Geoscience Reference
In-Depth Information
with CaCO
3
debris. The combination of equations [4.1], [4.4], [4.6]
and [4.7] can be summarized as follows:
CaSiO
3
+ CO
2
→ SiO
2
+ CaCO
3
[4.8]
When marine sediments and rocks on the seabed are transported to
continents by movements of tectonic plates, the high temperature and
pressure in the Earth's mantle combine SiO
2
and CaCO
3
into
metamorphic rocks:
SiO
2
+ CaCO
3
→ CaSiO
3
+ CO
2
[4.9]
With equation [4.9] being the opposite of equation [4.8], the
biogeochemical carbonate-silicate cycle is closed, which occurs after a
very long period of time (i.e. millions of years). On this time scale, the
release of CO
2
by volcanos maintains the concentration of this gas in
the atmosphere. The biogeochemical carbonate-silicate cycle continues
by the progressive uplifting of silicate rocks to the land surface, where
they are subjected to weathering by H
2
CO
3
(equation [4.6]).
Due to temporal irregularities in tectonic activity on Earth, which
includes the formation and fragmentation of supercontinents,
equations [4.8] and [4.9] are never in perfect equilibrium. On the time
scale of millions of years, the amount of CO
2
in the atmosphere and
the climate are governed by the carbonate-silicate biogeochemical
cycle, whereas on the time scale of Milankovitch paleoclimatic cycles,
they are governed by the biogeochemical carbonate cycle.
Long-term increases in atmospheric temperature are usually
accompanied by increases in precipitation. Higher precipitations cause
long-term increases in the weathering of rocks, including those that
contain silicate minerals (e.g. wollastonite in equation [4.6]). The
weathering of silicate rocks causes a net removal of atmospheric CO
2
(equation [4.8]), which leads to a decrease in temperature, which is an
example of a global-scale negative feedback.
A major industrial activity in modern societies is the calcination of
CaCO
3
[
11
] to obtain quick lime (CaO) for the production of cement:
CaCO
3
→ CaO + CO
2
[4.10]
