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accretion (Rowland and Shapiro 2002). As part of a
broader radiation of well-skeletonized animals,
sponges, rugose and tabulate corals, and bryozoans
renewed metazoan reef accretion during the
Ordovician (Harper 2006), and reefs constructed by
these organisms persisted with varying abundance
until the Frasnian-Famennian boundary in the late
Devonian, some 370 Myr ago. At this time, hyper-
calcifying animals collapsed again, ushering in a
brief interval of animal-poor microbial reefs (Copper
2002b). Animal-algal reefs occurred throughout the
later Carboniferous and Permian, with hypercalci-
fying sponges once again playing a particularly
important role in later Permian build-ups. Then, as
discussed above, hypercalcii ers suffered differen-
tially severe losses during the end-Permian mass
extinction.
Early Triassic reefs were microbial. Beginning in
the Middle Triassic, however, reef abundance
increased with the radiation of scleractinian corals
and sponges. Many hypercalcii ers disappeared,
once again, at the end of the Triassic, although
enough species survived to fuel renewed reef
expansion during the Jurassic (Lathuilière and
Marchal 2009). Another decline toward the end of
the Jurassic was followed by an extended interval
dominated by rudist bivalves. Only after the end-
Cretaceous mass extinction did modern reef ecosys-
tems begin to take shape.
Although there is good physiological reason to
connect the abundance and evolutionary history of
hypercalcii ers to state changes in Ω in ambient sea-
water (e.g. Veron 2008), the long (>10
5
yr) timescales
on which reef organisms have waxed and waned
introduces a new class of problem. As discussed
above, variables such as
p
CO
2
and temperature do
not appear to explain the stratigraphic pattern of
hypercalcii er evolution ( Kiessling 2009 ). This
should not be surprising, given the l exibility of
these parameters within the dynamic equilibrium
described in Section 4.2. What we require is a mech-
anism that is congruent with this dynamic equilib-
rium and yet can have an impact on marine
carbonate chemistry with a characteristic timescale
greater than that expected for ocean acidii cation,
bearing in mind the ever-present stabilizing
feedbacks.
Hypercalcifying organisms residing in reefs expe-
rience the Ω of regional surface seawater. Global Ω is
set by the overall marine carbonate system, but this
value (which is close to thermodynamic equilib-
rium) represents a cumulative parameter integrated
over the entire volume of global seawater. There
are, in spite of this, large gradients in Ω. These gra-
dients result in part from the hydrological cycle
(controlling salinity) and inorganic factors control-
ling the solubility of carbonate polymorphs (e.g.
temperature and pressure). An underappreciated
process promoting these gradients is the effect of
the biological pump (Higgins
et al
. 2009 ). CO
2
i xed
by primary producers in the surface ocean is aerobi-
cally respired in the deep, setting up a gradient in
C
T
that pushes Ω higher in surface seawater and
lower in deep seawater (Fig. 4.4).
We can imagine how the geobiological behaviour
of the biological pump differed in times past. The
pump could be stronger or weaker. A stronger bio-
logical pump means larger gradients in
C
T
, and
would translate into a world characterized by even
larger gradients and a higher Ω in surface seawater
than we observe today. We can also imagine a world
with a reduced depth gradient in Ω, including lower
surface-seawater Ω, due to the impact of anaerobic
metabolisms. In contrast to aerobic respiration, all
anaerobic metabolisms signii cantly affect
A
T
in
addition to
C
T
( Soetaert
et al
. 2007 ; Higgins
et al
.
2009 ).
4.3.3 Mechanisms to explain the pattern
of hypercalcii cation in reefs
Canonically recognized mass extinctions do not
fully explain the stratigraphic pattern of hypercalci-
i er evolution ( Kiessling 2009 , Kiessling and
Simpson 2011 ). Hypercalcii ers disappeared com-
pletely during the end-Permian mass extinction and
declined markedly in diversity and extent during
Late Devonian and Late Triassic extinctions. On the
other hand, mass extinctions at the end of the
Ordovician (Sheehan 2001) and Cretaceous do not
show the preferential loss of hypercalcii ers
observed for end-Permian collapse (Knoll
et al
.
2007); the proportional extinction of hypercalcii ers
was modest during the end-Ordovician and end-
Cretaceous mass extinctions, and the loss of meta-
zoan-built reefs was transient.
Anoxic
environments
characterized
by
