Geobiology (2007), 5, 303–309
DOI: 10.1111/j.1472-4669.2007.00130.x
Journal compilation © 2007 Blackwell Publishing
© Oxford,
The
Geobiology
1472-4677
Author
XXX
GBI
UK
Ltd
Editorial
The End-Permian mass extinction – how bad did it get?
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The current renaissance in study of the end-Permian mass
extinction can be traced back to the general interest in mass
extinctions fostered by the paper by Alvarez et al. (1980) and
the pioneering study by Holser and Magaritz (1995) of the
δ13C record of this interval (Holser et al., 1989; Holser &
Magaritz, 1995). They discovered a large, negative shift and
interpreted it as a signal of regression and oxidation of organic
C reservoirs. However, Erwin’s (1993) suggestion that it may
record a major pulse of methane release to the atmosphere
gained a rapid popularity that it still has today (e.g. Ryskin,
2003). The 1990s saw a series of major advances in our
understanding of the crisis, including the recognition that
the vast Siberian Traps flood basalt province was erupted
contemporaneously, and therefore a likely ultimate cause of
the crisis (Renne & Basu, 1991; Renne et al., 1995), and the
identification of a global oceanic anoxic event as the likely
proximate cause of the marine extinctions (Wignall &
Hallam, 1992; Isozaki, 1994). At the beginning of the 21st
century, the causal chain of events responsible for the world’s
greatest crisis had been developed into a working model
summarized in Wignall (2001; Fig. 1). The emphasis placed
on the different ‘kill mechanisms’ varies among protagonists,
although it is fair to say that widespread marine anoxia was a
leading causal mechanism (Wignall & Twitchett, 1996;
Isozaki, 1997) along with an associated collapse of ocean
productivity (Wang et al., 1994; Kajiwara et al., 1994) linked
to changes in the oceanic phosphorus inventory (Hallam &
Wignall, 1997). Elevated pCO2 values in the upper water
column are a possible corollary of an anoxic lower water
column that may also have been a factor in the marine
extinction losses (Knoll et al., 1996). Conversely, the role of
severe global cooling, and glacioeustatic regression, caused
by Siberian volcanism, was also championed (e.g. Campbell
et al., 1992; Renne et al., 1995).
The subsequent years of the 21st century have seen an
acceleration of research on the end-Permian crisis, with
papers now added to the literature on a nearly daily basis. This
editorial considers some of the more recent attempts to fully
understand this impressive global catastrophe. Not surprisingly many studies have confirmed the severity of the oceanic
anoxic event. Thus, the suggestion that euxinic conditions
extended into shallow marine waters above fair-weather wave
base (Wignall et al., 1998) is supported by the discovery of
biomarkers for green-sulphur bacteria and other distinctive
biomarkers in the extinction interval (Grice et al., 2005; Xie
et al., 2007). The complexity and duration of the crisis has
also been clarified. For much of the 1980s and early 1990s,
the mass extinction was considered to be a protracted event
spread over as much as 10 myr of the late Permian. However,
field studies suggested a geologically abrupt mass extinction
that occurred during only tens or hundreds of thousands
of years (Wignall & Hallam, 1992; Rampino et al., 2000;
Fig. 1 Flow chart summarizing the proposed
chain of events required to explain the link
between the Siberian Traps eruptions and
the end-Permian mass extinction, based on
literature reviewed in Wignall (2001). The
dashed line shows the positive feedback
between a global warming trend and a gas
hydrate dissociation that releases CH4 to the
atmosphere where it rapidly oxidizes to CO2.
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Journal compilation © 2007 Blackwell Publishing Ltd
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P. B. WIGNALL
Twitchett et al., 2001). This work culminated in the suggestion
that the extinction was so abrupt (i.e. instantaneous) that it
was possibly caused by meteorite impact (Jin et al., 2000). In
recent years the pendulum of opinion has tended to swing
back in favour of an extinction spread over a measurable
interval of geological time, thereby ruling out the possibility
of an impact-generated crisis (Yin et al., 2007), although this
idea still has its adherents (Becker et al., 2001).
