An Introduction to Environmental Chemistry

bonate (CO 32 - ). Measured Ca^2 +and CO 32 - ocean surface concentrations are 0.01

and 0.000 29 mol l-^1 respectively and thus the ion activity product (IAP) (see Box

4.12) can be calculated:

eqn. 6.5

This value is much greater than the solubility product (see Box 4.12) of calcite

(CaCO 3 ), which is 3.3¥ 10 -^9 mol^21 -^2 at 25°C and 1 atmosphere pressure, a suit-

able choice for tropical surface seawater. The degree of saturation (see Box 4.12)

is:

eqn. 6.6

An Wvalue of 1 indicates saturation, values greater than 1 indicate supersatura-

tion, and values less than 1 indicate undersaturation. Using the values for calcite

above we get:

eqn. 6.7

a value that implies surface seawater in the tropics is highly supersaturated with

respect to calcite. This approach neglects the effects of ion pairing (Box 6.4).

W=

¥

¥

=

• 
  


• 
  

15 10

33 10

45 4

7

9

.

.

.

Degree

IAP

sp

of saturation==W

K

IAP Ca CO Ca CO

mol l

=¥ =◊¥◊

=¥ ¥ ¥

=¥

+-+ -

--

aa c c^232232

722

0 01 0 26 0 00029 0 2

15 10

gg

....

.

The Oceans 197

(a) (b)

• 5


• 4.5


• 4


• 3.5


• 3


• 2.5


• 2


• 1.5


• 1


• 0.5

0

pH

Log [concentration (mol kg

–1

)] H+ OH–

Surface seawater pH

0 2 4 6 8 10 12 14

CO 2 HCO 3 – CO 3 2–

Sediment

Corals

CaCO 3

CO2(g)

CaCO 3

Phytoplankton

OH– + HCO 3

Ca2+

2– –

• 
  

+

CO 3 + H 2 O

H 2 O

Sample

seawater

@ 25°C

S = 35

CO2(aq) + H 2 OHH 2 CO 3 + + HCO 3

Fig. 6.8(a) Schematic diagram to illustrate the buffering effect of CaCO 3 particles (suspended in the water

column) and bottom sediments on surface seawater HCO 3 - concentrations (after Baird 1995, with permission

from W.H. Freeman and Company.). (b) A sample of the seawater in (a) will have a pH very close to 8 because

of the relative proportions of CO 2 , HCO 3 - and CO 32 - , which in seawater is dominated by the HCO 3 - species.

Increased CO 2 concentrations in the atmosphere from anthropogenic sources could induce greater dissolution

of CaCO 3 sediments including coral reefs.