however, thought to be double the long-term values due to the effects of abnor-
mally high postglacial suspended-solid input rates (see introductory text to
Section 6.4). The modern values are halved in Table 6.2 to account for this.6.4.4 Calcium carbonate formation
It is surprisingly difficult to calculate whether seawater is supersaturated or under-
saturated with respect to CaCO 3. There are a number of different approaches to
the problem all based on equilibrium relationships that describe the precipitation
(forward reaction) or dissolution (back reaction) of CaCO 3.
eqn. 6.4
We have already noted the importance of this reaction in buffering the pH of
continental waters (see eqn. 5.12), and it behaves in exactly the same way in sea-
water. The Le Chatelier Principle (see Box 3.2) predicts that any process that
decreases the concentration of HCO 3 - in equation 6.4, will encourage dissolution
of CaCO 3 to restore the amount of HCO 3 - lost (Fig. 6.8). The oceans contain an
effectively infinite amount of CaCO 3 particles suspended in surface waters and
in bed sediments (see below). This CaCO 3 helps maintain (buffer) the pH of the
oceans at values between 7.9 and 8.1, in much the same way as pH in continen-
tal waters is buffered by reacting with limestone (see Section 5.3.1 & Fig. 5.5).
If, for example, more atmospheric CO 2 is taken up by the oceans as a result of
global warming (see Sections 7.2.2 & 7.2.4), the acidity added to the oceans is
largely neutralized by dissolution of CaCO 3 , such that the HCO 3 - ion concen-
tration is unchanged (Fig. 6.8), maintaining the pH around 8.
Seawater is a concentrated and complex solution in which the ions are close
together, compared with those in a more dilute solution. Electrostatic interaction
occurs between closely neighbouring ions and this renders some of these ions
‘inactive’. We are interested in the available or ‘active’ ions and we correct for
this effect, using activity coefficients, denoted g(see Section 2.6).
Activity coefficients are notoriously difficult to measure in complex solutions
like seawater, but are thought to be around 0.26 for Ca^2 +and around 0.20 for car-Ca^2 ()aq+ ++ 2 HCO 3322 - ()aq ªCaCO()aq CO()g+H O() 1196 Chapter Six
Table 6.3Additions to the river flux from ion exchange between river-borne clay and
seawater. From Drever et al. (1988).
Laboratory studies Amazon Average* Percentage of
(10^12 mol yr-^1 ) (10^12 mol yr-^1 ) (10^12 mol yr-^1 ) river flux†
Na+ -1.58 -1.47 -1.53±0.06 26
K+ -0.12 -0.27 -0.20±0.08 17
Mg^2 + -0.14 -0.49 -0.31±0.08 7
Ca^2 + 0.86 1.05 0.96±0.10 8
* The averages are based on modern suspended sediment input to the oceans, which is probably double the
long-term input rate (see Section 4.4). These values are thus halved when used in Table 6.2.
†Corrected for sea-salt and pollution inputs.
Minus sign indicates removal from seawater.