Geoscience Reference
In-Depth Information
years, it was necessary that the life forms were able to extend their
area of distribution very greatly and to diversify, as they had done in
the oceans, on the continents. This could not occur without a
minimum greenhouse effect, of which the regulation was ensured, and
still is, by the ocean and its exchanges with the atmosphere on the one
hand and sediments on the other.
1.4.2.
The regulation of the greenhouse effect by the ocean
Firstly, it is useful to recall that today carbon is very unequally
distributed between the atmosphere, ocean and sediments. For one atom
of carbon present in the atmosphere (in the form of CO
2
), 65 are
present in the ocean (mainly in the dissolved forms HCO
3
-
, CO
3
2-
and
CO
2
) and 150,000 are present in the sediments (mainly in the form of
calcium carbonate and organic matter).
On the one hand, the level of CO
2
in the atmosphere results from
carbon dioxide exchanges, which tend to permanently establish
equilibrium between the partial pressure of CO
2
in the lower
atmosphere and the partial pressure of CO
2
dissolved in the surface
ocean. On the other hand, the ocean can exchange carbon with the
sediments through the precipitation of calcites (carbonates of calcium,
CaCO
3
) from dissolved CO
3
2-
or, conversely, through them being
dissolved.
The exchanges of inorganic carbon between the ocean and
atmosphere (exchanges of CO
2
between its dissolved form in the
surface ocean and its gas form in the atmosphere) and between the
ocean and the sediment (precipitation or dissolving of CaCO
3
) depend
largely on chemical equilibriums which tend to establish themselves
between the different forms of inorganic carbon dissolved in the
ocean. A unique and simplified expression of these equilibriums is as
follows:
2HCO
3
-
↔ CO
3
2-
+ CO
2
+ H
2
O
[1.4]
In the current conditions of weak acidity in the ocean (pH = 8.2),
these equilibriums lead to the significant presence of carbon in
the form of dissolved bicarbonate (HCO
3
-
) and minor proportions in
