Geoscience Reference
In-Depth Information
Several episodes of ocean acidification have been identified at a
geological scale (Figure 5.3, [ZEE 11]). The major ones are the
Paleocene-Eocene Thermal Maximum (PETM, ~56 Myr ago), the
Cretaceous-Tertiary transition (~65 Myr), multiple oceanic anoxic
events (OAE) and the Permo-Triassic (P-T, ~252 Myr). The PETM
and the P-T are considered to be among the closest to current episode
[HON 12] in terms of the magnitude of the change in pH, but what
distinguishes the current event is its very high rate. The P-T
corresponds to the biggest mass extinction of the Phanerozoic Eon,
following a release of CO 2 of 13,000-43,000 Gt C in 20-400 ka, thus
an annual release of carbon of 0.1-1 Gt C [HON 12] compared to
10 Gt C in 2010 [PET 12]. The current and future changes are without
precedent in the last 300 million years.
CENOZOIC ERA
MESOZOIC ERA
PALEOZOIC ERA
PHANEROZOIC ERA
Main changes of the carbonate
chemistry
Modern pH (pre-industrial)
Time (in millions of years before present)
Figure 5.3. Geological context of the acidification of oceans (according to
[ZEE 11]): the major periods of acidification and the corresponding pH of surface
waters on the “seawater scale” (see color section)
5.2.2. Recent changes
Direct measurements of the carbonate chemistry confirm without any
ambiguity that ocean acidification is a real phenomenon and that it is
in progress. Orr [ORR 11] has reported on the available data. Most of
the data were obtained in three long-term series that have been
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