Geoscience Reference
In-Depth Information
The production of organic compounds (here, CaCO
3
) by biological
processes (here, calcification) is called biomineralization, which is a
very widespread phenomenon in living organisms (e.g. siliceous
frustules of diatoms, carbonate shells of mollusks, coral reefs and
bones and teeth of vertebrates). In the ocean, part of the carbonate
debris from organisms (PIC, see above) may dissolve in the water
column [
5
]. The process of CaCO
3
dissolution is the counterpart of
calcification:
CaCO
3
+ CO
2
+ H
2
O → Ca
2+
+ 2 HCO
3
-
[4.5]
The CaCO
3
that does not dissolve in the water column is deposited
on the seafloor [
6
], where carbonate sediments are slowly transformed
into sedimentary rocks [
7
]. Over millions of years, the movements of
tectonic plates (orogenesis: formation of mountains) carry part of
these rocks to the continents [
8
], where they may be altered by H
2
CO
3
(equation [4.3]). Tectonic movements also carry carbonate rocks to
volcanoes where they are recycled [
9
], which releases CO
2
into the
atmosphere [
10
]. This closes the carbonate cycle.
The biogeochemical carbonate cycle above can regulate the climate
in the medium term (time scale of approximately 100,000 years,
which corresponds to astronomic and paleoclimatic Milankovitch
cycles), but the CO
2
balance of this cycle is null in the longer term.
Indeed:
- the formation of one molecule of carbonic acid (H
2
CO
3
) on
continents removes one molecule of CO
2
from the atmosphere
(equation [4.1]);
- the weathering of CaCO
3
by this H
2
CO
3
molecule on land
produces two HCO
3
-
molecules (equation [4.3]);
- the precipitation of CaCO
3
in the ocean combines two molecules
of HCO
3
-
with one molecule of calcium (Ca
2+
), producing one
molecule of CaCO
3
, one molecule of water and one molecule of CO
2
(equation [4.4]), which is released to the atmosphere.
Thus, the molecule of CO
2
that is removed from the atmosphere
during the weathering of CaCO
3
is returned to the atmosphere during
