Geoscience Reference
In-Depth Information
An increase in pCO
2
in the atmosphere causes a rapid increase in
the dissolution of CO
2
into the ocean, which causes an increase in
the concentration of H
+
ions in seawater as shown in equation [4.1].
Because the pH is inversely proportional to the concentration of H
+
ions, the current increase in atmospheric CO
2
causes a decrease in pH
or, in other words, ocean acidification. The sequestration of
anthropogenic carbon by the ocean, described in the previous
paragraph, is therefore not “free”: the price to pay is the acidification
of seawater. For more information, refer to the special edition of the
journal
Oceanography
dedicated to
Ocean Acidification
[AUT 09].
Note that seawater is not currently acidic (acidity is defined as
pH < 7.0), since its pH, which is slowly but progressively decreasing,
is still usually >8.0.
4.3.2.2.
Long inorganic carbon cycle
The long inorganic carbon cycle begins with CO
2
dissolving into
continental waters [
3
], where it forms H
2
CO
3
(see the short cycle,
above), which reacts with continental rocks (including limestone,
CaCO
3
) thereby producing calcium ions and bicarbonate:
CaCO
3
+ H
2
CO
3
→ Ca
2+
+ 2 HCO
3
-
[4.3]
In the sea, ions transported by rivers [
4
] and those already in the
ocean may be precipitated by calcification, which is essentially
biological in the current ocean:
Ca
2+
+ 2 HCO
3
-
→ CaCO
3
+ CO
2
+ H
2
O
[4.4]
This equation summarizes several chemical and biological
phenomena. Indeed, at the final stage of CaCO
3
precipitation,
calcifying organisms combine Ca
2+
and CO
3
2-
. However, since CO
3
2-
is not very abundant in seawater and its transport across biological
membranes is not easy, it is often HCO
3
-
that is taken from seawater
and transported in the organisms (left part of equation [4.4]), where it
dissociates into CO
3
2-
and H
+
. The latter H
+
ion chemically combines
with an HCO
3
-
ion in seawater (i.e. one of two HCO
3
-
ions in the left
part of equation [4.4]), which generates CO
2
and H
2
O (in the right part
of equation [4.4]).
