Geoscience Reference
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side of this maximum [LAN 06]. Other species are particularly resistant
to changes in pCO
2
, and their rate of calcification does not vary when
pCO
2
increases (e.g. [LAN 06]). Finally, two papers suggest a
stimulating effect of ocean acidification on calcification by
coccolithophore [IGL 08, SHI 09]. However, these results have been
seriously questioned [RIE 11].
Given the experimental results available, it seems difficult, or even
impossible, to predict the effect of the acidification of oceans on
calcifying planktonic species. A field study has recently provided very
interesting pieces of information on this question. Beaufort
et al.
[BEA 11] have collected coccoliths and seawater in several regions in
order to document the variations in the weight of coccoliths over the
last 40,000 years (Figure 5.7). They concluded that coccolithophores
are generally less calcified when pH and the concentration in
CO
−
decrease, which suggests that they could be seriously affected in the
coming decades. However, this work also highlighted a resistant
strain, the coccoliths of which have a relatively high weight at
relatively low pH values. It therefore seems that this strain has
succeeded in naturally adapting to an environment which does not
favor calcification. It remains to be seen whether the species exposed
to normal pCO
2
will be able to adapt to the rapid decrease of pH in the
surface ocean projected for the coming decades.
3
Other planktonic organisms, such as foraminifera and pteropods,
contribute significantly to the global production of calcium carbonate.
Planktonic foraminifera, which have a calcitic shell, can represent up
to 50% of the CaCO
3
present in sediments. They are, therefore,
instrumental in the carbonate counter-pump [SCH 02]. The effect of
changes in the carbonate chemistry on the physiology and ecology of
planktonic foraminifera has not been the subject of active research
until recently. Nevertheless, a positive correlation between the weight
of the shell and the concentration in
CO
−
has been reported in
sediment samples covering multiple glacial-interglacial cycles
[BAR 02]. Similar observations have been made for much more recent
periods. The shell weight in recent sediments is smaller than in
Holocene sediments [MOY 09]. These observations are in agreement
2
3
