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Finally, one could ask whether long-term changes
in ocean carbonate chemistry could have inl u-
enced, for instance, the distribution of coral reefs
throughout the Cenozoic. Based on geological evi-
dence in the continental United States, Opdyke and
Wilkinson (1993) suggested a roughly 10° latitudi-
nal reduction in areal extent of reefal/oolitic car-
bonate accumulation between the Cretaceous and
the Holocene, with a gradual decrease toward the
present. These authors mostly focused on carbonate
mineral saturation state and sea-surface tempera-
ture as environmental parameters that control the
latitudinal extent of reefs. In contrast, based on an
extensive dataset of palaeolatitudinal distribution
of reef sites, Kiessling (2001) concluded that neither
the width of the tropical reef zone nor the total lati-
tudinal range of reefs is correlated with palaeotem-
perature estimates. He inferred a fairly wide reef
zone during the Cretaceous and early Palaeogene
and an exceptionally wide tropical reef zone in the
late Palaeocene and Eocene, relative to the modern
latitudinal boundaries. The bottom line is that these
studies do not seem to indicate any obvious rela-
tionships between the distribution of reefs and
changes in, for example, seawater CO 2 or pH over
the Cenozoic (see Fig. 2.3). As mentioned above, no
signii cant relationships are in fact to be expected
because the long-term saturation state of the ocean
appears to have been nearly constant throughout
the Cenozoic. In contrast, rapid short-term ocean
acidii cation events such as the Palaeocene-Eocene
Thermal Maximum (~55 Myr) have been identii ed
as the cause for ancient reef crises (e.g. Kiessling
and Simpson 2011 ).
Long-term changes in ocean carbonate chemistry
over the past 500 Myr are summarized in Fig. 2.4
(see Ridgwell 2005 ; Ridgwell and Zeebe 2005 ).
formations that consist of coccolithophore calcite.
Given the basics of carbon cycling and controls on
seawater carbonate chemistry as reviewed above, it
is obvious that such comparisons are invalid (see
also Zeebe and Westbroek 2003; Ridgwell and
Schmidt 2010). This applies not only to the
Cretaceous, but in general to long-term, high-CO 2
steady states in the past. Briel y, because two CO 2
system parameters are required to set the carbonate
chemistry, similar CO 2 concentrations do not imply
similar carbonate chemistry conditions because of
differences in, for example, saturation state (Section
2.2.2). The anthropogenic perturbation represents a
transient event with massive carbon release over a
few hundred years. In contrast, the Cretaceous, for
instance, represents a long-term steady-state inter-
val over millions of years. As a result, the timescales
involved (centuries versus millions of years), reser-
voir sizes (a few thousand Pg C versus 10 8 Pg C),
and controls on carbonate chemistry are fundamen-
tally different (Section 2.3).
Ocean carbonate saturation state is generally
well regulated by the requirement that on 'long'
(>10 000 yr) timescales, sources (weathering) and
sinks (shallow- and deep-water CaCO 3 burial) must
balance (Ridgwell and Schmidt 2010). In contrast,
as pH rel ects the balance between dissolved CO 2
and carbonate ion concentration, it is governed pri-
marily by p CO 2 (controlling CO 2 for given tempera-
ture) and Ca 2+ /Mg 2+ (controlling CO 3 2- for given Ω)
rather than weathering. It follows, for instance, that
there was no late Mesozoic carbonate crisis because
Ω was probably high and decoupled from pH. Only
events involving geologically 'rapid' (<10 000 yr)
CO 2 release will overwhelm the ability of the ocean
and sediments to regulate Ω, producing a coupled
decline in both pH and saturation state, and hence
providing an ocean acidii cation analogue relevant
to the future.
2.4.5 Comparisons between the Cretaceous
and the near future are invalid
2.5 Ocean acidii cation events in earth's
history
As mentioned above, comparisons between the
Cretaceous and the near future are frequently made
to suggest that marine calcii cation will not be
impaired in a future high-CO 2 world. The evidence
cited for this is usually based on the occurrence of
massive carbonate deposits during the Cretaceous
such as the White Cliffs of Dover—carbonate
We will use the term 'ocean acidii cation events' to
describe episodes in earth's history that involve
geologically 'rapid' changes of ocean carbonate
chemistry on timescales shorter than ~10 000 years.
In the following, we will limit our discussion to a
 
 
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