One of the more intriguing recent discoveries is the
possibility that the planktonic crisis began before the benthic
one. Deep-water chert sections in British Columbia reveal that
the radiolarian mass extinction preceded the loss of benthic
invertebrates (consisting of sponge spicules and bioturbation),
by a short interval represented by a few decimetres of chert
accumulation (Wignall & Newton, 2003). This two phase
extinction is not seen in the Japanese accreted chert terranes,
because they lack a benthic fossil record, but they do reveal a
protracted radiolarian extinction event. The main phase of
extinction occurs at the top of the Albaillella yaoi radiolarian
zone with the survivors, an impoverished fauna of mutated
forms, struggling on to the top of the Albaillella simplex zone
(Xia et al., 2004). Correlation of the Japanese sections with
the well-known global stratotype at Meishan in South China
reveals that the initial extinction occurs at the top of the
Clarkina changxingensis–Clarkina deflecta zone (Xia et al.,
2004). This level is below the benthic extinction level which
occurs in the latest part of the Clarkina yini Zone (Jin et al.,
2000, 2007). The final coup de grãce of the radiolarians was
after the main phase of benthic extinctions within the final
conodont zone of the Permian (Xia et al., 2004). A protracted
phase of radiolarian extinctions is also seen in the deep-water
chert sections of South China where their sequential losses
culminate in an impoverished assemblage of Entactinaria and
Spumellaria (He et al., 2007). Interestingly, Chinese researchers
have attributed these planktonic losses to a major regressive
event (He et al., 2007; Yin et al., 2007), thus resurrecting an
extinction mechanism that has rather fallen out of favour in
recent years. Alternatively, modern radiolarians often show
adaptations to discrete depth zones within the water column
(Lazarus, 2005), and it is possible that their Permian extinctions may reflect habitat loss within the water column due to
major changes in the oceanic density structure. Evidence for
water-column conditions is hard to extract from the geological
record but several disparate clues are available. In the British
Columbia section studied by Wignall & Newton (2003), the
radiolarian extinction event coincides with the onset of rapid
alternations of anoxic and oxic facies, suggesting geologically
rapid changes in the water column. This interval immediately
precedes the development of persistently anoxic facies in the
Early Triassic. The acritarchs are the only other planktonic
group with a significant fossil record and, at Meishan, they
suddenly become abundant immediately before and during
the radiolarian crisis (Li et al., 2004), indicating stressed
water-column conditions. Large-scale oscillations of the
oceanic sulphur isotope record, recorded in carbonateassociated sulphate (CAS), have been identified in the interval
leading up to the marine mass extinction (Newton et al.,
2004). This too points to major oceanic redox change (and an
associated planktonic crisis?) before the main extinction.
Probably the most important advance in end-Permian mass
extinction studies in the past few years has been the correlation
of the terrestrial and marine extinction events using a combination of palynological and organic geochemical (biomarker)
approaches. Remarkably, this has revealed a story rather like
that of the radiolarians with the onset of the environmental
crisis pre-dating the benthic extinctions and the final phase of
crisis occurring after. Thus, in Greenland, South China and
Antarctica, there are at least two phases to the terrestrial crisis,
with the initial change in palynomorph assemblages seen to
pre-date the marine crisis (Looy et al., 2001; Twitchett et al.,
2001; Collinson et al., 2006; Wang & Visscher, 2007). These
indicate the loss or dieback of forest communities, dominated
by gymnosperms, and replacement by herbaceous communities
with common lycopsids. However, for the most part, this first
crisis (or the onset of the crisis, depending on the semantics
used) is a major turnover in the composition of terrestrial
floras, while the main extinction of Permian fossil plants does
not occur until after the marine extinction. Thus, in southwest
China the last elements of the distinctive Gigantopteris flora
survived into the earliest Triassic (Yin et al., in press).
