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In-Depth Information
extinction we revealed only half the story—the
other half may be what ensured the unusual sever-
ity of this largest mass extinction.
In some ways, the late Permian world into which
the Siberian Traps erupted was almost maximally
different from the earth we experience today.
Continental masses were aggregated into the super-
continent Pangaea, resulting in a Panthalassic ocean
more than a hemisphere in extent. With the mid-
Permian decay of late Palaeozoic continental ice
sheets and climate warming, physical circulation in
this ocean may have been relatively sluggish, pro-
moting extensive subsurface hypoxia (e.g. Meyer
et al
. 2008 ). Both deep-sea black shales, preserved in
obducted slivers of late Permian seal oor (Isozaki
1997), and biomarker lipids that document anoxy-
genic photosynthetic bacteria in latest Permian seas
( Cao
et al
. 2009) record oxygen depletion in subsur-
face water masses. Global warming associated with
volatile release from Siberian volcanism appears to
have tipped the oceans into a state of widespread
anoxia beneath the mixed layer (Wignall and
Twitchett 1996), generating additional l uxes of CO
2
from upwelling waters (Knoll
et al
. 1996 ). In short,
as observed at some level today, the end-Permian
extinction was not a crisis fomented by hypercap-
nia/ocean acidii cation, global warming,
or
subsur-
face anoxia, it was a crisis in which all three occurred
simultaneously (Knoll
et al.
2007 ). Given the inter-
connected nature of the earth system, it could hardly
be otherwise.
Subsurface anoxia would have impacted on the
biota in several ways. Most obviously, shoaling of
the oxycline would have stressed benthic popula-
tions, much as it does in seal oor 'dead zones'
today. Secondly, the physiological effects of warm-
ing, hypercapnia, and hypoxia are not independ-
ent, but rather are synergistic, amplifying
physiological stress ( Pörtner 2008 ; see Chapter 8 ).
Many organisms in end-Permian oceans probably
died of asphyxiation; nonetheless, end-Permian
skeletons suggest marked selectivity consistent
with the physiological effects of hypercapnia/
ocean acidii cation.
Recently, Higgins
et al
. ( 2009 ) explored the
consequences of widespread subsurface anoxia for
the carbonate system. As discussed in more detail
below, anaerobic heterotrophs generate total
alkalinity as they remineralize organic matter. Thus,
in oceans with widespread subsurface anoxia, sub-
surface water masses should be expected to have
higher Ω than at present, while the Ω of overlying
surface waters should be reduced. Reduction of
surface-water Ω should, in turn, make skeleton for-
mation by hypercalcii ers more difi cult (e.g.
Gattuso
et al
. 1999 ), increasing the physiological
stress on latest Permian corals and hypercalcifying
sponges and other organisms with limited ability to
modulate internal l uid composition.
The punchline for end-Permian extinction, then,
is that the ability of marine organisms to precipitate
calcium carbonate skeletons was impeded by
two
circumstances. Expanding subsurface anoxia and
rapidly rising
p
CO
2
would both have lowered Ω in
the surface ocean; operating in tandem, they appear
to have depressed Ω strongly for a biologically pro-
tracted interval of time. To the extent that this is cor-
rect, it suggests that the end-Permian extinction can
inform current research in terms of taxonomic, eco-
logical, and physiological vulnerability to 21st-cen-
tury global change. Perhaps mercifully, however,
the extent of the end-Permian catastrophe appears
to rely on concatenated factors only partially in play
today.
4.3 Is there a more general historical
pattern?
Reefs are a striking component of the carbonate
sediments through geological time, and the record
of biological calcii cation is well written in the fossil
composition of ancient reefs. The preceding sections
examined two historical events wherein ocean acid-
ii cation occurred due to rapid inl ux of CO
2
from
the solid earth; the effects of these events on the
biota were variable, but in one case, at least, devas-
tating for marine hypercalcii ers. Using historical
metrics, one can ask a set of broader questions about
the processes that have controlled the abundance
and diversity of reef-building organisms through
time. What conditions are responsible for observed
long-term patterns in the evolution of hypercalcii -
ers? And how might these trends rel ect long-term
changes in the nature and behaviour of the marine
carbonate system that extend beyond short-term
ocean acidii cation events?
