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reef crises be related to rapid decreases in Ω associ-
ated with ocean acidii cation, the expansion of sub-
surface anoxia, or both?
Recent radiometric determinations suggest that
the Cambrian extinction of archaeocyathids may
coincide with emplacement of the Kalkarindji Large
Igneous Province, Australian l ood basalts compa-
rable in scale to the end-Permian Siberian Traps
( Evins
et al.
2009). Citing the extensive deposition of
black shales, Zhuravlev and Wood (1996) linked
Cambrian hypercalcifer extinctions to the expan-
sion of anoxic subsurface waters. In turn, Glass and
Phillips ( 2006 ) and Hough
et al
. ( 2006 ) related anoxia
and extinction to the Kalkarindji eruptions. In fact,
the palaeobiological particulars of this extinction
suggest that we should focus on the modulation of
Ω discussed in the previous section. Early Cambrian
hypercalcii ers were widespread in shallow shelf
and platform environments, and they disappeared
despite the limited incursion of anoxic water masses
into shallow marine environments. Trilobites, the
dominant organisms recorded in Cambrian strata,
suffered major extinctions, but because i rst appear-
ances kept pace, the overall pattern is one of marked
turnover, not loss of diversity (Bambach
et al
. 2004 ).
Small shelly fossils of phylogenetically diverse ori-
gins declined across this interval, but as their record
is closely tied to preservational circumstances that
also change, it is difi cult to quantify their evolu-
tionary pattern (Porter 2004). The renowned Burgess
Shale in British Columbia documents the persist-
ence of diverse animal groups into mid-Cambrian
oceans, but few of these made robust carbonate
skeletons. In fact, in later Cambrian carbonates,
skeletons account for only a few per cent of known
carbonate volume—more like the Early Triassic
than any other time of Phanerozoic history (Pruss
et al
. 2010). As in the end-Permian example, massive
basaltic volcanism appears to have been visited on
a planet whose oceans were already characterized
by subsurface hypoxia. Volcano-driven warming
and expansion of subsurface anoxia caused mass
mortality in deeper marine environments, but it
may have been the associated decline in surface-
water Ω that selectively removed hypercalcii ers
from shallow shelves and platforms.
Rapidly accumulating geochemical data (Hurtgen
et al
. 2009 ; Gill
et al.
2011) suggest that the ensuing
1
W
under-
saturated
aerobic
anaerobic
no biological pump
supersaturated
Figure 4.4
Schematic cross-section of gradients in Ω in seawater under
different scenarios. Global seawater tends to arrange gradients around a
mean value controlled by carbonate compensation (Ω ~ 1). Today, large
gradients exist with depth due to the biological pump and aerobic
respiration. Surface seawater is strongly supersaturated and deep seawater
is undersaturated. A world without a biological pump would still have
gradients in Ω in seawater due to the effects of temperature, pressure,
and salinity. An idealized world with a biological pump, but anaerobic
metabolisms at depth, will have subdued gradients in Ω, due to metabolic
gradients in total alkalinity.
anaerobic respiration tend to ameliorate depth
gradients in Ω, while aerobic metabolisms tend to
promote them. This means that a world in which a
signii cant proportion of biological pump electrons
pass through anaerobic metabolisms will tend to
have subdued gradients in Ω, even though global
seawater may not vary (Fig. 4.4). The ability of the
marine carbonate system to accommodate this reor-
ganization is, in principle, intimately tied to the
processes that control the long-term and large-scale
development of marine anoxia.
It is now possible to develop the logic for hypoth-
eses to explain the abundance and diversity of
marine hypercalcifying organisms through time,
making reference to both short-term events of rapid
CO
2
inl ux that cause Ω to deviate briel y (in geo-
logical terms) from a dynamic equilibrium, and also
long-term transitions due to the waxing and wan-
ing of ocean basin anoxia. Might multiple ancient
