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later Cambrian to earliest Ordovician oceans expe-
rienced persistent or recurring subsurface hypoxia,
at least episodically expanding to widespread
anoxia. Thus, the protracted post-extinction inter-
val marked by limited skeletal contributions to
accumulating carbonates, a dearth of hypercalcii -
ers, and few metazoan contributions to reefs may
have been governed, at least in part, by the redox-
modulated depression of Ω in surface seawater.
Depression of surface Ω may also explain hyper-
calcii er loss at the boundary between the Frasnian
and Famennian stages of the Devonian. Although
commonly included as one of the 'big i ve' mass
extinctions, extinction rates were not unusually
high during this event. Rather origination rates
declined, resulting in a 'mass depletion' of standing
diversity (Bambach
et al
. 2004). In fact, the Frasnian-
Fammenian loss of reef-building hypercalcifers,
especially calcareous sponges, sits in the middle of
a protracted interval of diversity decline. As dis-
cussed earlier for Permian and Cambrian hypercal-
cii er extinctions, this entire mid-Devonian to basal
Carboniferous interval is characterized by wide-
spread black shales as well as high taxonomic turn-
over. Once again, then, anoxia at depth may have
inl uenced diversity through a protracted interval,
with the Fransian-Famennian boundary represent-
ing an extreme perturbation that exceeded the
capacity of hypercalcii ers to respond.
Thus, a case can be made that the three Palaeozoic
intervals marked by hypercalcifer extinction and
subsequent gaps in metazoan reef accretion may
share the environmental circumstance of marked
reduction of Ω in surface waters. Although in need
of geochemical testing, this hypothesis can account
for both the timing and taxonomic/physiological
selectivity of extinctions at these moments in time.
Transient ocean acidii cation triggered by erup-
tion of the Central Atlantic Magmatic Province has
also been argued to be the trigger mechanism for
pronounced extinctions at the end of the Triassic
(200 Myr ago; Hautmann 2004 ; Hautmann
et al
.
2008a ). Coral diversity declined strongly ( Lathuilière
and Marchal 2009), but not completely (Kiessling
et al.
2009), so that relatively diverse communities,
including reefs, became re-established on a million-
year timescale (Hautmann
et al.
2008b ). Interestingly,
l ood basalts and transient subsurface anoxia recur
about 17 Myr later, at the Pliensbachian-Toarcian
boundary of the early Jurassic; in this case, corals
show elevated turnover rates, but not strong diver-
sity decline (Lathuilière and Marchal 2009). Ocean
acidii cation has been proposed to explain both
Mesozoic events (Kiessling and Simpson 2011, and
references therein). These events make it clear that
while several episodes of hypercalcii er extinction
coincide with large igneous eruptions (e.g. Courtillot
and Olson 2007), large igneous provinces do not
invariably result in mass extinction of reef-building
metazoans (Wignall 2001). The hypothesis enter-
tained here suggests that massive volcanism
affected hypercalcii ers most strongly when the
redox state of the oceans was prone to subsurface
anoxia (Fig. 4.1), facilitating a combined inl uence
of ocean acidii cation and redox-driven redistribu-
tion of total alkalinity on surface-water Ω. Of indi-
vidually moderate effect, ocean acidii cation and
subsurface anoxia in tandem provide a lethal cock-
tail for organisms with limited physiological
capacity to buffer the l uids from which they pre-
cipitate carbonate skeletons.
Earth scientists commonly argue the merits of
factors X
versus
Y in affecting the history of life;
more realistic approximations may occur when we
discuss the effects of X
and
Y, occurring together or
in series. The point here is not to argue that satura-
tion level was the sole inl uence on hypercalcii er
evolution through time. Additional aspects of sea-
water chemistry (e.g. Mg/Ca), biological interac-
tions, and other inl uences may well have affected
hypercalcii er evolution (Kiessling 2009). But we do
argue that emerging geochemical tools provide a
means of exploring both short- and long-term
changes in Ω through time and that results to date
support the hypothesis that episodic declines in Ω
have played a major role in governing the strati-
graphic distribution of hypercalcii ers and, hence,
metazoan reefs.
4.4 Summary, with lessons for the
future
Several events in earth history bear the i ngerprints
of ocean acidii cation. The two examples discussed
here in detail (the PETM and Permian-Triassic
mass extinction) reveal a complex, and sometimes