Biomarkers have also provided tell-tale evidence of terrestrial ecosystem catastrophe. In the Dolomites sections of
northern Italy the sudden spike abundance of polysaccharides
led Sephton et al. (2005) to infer a brief phase of intense soil
erosion following vegetation dieback. This spike occurs in the
late regressive portion of the Bellerophon Formation below
the flooding surface that marks the final Permian transgression.
The marine extinction occurs early within this transgressive
record (Wignall & Hallam, 1992; Groves et al., 2007), clearly
after the terrestrial disaster. Similarly at Meishan, the enrichment of dibenzofuran homologues and an increase of the
moretanes to hopanes ration have been used to infer intervals
of increased terrestrial runoff caused by acidification of soils
(Wang & Visscher, 2007; Xie et al., in press). Interestingly,
increased soil erosion appears to be the first signal of the
impending crisis at Meishan, pre-dating even the crisis among
the plankton, but the main pulse of this terrestrial runoff
occurred in the Early Triassic (Xie et al., in press), perhaps at
the same time as the final extinction of ‘Permian’ flora noted
by Yin et al. (2007). The presence of a claystone breccia in
Antarctica terrestrial sections has also been used as evidence of
catastrophic soil erosion (Retallack et al., 2003), where the
total duration of the terrestrial extinction crisis has been
calculated to be 200 kyr (Collinson et al., 2006).
The study of the end-Permian mass extinction has also
benefited from impressive improvements in radiometric dating
of zircons. Thus, the latest study of the extinction horizon
reveals an age of 252.6 ± 0.2 Ma (Mundil et al., 2004), over
© 2007 The Author
Journal compilation © 2007 Blackwell Publishing Ltd
Editorial
a million years older than previously thought, although the
temporal link with the Siberian Traps eruptions is maintained.
A more significant change to result from improved radiometric
dating is the discovery that the first three stages of the Triassic
have a very brief duration of only 2 myr (Ovtcharova et al.,
2007), making them of shorter duration than a typical Jurassic
ammonite zone. In contrast, the fourth Stage, the Spathian,
has become much longer, having more than doubled in
duration to 3.5 myr. The full significance of this has yet to be
fully integrated into extinction and recovery scenarios.
However, the much discussed and supposedly long-delayed
recovery interval spanning the first three Triassic stages has
now become much shorter, and is probably comparable to that
seen after other mass extinctions such as the end-Ordovician
and Late Devonian events. Also, those few groups that did
radiate in the earliest Triassic, notably the conodonts and
ammonoids, are now seen to have done so at a spectacular rate.
Sediment accumulation rates must also be revised. Thus, the
moderately high organic C accumulation rates calculated for
oceanic black shales of the Early Triassic (Wignall & Twitchett,
2002) are now seen to be very impressive. This calls into doubt
claims (e.g. Twitchett, 2001) for low productivity oceans at
this time. Similarly, claims for low carbonate productivity of
shelly benthos (Payne et al., 2006) require revisiting.
Hand-in-hand with the new discoveries from the field and
laboratory has been the development of increasingly more
sophisticated computer modelling scenarios, culminating in
fully coupled ocean–land–atmosphere models that successfully
replicate the global oceanic anoxia hypothesis (Kiehl &
Shields, 2005). These have highlighted the importance of
surface–water temperature in Boreal shelf seas in controlling
deep-water generation and oceanic circulation, although they
do not currently have the ability to replicate the diachroneity
of environmental change seen in the rock record.
Hitherto, the origin of the terrestrial extinctions has been
linked to climatic changes such as global warming (Retallack,
1995), and a consequent loss of environmental heterogeneity
(Hallam & Wignall, 1997). Such effects can be severe, particularly
for cold-adapted forests, but the development of mild, wet
conditions in high latitude settings is hardly the stuff of mass
extinction, something more terrible must have happened.
Global mechanisms, such as changes to atmospheric
chemistry, have therefore have been proposed in recent years.
