158
6.2.2 pCO 2 and Carbonate Chemistry in Water
The pCO 2water is influenced by several factors. First, even if the CO 2 concentration in
water is constant, pCO 2 changes when the CO 2 solubility changes (Eq. 6.2). The
solubility of a gas in water is the amount of the gas that can be dissolved in water
under specified conditions, and it tends to decrease as the water temperature rises.
This behavior reflects the fact that the dissolved gas molecules have higher kinetic
energy (pressure) in warmer water, and a higher percentage of the molecules is able
to escape from the water. This tendency applies to CO 2 . The solubility therefore
decreases and pCO 2 increases as the temperature increases (Fig. 6.3). The kinetic
energy of atmospheric CO 2 also rises as the temperature rises, but the effect is not
as great as the temperature effect on the kinetic energy of CO 2 molecules in water
because CO 2 molecules in air already have a much higher kinetic energy than CO 2
molecules dissolved in water.
Changes of salinity also affect CO 2 solubility, but the influence of salinity is
smaller than the effect of temperature in normal seawater. However salinity indirectly influences pCO 2 via changes of carbonate system equilibria; that effect (see
below) is much stronger than the direct effect of salinity on solubility. This indirect
salinity effect complicates carbonate chemistry in shallow coastal waters.
It is important to note here that HCO 3
−
and CO 3
2−
are present in water in addition
to dissolved CO 2 , and these ions partially compensate for changes of pCO 2water .
There are two parameters related to the compensation process, dissolved inorganic
carbon (DIC) and total alkalinity (TA). DIC is the sum of all inorganic carbon species dissolved in water: dissolved CO 2 , HCO 3
−
, and CO 3
2−
. In the case of seawater,
HCO 3
−
accounts for more than 90% of DIC at typical seawater pH values. TA is
determined by mineral components such as calcium ion (Ca
2+
) and magnesium ion
(Mg
2+
) in water. Dickson et al. (2007) give an accurate and precise definition of
TA. TA gives a rough indication of the potential amount of dissolved CO 2 that can
dissociate into bicarbonate or carbonate ions. To be more specific, TA indicates the
excess positive electrical charge contributed by the conservative cations (e.g., Ca
2+
,
Mg
2+
, K
+
, and Na
+
) versus the conservative anions (e.g., Cl
−
and SO 4
2−
). When the
excess positive charge is large, a greater percentage of DIC is in the form of negative
bicarbonate and carbonate ions so that positive and negative charges are in balance.
As a result, the pCO 2 in high-TA water tends to decrease (Fig. 6.3). Furthermore, the
buffer capacity against changes of pCO 2 is greater in high-TA water. The pCO 2
affects the air–water CO 2 flux, but carbonate and bicarbonate ions do not because
they are electrically isolated in water. Water therefore tends to be an atmospheric
sink for CO 2 if the TA is high, even if the DIC is also high.
Biological processes such as photosynthesis, respiration, and remineralization
also change pCO 2water by changing the distribution of carbonate species in the water.
Photosynthesis and respiration/remineralization basically change only DIC, but calcification (i.e., formation of the calcium carbonate that makes up the hard parts of
organisms such as corals, foraminifera, pteropods, and coccolithophores) changes
T. Tokoro et al.
6.2.2 pCO 2 and Carbonate Chemistry in Water
The pCO 2water is influenced by several factors. First, even if the CO 2 concentration in
water is constant, pCO 2 changes when the CO 2 solubility changes (Eq. 6.2). The
solubility of a gas in water is the amount of the gas that can be dissolved in water
under specified conditions, and it tends to decrease as the water temperature rises.
This behavior reflects the fact that the dissolved gas molecules have higher kinetic
energy (pressure) in warmer water, and a higher percentage of the molecules is able
to escape from the water. This tendency applies to CO 2 . The solubility therefore
decreases and pCO 2 increases as the temperature increases (Fig. 6.3). The kinetic
energy of atmospheric CO 2 also rises as the temperature rises, but the effect is not
as great as the temperature effect on the kinetic energy of CO 2 molecules in water
because CO 2 molecules in air already have a much higher kinetic energy than CO 2
molecules dissolved in water.
Changes of salinity also affect CO 2 solubility, but the influence of salinity is
smaller than the effect of temperature in normal seawater. However salinity indirectly influences pCO 2 via changes of carbonate system equilibria; that effect (see
below) is much stronger than the direct effect of salinity on solubility. This indirect
salinity effect complicates carbonate chemistry in shallow coastal waters.
It is important to note here that HCO 3
−
and CO 3
2−
are present in water in addition
to dissolved CO 2 , and these ions partially compensate for changes of pCO 2water .
There are two parameters related to the compensation process, dissolved inorganic
carbon (DIC) and total alkalinity (TA). DIC is the sum of all inorganic carbon species dissolved in water: dissolved CO 2 , HCO 3
−
, and CO 3
2−
. In the case of seawater,
HCO 3
−
accounts for more than 90% of DIC at typical seawater pH values. TA is
determined by mineral components such as calcium ion (Ca
2+
) and magnesium ion
(Mg
2+
) in water. Dickson et al. (2007) give an accurate and precise definition of
TA. TA gives a rough indication of the potential amount of dissolved CO 2 that can
dissociate into bicarbonate or carbonate ions. To be more specific, TA indicates the
excess positive electrical charge contributed by the conservative cations (e.g., Ca
2+
,
Mg
2+
, K
+
, and Na
+
) versus the conservative anions (e.g., Cl
−
and SO 4
2−
). When the
excess positive charge is large, a greater percentage of DIC is in the form of negative
bicarbonate and carbonate ions so that positive and negative charges are in balance.
As a result, the pCO 2 in high-TA water tends to decrease (Fig. 6.3). Furthermore, the
buffer capacity against changes of pCO 2 is greater in high-TA water. The pCO 2
affects the air–water CO 2 flux, but carbonate and bicarbonate ions do not because
they are electrically isolated in water. Water therefore tends to be an atmospheric
sink for CO 2 if the TA is high, even if the DIC is also high.
Biological processes such as photosynthesis, respiration, and remineralization
also change pCO 2water by changing the distribution of carbonate species in the water.
Photosynthesis and respiration/remineralization basically change only DIC, but calcification (i.e., formation of the calcium carbonate that makes up the hard parts of
organisms such as corals, foraminifera, pteropods, and coccolithophores) changes
T. Tokoro et al.