H 2 CO 3
0
aq ⇔ H + + HCO 3
-
(9.3)
This equilibrium is described by the equation:
K H2CO3 = {[H + ] · [HCO 3
- ]} / [H 2 CO 3
0
aq ] = 4.48E-7
(25 ° C, 1 atm)
(9.4)
The second step of dissociation is determined by:
HCO 3
- ⇔ H + + CO 3
2(9.5)
This equilibrium is described by the equation:
K HCO3 = {[H + ] · [CO 3
2- ]} / [HCO 3
- ] = 4.68E-11
(25 ° C, 1 atm)
(9.6)
Furthermore, the dissociation of water needs to be
included:
H 2 O ⇔ H + + OH -
(9.7)
with the constant K H20 :
K H2O = [H + ] · [OH - ] = 1.00E-14
(25 ° C, 1 atm)
(9.8)
Marine Carbonates: Their Formation and Destruction
318
the fact, that for instance only part of the measured calcium is present in the form of Ca
2+
-ions,
whereas the rest forms complexes or ion pairs
(CaSO 4
0
aq
, CaHCO 3
+
aq
, or others). This is true for
seawater, since the activity coefficients of these
complexes or ion pairs with sufficient accuracy
exert a constant influence.
The other approach employs a geochemical
computer model, such as PHREEQC (Parkhurst
1995; also Chap. 15) with an input of a complete
seawater analysis. Such a model will then calculate the activity coefficients and the species distribution of the solution according to the complete
analysis and the constants of the thermodynamic
database used. These constants are well known
with an accuracy which is usually better than the
accuracy of most of our analyses at least for the
major aquatic species. Together with the 'real' constant of the solubility product a reliable saturation
index (SI = log Ω) is then calculated. The
constants of solubility products are not accurately known for some minerals, but for calcite,
and also for most other carbonates, these constants and their dependence on temperature and
pressure are very well documented.
Both approaches used in the analysis of
seawater lead to identical results, thus leaving it
undetermined which method should be preferred
for seawater analysis. However, for marine pore water the first approach is not valid, since quite
different concentrations of complexes or ion pairs
are likely to occur, for instance, sulfate reduction
and/or an increase of alkalinity. Therefore we rely
on the second approach in our examples and
strongly recommend it for the marine geochemistry.
9.3.1
Primary Reactions of the CalciteCarbonate-Equilibrium with
Atmospheric Contact in Infinitely
Diluted Solutions
Upon making contact with the atmosphere, the
partial pressure (pCO 2 ) of carbon dioxide (CO 2,g )
determines the concentration of carbonic acid
(H 2 CO
0
3,aq
) in solution. The equilibrium reaction is
written as:
CO 2,g + H 2 O ⇔ H 2 CO 3
0
aq
(9.1)
The equilibrium is determined by the equation:
K CO2 = [H 2 CO 3
0
aq ] / {pCO 2 · [H 2 O]} = 3.38E-2
(25 ° C, 1 atm)
(9.2)
In this form the reaction combines two
different steps: One leads to the formation of
CO 2,aq in aqueous solution, and a second leads
from CO 2,aq to the formation of H 2 CO 3
0
aq
. The
brackets [...] denote activities, whereas parentheses embrace molar concentrations. The
activity of water [H 2 O] is equal to 1.0 in infinitely diluted solutions, whereas it is slightly
less in seawater (0.981). The value of the constant K CO2 – as for all other constants in the
following – is determined at a temperature 25
°
C
and under a pressure of 1 atm. The temperature
dependency of the constants was empirically
investigated by Plummer et al. (1978) for the
range between 5
°
C and 60
°
C. The computer
program PHREEQC (Parkhurst 1995) also
accounts for this temperature dependence. The
pressure dependence, which is important in
great water depths, is considered only in the
model SOLMINEQ (Kharaka et al. 1988). The
constants belonging to this program, corrected
for specific water depths, can be extracted and
explicitly entered into the database of the
generally more versatile model PHREEQC.
The aqueous complex H 2 CO 3
0
aq
dissociates into
protons and bicarbonate ions:
H 2 CO 3
0
aq ⇔ H + + HCO 3
-
(9.3)
This equilibrium is described by the equation:
K H2CO3 = {[H + ] · [HCO 3
- ]} / [H 2 CO 3
0
aq ] = 4.48E-7
(25 ° C, 1 atm)
(9.4)
The second step of dissociation is determined by:
HCO 3
- ⇔ H + + CO 3
2(9.5)
This equilibrium is described by the equation:
K HCO3 = {[H + ] · [CO 3
2- ]} / [HCO 3
- ] = 4.68E-11
(25 ° C, 1 atm)
(9.6)
Furthermore, the dissociation of water needs to be
included:
H 2 O ⇔ H + + OH -
(9.7)
with the constant K H20 :
K H2O = [H + ] · [OH - ] = 1.00E-14
(25 ° C, 1 atm)
(9.8)