Damage to the ozone layer and the consequent increase of
ultraviolet-B radiation have become an especially popular
terrestrial extinction cause, albeit with different proposed
origins (e.g. Visscher et al., 2004; Kump et al., 2005; Sephton
et al., 2005; Collinson et al., 2006; Beerling et al., 2007). In
a model that neatly links the oceanic crisis with atmospheric
changes, Kump et al. (2005) suggested that oceanic anoxia
became sufficiently severe that hydrogen sulphide began to
leak into the atmosphere. The consequences were direct
poisoning of the terrestrial biota and also damage to the ozone
layer due to the suppression of OH and O radicals in the
© 2007 The Author
Journal compilation © 2007 Blackwell Publishing Ltd
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atmosphere. The latter effect would also prolong the lifespan
of methane in the atmosphere and thus increase global
warming (Kump et al., 2005). This elegantly catastrophic
models requires supporting evidence, and Kump et al. highlight
the recent discovery of isorenieratane, a biomarker for green
sulphur bacteria, in boundary sediments (Grice et al., 2005),
and S enrichment in the soils of the Karoo Basin (Maruoka
et al., 2003). The presence of isorenieratane suggests photic
zone euxinia and is therefore good evidence for anoxic conditions
in shallow water. However, it should be remembered that this
biomarker is near ubiquitous in black shale facies of many ages,
whereas the end-Permian terrestrial crisis is unique. The terrestrial
S data are more unusual and this is an avenue of research that
should be pursued. Field evidence, such as the development of
euxinic conditions in very shallow water conditions above fairweather wave base (Wignall et al., 1998), also lends credence
to the Kump et al. model. More recently, Kump et al. have
suggested that fluctuations in the oceanic S isotope record also
support their H2S degassing model (Riccardi et al., 2007).
However, many of their reported δ34SCAS values are incredibly
low, reaching values of –35‰, which is the same as contemporary
pyrite. This, together with very high CAS values (averaging
1000–1500 p.p.m.), suggests that there may have been oxidation of pyrite in the samples. It remains to be seen if these very
low δ34SCAS values are replicated in studies of other sections.
Kaiho et al. (2006; p. 44) also favour ‘massive H2S outgassing’ for which their offered evidence includes a gypsum layer
at the Meishan extinction boundary and a negative excursion
of δ34SCAS that this time reaches a more credible low point of
+5‰. The gypsum layer is explained to be a consequence of
‘oceanic mixing leading to oxygenation’. The gypsum layer
at Meishan has misled several previous teams of geologists at
Meishan, but it is more prosaically (and truthfully!) interpreted
as a weathering product of the pyrite layer that caps the extinction
horizon (Wignall & Hallam, 1993). The other line of argument,
based on the δ34SCAS excursion, is also somewhat enigmatic
because they consider the H2S to have both outgassed and at
the same time to have oxidized within the upper water column
and left a signature in the sulphate record.
Providing a mechanism to cause extreme damage to the
ozone layer requires an understanding of the complexities of
atmospheric chemistry and several alternatives have been
mooted in recent years. Kump et al. (2005) favoured the
suppression of oxidizing radicals by H2S as a direct cause of
ozone depletion, although this effect may be more important
as a means of prolonging the residence time of methane in the
atmosphere and thereby indirectly reduces ozone generation
rates (Lamarque et al., 2007). In fact, an atmospheric ‘double
whammy’ of high H2S and CH4 fluxes may be needed to cause
extinctions because calculations suggest that neither gas alone
is capable of doing sufficient damage to the ozone layer.
Lamarque et al. (2006) calculated that raising atmospheric
CH4 concentrations to 5000 times preindustrial values would
damage the ozone layer sufficiently to increase ultraviolet-B
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P. B. WIGNALL
radiation by sevenfold. However, to achieve this would require
the instantaneous release of (an improbably large) 8000 Gt
(C) from hydrate reservoirs. It is also noteworthy that this
intensity of depletion would be very short-term (several
decades), due to the short residence time of CH4 in the
atmosphere, implying that the terrestrial extinctions should be
equally abrupt if this was the cause. This is not observed in the
fossil record.
Beerling et al. (2007) have highlighted an alternative source
of ozone damage – the release of CH3Cl, caused by baking
organic C in sediments at the site of Siberian eruptions.
