Geoscience Reference
In-Depth Information
requirement has led to misunderstandings in the literature. For
example, the future concentration of CO
2
in the atmosphere could reach
a value close to that of the Cretaceous period (145-65 Myr ago), about
2,000 ppmv. However, the Cretaceous period is a geological period
during which calcification was very intense, as proven by the
limestone cliffs of the Pas-de-Calais and Dover, which were formed at
that time. It would, however, be a serious mistake to conclude that
calcification will not be affected in a high-CO
2
future because the
carbonate system will be very different from that of the Cretaceous
period. Seawater with a concentration of dissolved inorganic carbon
(C
T
) of 2,400
10
-6
mol kg
-1
in equilibrium with an atmosphere with a
concentration of CO
2
of 2,000 ppmv exhibits a calcite saturation state
of 1.1 (at a temperature of 15°C and a salinity of 35). However, the C
T
of surface waters in the Cretaceous period were higher (4,900
×
10
-6
mol kg
-1
), and the calcite saturation state was 4.5 (at the same
temperature and salinity). This example, taken from Zeebe and
Ridgwell [ZEE 11], shows that despite their high CO
2
content, the
surface waters of the Cretaceous period were highly favorable to
calcification due to their high saturation in CaCO
3
, which will not be
the case in the future ocean.
×
The main reason for which the pH and the saturation of CaCO
3
are
coupled today and in the near future, as opposed to the Cretaceous
period where they were decoupled, is due to CaCO
3
compensation.
The depth of the limit between waters that are saturated and under-
saturated in CaCO
3
(saturation horizon (SH)) is subject to large
vertical changes such that the supply of
CO
−
and of calcium (Ca
2+
)
by rivers are, at equilibrium, equal to the sedimentation of CaCO
3
. If
CaCO
3
deposition is higher than its supply, then the SH moves
upward, exposing a larger area of sediment to corrosive waters and
reducing CaCO
3
burial. In the opposite case, if CaCO
3
deposition is
lower than its supply, the SH moves downward, exposing a smaller
area of sediment to corrosive waters and increasing CaCO
3
burial in
the sediment. This compensation mechanism is very efficient over
timescales longer than 10,000 years [ZEE 11] but does not operate in
the current period of acidification, which is much too fast.
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