323
ingly, a state of supersaturation ensues equivalent
to a SI calcite value of 0.43 or an Ω calcite value of 2.7.
As a theoretical example (lower part of Table 9.4)
for the modeling in this case a constant state of
supersaturation according to a SI calcite value of 0.50
or an Ω calcite value of 3.2 was calculated.
1
In the calculated model summarized in Table
9.5, the equilibrium constants for calcite was
adjusted to temperature (2°C) and pressure (600
atm) of a water depth of about 6000 m. In this example as well, the pressure-corrected constant
derived from the model SOLMINEQ (Kharaka et
al. 1988) was entered into the database of the
model PHREEQC, and the correction procedure
for temperature of this model was used.
Calculation was run under the boundary
conditions of a system closed with regard to CO 2 ,
i.e. the concentrations of the previous example in
Table 9.4 were regarded as 'initial'. The partial
pressure pCO 2 is not a fixed boundary condition
of an open system, and it did not change as in
the example shown in Table 9.3, since no
reactions were determined. A comparable
decomposition of organic matter as in Table 9.3
was thus excluded from this example. After
pressure-correction, the saturation index of
calcite documents an undersaturation displaying
a value of –0.16 (equivalent to a Ω calcite value of
0.69). In the case of an additional decomposition
of organic matter, this undersaturation of calcite
is likely to increase further. Such a reaction could
be in fact applicable to deep-sea waters of
pelagic deep ocean areas.
M odel of low latitude seawater at 1000 m depth
input concentrations:
model c alculation of warm surfac e seawater
boundary conditions:
temperature
6 °C
log k c alc ite
-8.26
(at 6 °C and 100 atm pressure)
input situation without calcite-carbonate-equilibrium:
pH
8.08
sum of c arbonate spec ies (TIC)
2.26 mmol/l
sum of c alc ium spec ies
10.61 mmol/l
Sicalcite
0.26
(i.e. Ω c alc ite = 1.8)
reactions:
(CH2O)106(NH3)16(H3PO4) reac ts with dissolved O2
calc ite supersaturation c onstant at SI = 0.26
PHREEQC model results:
pH
8.01
sum of c arbonate spec ies (TIC)
2.60 mmol/l
sum of c alc ium spec ies
10.79 mmol/l
Sicalcite
0.26
(i.e. Ω c alc ite = 1.8)
Table 9.3 Model calculation applying the computer program PHREEQC (Parkhurst 1995) to a sample of ocean water in
a water depth of approximately 1000 m, near the equator. The constant of the solubility product for calcite is corrected
for temperature and pressure. The decomposition of organic substance due to the presence of free oxygen in the water
column has been included.
9.3
The Calcite-Carbonate-Equilibrium in Marine Aquatic Systems
1 Ω calcite is still in use in chemical oceanography for describing the state of saturation. It corresponds to the
saturation index without the logarithm, hence: log
Ω calcite = SI calcite . However, the SI value is more useful,
since the same amounts of undersaturation and supersaturation respectively display the same SI values,
only distinguished by different signs (+) for supersaturation and (-) for undersaturation. A SI value of zero
describes the state of saturation.
ingly, a state of supersaturation ensues equivalent
to a SI calcite value of 0.43 or an Ω calcite value of 2.7.
As a theoretical example (lower part of Table 9.4)
for the modeling in this case a constant state of
supersaturation according to a SI calcite value of 0.50
or an Ω calcite value of 3.2 was calculated.
1
In the calculated model summarized in Table
9.5, the equilibrium constants for calcite was
adjusted to temperature (2°C) and pressure (600
atm) of a water depth of about 6000 m. In this example as well, the pressure-corrected constant
derived from the model SOLMINEQ (Kharaka et
al. 1988) was entered into the database of the
model PHREEQC, and the correction procedure
for temperature of this model was used.
Calculation was run under the boundary
conditions of a system closed with regard to CO 2 ,
i.e. the concentrations of the previous example in
Table 9.4 were regarded as 'initial'. The partial
pressure pCO 2 is not a fixed boundary condition
of an open system, and it did not change as in
the example shown in Table 9.3, since no
reactions were determined. A comparable
decomposition of organic matter as in Table 9.3
was thus excluded from this example. After
pressure-correction, the saturation index of
calcite documents an undersaturation displaying
a value of –0.16 (equivalent to a Ω calcite value of
0.69). In the case of an additional decomposition
of organic matter, this undersaturation of calcite
is likely to increase further. Such a reaction could
be in fact applicable to deep-sea waters of
pelagic deep ocean areas.
M odel of low latitude seawater at 1000 m depth
input concentrations:
model c alculation of warm surfac e seawater
boundary conditions:
temperature
6 °C
log k c alc ite
-8.26
(at 6 °C and 100 atm pressure)
input situation without calcite-carbonate-equilibrium:
pH
8.08
sum of c arbonate spec ies (TIC)
2.26 mmol/l
sum of c alc ium spec ies
10.61 mmol/l
Sicalcite
0.26
(i.e. Ω c alc ite = 1.8)
reactions:
(CH2O)106(NH3)16(H3PO4) reac ts with dissolved O2
calc ite supersaturation c onstant at SI = 0.26
PHREEQC model results:
pH
8.01
sum of c arbonate spec ies (TIC)
2.60 mmol/l
sum of c alc ium spec ies
10.79 mmol/l
Sicalcite
0.26
(i.e. Ω c alc ite = 1.8)
Table 9.3 Model calculation applying the computer program PHREEQC (Parkhurst 1995) to a sample of ocean water in
a water depth of approximately 1000 m, near the equator. The constant of the solubility product for calcite is corrected
for temperature and pressure. The decomposition of organic substance due to the presence of free oxygen in the water
column has been included.
9.3
The Calcite-Carbonate-Equilibrium in Marine Aquatic Systems
1 Ω calcite is still in use in chemical oceanography for describing the state of saturation. It corresponds to the
saturation index without the logarithm, hence: log
Ω calcite = SI calcite . However, the SI value is more useful,
since the same amounts of undersaturation and supersaturation respectively display the same SI values,
only distinguished by different signs (+) for supersaturation and (-) for undersaturation. A SI value of zero
describes the state of saturation.