However, their modelling suggests that the only significant
damage to the ozone layer would occur at high latitudes,
where it could explain the occurrence of mutated pollen and
spores, whereas the lower latitude stratosphere has ‘selfhealing’ properties. This rather negates the significance of this
potential extinction mechanism, although other potentially
damaging influences on the ozone layer, such as methane and
hydrogen sulphide fluxes, are not incorporated in Beerling
et al.’s (2007) model. As a word of caution though, it should
be highlighted that the Siberian intrusives are not unique a
phenomenon. There have been several large igneous province
eruptions in the Phanerozoic, many of similar scale to that of
the Siberian Traps, such as the Karoo-Ferrar Province and the
North Atlantic Igneous Province, that have not been associated with catastrophic terrestrial ecosystem collapse (Wignall,
2005). Indeed, there is good evidence for significant thermogenic hydrocarbon release during both of these igneous
episodes (Svensen et al., 2004, 2007), but neither is associated
with terrestrial extinctions. The end-Permian terrestrial
catastrophe is unique and therefore requires a cause that is
either unique in magnitude or causation.
Lower atmospheric oxygen levels of the Permo-Triassic
interval, suggested by Berner’s modelling (e.g. Berner, 2002,
2005), may also have exacerbated the ozone-damaging effects
of methane and hydrogen sulphide release. However, for some
researchers, low atmospheric oxygen levels are themselves a direct
cause of the extinction (Retallack et al., 2003; Huey & Ward,
2005). Considerable attention has been paid to Lystrosaurus,
a tetrapod that survived the extinction and proliferated, in
true ‘disaster taxon’ style, in the immediate aftermath. In a
‘Kipling-esque’ study of the biology of this beast, Retallack
et al. (2003; p.1133) suggest that its ‘short internal nares,
barrel chest and high neural spines would have given it greater
aerobic scope’. If these are indeed adaptations to low O2
levels, then they could simply reflect the efficient respiration
requirements for the well-known burrowing habit of Lystrosaurus rather than be evidence for low atmospheric oxygen per
se. Other evidence for low oxygen levels in the atmosphere,
from palaeosol mineralogy (Sheldon & Retallack, 2002), is
equally equivocal (Huggett & Hesselbo, 2003).
Having first mooted the possibility of low atmospheric O2
levels as the cause of the terrestrial extinctions in Wignall
(1992), I have become increasingly unconvinced ever since!
The Permo-Triassic values proposed are no worse than those
encountered on the Tibetan plateau today and, as any visitor
to the region knows, it does not take too long to become
acclimatized to the thin atmosphere. The main difficulty with
this extinction mechanism is achieving atmospheric oxygen
decline faster than the evolutionary response of the terrestrial
biota. Oxygen levels as low as 12% are not harmful to animals
(Engoren, 2004), although potentially, the sudden establishment of these values could be. Retallack et al. (2003) suggest
that atmospheric oxygen values fell from 30% to 12% in only
a few tens of thousands of years due to combustion with
methane. However, this would result in atmospheric CO2
levels reaching utterly implausible values of 90 000 ppmV
(Engoren, 2004). The unlikelihood of such rapid atmospheric oxygen changes is tacitly admitted by Huey & Ward
(2005; p. 400), who instead suggest that generally low
atmospheric O2 values in the entire Permian–Triassic interval
‘contributed to the high background extinctions and high
turnover before the mass extinction’. It is a moot point
whether extinction/turnover rates were unusually high in the
Permian, as is their further claim that low O2 levels persisted
‘into the early Cretaceous and may have contributed to a slow
recovery after the mass extinction’ (Huey & Ward, 2005; p.399).
Few have claimed that there is a suppression of origination
rates for the entire 100 myr interval after the end-Permian
mass extinction.
Ignoring the nasal passages of Lystrosaurus for the moment,
the principal ‘evidence’ for atmospheric oxygen levels is derived
from modelling calculations (Berner, 2002, 2005). Values as
low as 12% should severely inhibit wildfires and yet charcoal is
known from latest Permian sediments (Thomas et al., 2004)
and the palynofacies from the Meishan section reveal black
organic particles (derived from wildfires) increase in abundance
during the extinction (Xie et al. in press). Therefore, some of
the lower estimates for Permo-Triassic atmospheric oxygen
levels, such as Berner’s (2002) 12%, may be too low. A systematic
palynofacies investigation of Permo-Triassic sections for
charcoal concentrations remains to be done, but it is clearly
merited.
A final catastrophe blamed on atmospheric chemistry is the
increase of atmospheric CO2 levels to the point where they
may have caused an acidification crisis in the oceans (Heydari
et al., 2003; Fraiser & Bottjer, 2007; Payne et al., 2007).
Borrowing the model proposed for the calcification crisis at
the time of the Palaeocene–Eocene Thermal Maximum
(PETM), Payne et al. (2007) suggest that a brief phase of
intense dissolution was mostly responsible for the endPermian marine mass extinction caused by the usual culprits,
volcanogenic CO2 and methane release. It is worth highlighting
that the ‘type example’ PETM calcification crisis was only
associated with trivial extinction losses compared to the endPermian holocaust. This rather calls into doubt the efficacy of
such changes as an agent of the Permian event, unless pH
values become exceptionally low at this time. In fact Payne
© 2007 The Author
Journal compilation © 2007 Blackwell Publishing Ltd
Editorial
307
Fig. 2 Flow chart, modified from Fig. 1,
summarizing the cascade of environmental
consequences caused by the eruption of the
Siberian Traps and the latest additions, since
2001, to current thought (shown in italics).
The increase in viable terrestrial extinction
mechanisms is especially notable. Although
this diagram attempts to show a current
consensus, all aspects of the chart are
actively debated. Boxes with a ‘?’ denote
proposed causes and effects that, for this
author at least, should be treated with
skepticism.
et al.’s evidence for this crisis, a dissolution surface seen in
some shallow-water carbonate sections, could also be interpreted
as a karstic surface formed during a brief interval of sea-level
fall. However, Payne et al. (2007) note that there is no tell-tale
diagenetic evidence for emergence. The clearest indication of
pH change in the world’s ocean at this time is the proliferation
of microbial and abiotic carbonate precipitates in the immediate
aftermath of the extinction, suggesting an alkalinity increase
perhaps due to the loss of shelly taxa and/or the upwelling of
bicarbonate-rich waters from an anoxic deep ocean (e.g. Knoll
et al., 1996; Baud et al., 2007).
So how have the new advances changed our understanding
of end-Permian mass extinction mechanisms since the 20th
century? The principal new additions have resulted from
consideration of predicted changes in atmospheric chemistry
(Fig. 2). However, it should be stressed that it is difficult to
study atmospheric composition 250 Ma and all postulated
changes are still ideas in search of evidence. Nearly all studies
recognize a cascade of environmental change, and future
work, both in the field and using computer modelling, should
attempt to produce a consistent cause-and-effect chain of
events. It is already clear that the initial ecosystem stress, seen
in the planktonic and terrestrial records pre-dates the onset of
the negative C isotope excursion (Twitchett et al., 2001; Yin
et al., 2007), an observation that should make workers more
wary of attributing catastrophic causes, such as methane
release, to this perturbation. The excursion is probably more
related to the consequences of the crisis rather than the causes
of it. Suggestions that the negative δ13C shift records the neartotal shutdown of terrestrial and marine productivity should
be given credence (Broecker & Peacock, 1999; Rampino &
Caldeira, 2005). High amplitude δ13C oscillations persist into
© 2007 The Author
Journal compilation © 2007 Blackwell Publishing Ltd
the Early Triassic and the new, briefer timescale indicates that
this was a time of exceptional global environmental instability,
a legacy of the greatest crisis to affect global ecosystems in the
Phanerozoic.
ACKNOWLEDGEMENTS
I thank Lee Kump and Dave Beerling for their comments on
this editorial.
P. B. Wignall
School of Earth and Environment, University of Leeds,
Leeds LS2 9JT, UK
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