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The Alvarez Impact Theory of Mass Extinction; Limits to its Applicability and the “Great Expectations Syndrome”


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For the past three decades, the Alvarez impact theory of mass extinction, causally related to catastrophic meteorite impacts, has been recurrently applied to multiple extinction boundaries. However, these multidisciplinary research efforts across the globe have been largely unsuccessful to date, with one outstanding exception: the Cretaceous–Paleogene boundary. The unicausal impact scenario as a leading explanation, when applied to the complex fossil record, has resulted in force−fitting of data and interpretations (“great expectations syndrome”). The misunderstandings can be grouped at three successive levels of the testing process, and involve the unreflective application of the impact paradigm: (i) factual misidentification, i.e., an erroneous or indefinite recognition of the extraterrestrial record in sedimentological, physical and geochemical contexts, (ii) correlative misinterpretation of the adequately documented impact signals due to their incorrect dating, and (iii) causal overestimation when the proved impact characteristics are doubtful as a sufficient trigger of a contemporaneous global cosmic catastrophe. Examples of uncritical belief in the simple cause−effect scenario for the Frasnian–Famennian, Permian–Triassic, and Triassic–Jurassic (and the Eifelian–Givetian and Paleocene–Eocene as well) global events include mostly item−1 pitfalls (factual misidentification), with Ir enrichments and shocked minerals frequently misidentified. Therefore, these mass extinctions are still at the first test level, and only the F–F extinction is potentially seen in the context of item−2, the interpretative step, because of the possible causative link with the Siljan Ring crater (53 km in diameter). The erratically recognized cratering signature is often marked by large timing and size uncertainties, and item−3, the advanced causal inference, is in fact limited to clustered impacts that clearly predate major mass extinctions. The multi−impact lag−time pattern is particularly clear in the Late Triassic, when the largest (100 km diameter) Manicouagan crater was possibly concurrent with the end−Carnian extinction (or with the late Norian tetrapod turnover on an alternative time scale). The relatively small crater sizes and cratonic (crystalline rock basement) setting of these two craters further suggest the strongly insufficient extraterrestrial trigger of worldwide environmental traumas. However, to discuss the kill potential of impact events in a more robust fashion, their location and timing, vulnerability factors, especially target geology and palaeogeography in the context of associated climate−active volatile fluxes, should to be rigorously assessed. The current lack of conclusive impact evidence synchronous with most mass extinctions maystill be somewhat misleading due to the predicted large set of undiscovered craters, particularly in light of the obscured record of oceanic impact events.
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The Alvarez impact theory of mass extinction; limits to
its applicability and the “great expectations syndrome”
Racki, G. 2012. The Alvarez impact theory of mass extinction; limits to its applicability and the “great expectations syn−
drome”. Acta Palaeontologica Polonica 57 (4): 681–702.
For the past three decades, the Alvarez impact theory of mass extinction, causally related to catastrophic meteorite im−
pacts, has been recurrently applied to multiple extinction boundaries. However, these multidisciplinary research efforts
across the globe have been largely unsuccessful to date, with one outstanding exception: the Cretaceous–Paleogene
boundary. The unicausal impact scenario as a leading explanation, when applied to the complex fossil record, has resulted
in force−fitting of data and interpretations (“great expectations syndrome”). The misunderstandings can be grouped at
three successive levels of the testing process, and involve the unreflective application of the impact paradigm: (i) factual
misidentification, i.e., an erroneous or indefinite recognition of the extraterrestrial record in sedimentological, physical
and geochemical contexts, (ii) correlative misinterpretation of the adequately documented impact signals due to their in−
correct dating, and (iii) causal overestimation when the proved impact characteristics are doubtful as a sufficient trigger
of a contemporaneous global cosmic catastrophe. Examples of uncritical belief in the simple cause−effect scenario for the
Frasnian–Famennian, Permian–Triassic, and Triassic–Jurassic (and the Eifelian–Givetian and Paleocene–Eocene as
well) global events include mostly item−1 pitfalls (factual misidentification), with Ir enrichments and shocked minerals
frequently misidentified. Therefore, these mass extinctions are still at the first test level, and only the F–F extinction is po−
tentially seen in the context of item−2, the interpretative step, because of the possible causative link with the Siljan Ring
crater (53 km in diameter). The erratically recognized cratering signature is often marked by large timing and size uncer−
tainties, and item−3, the advanced causal inference, is in fact limited to clustered impacts that clearly predate major mass
extinctions. The multi−impact lag−time pattern is particularly clear in the Late Triassic, when the largest (100 km diame−
ter) Manicouagan crater was possibly concurrent with the end−Carnian extinction (or with the late Norian tetrapod turn−
over on an alternative time scale). The relatively small crater sizes and cratonic (crystalline rock basement) setting of
these two craters further suggest the strongly insufficient extraterrestrial trigger of worldwide environmental traumas.
However, to discuss the kill potential of impact events in a more robust fashion, their location and timing, vulnerability
factors, especially target geology and palaeogeography in the context of associated climate−active volatile fluxes, should
to be rigorously assessed. The current lack of conclusive impact evidence synchronous with most mass extinctions may
still be somewhat misleading due to the predicted large set of undiscovered craters, particularly in light of the obscured re−
cord of oceanic impact events.
Key w o r d s : Bolide impacts, extraterrestrial markers, impact craters, mass extinctions, Cretaceous–Paleogene bound−
ary, Triassic–Jurassic boundary, Frasnian–Famennian boundary.
Grzegorz Racki [], Department of Earth Sciences, Silesian University, ul. Będzińska Str. 60,
PL−41−200 Sosnowiec, Poland.
Received 9 July 2011, accepted 18 December 2011, available online 24 February 2012.
Copyright © 2012 G. Racki. This is an open−access article distributed under the terms of the Creative Commons Attribu−
tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author
and source are credited.
Initially, the impact theory of mass extinction (or the theory of
impact crises) was outlined in the outstanding 1980 Science
paper by the Alvarez group, who presented reasonable geo
chemical evidence of a massive meteorite impact (i.e., abnor
mally high iridium abundances) and a spectacular scenario of
an impact−induced environmental disaster recorded in the thin
boundary clay at the Cretaceous–Paleogene boundary (K–Pg;
formerly K–T), dated at ~65.5 Ma. This theory was first for
mally introduced in the next papers by Alvarez and co−authors
(1982, 1984, 1989); they confirmed the theoretical palaeonto
logical prediction of the worldwide cataclysm reflected in in
stantaneous mortality among numerous, unrelated groups of
fossil organisms exactly at the Ir anomaly horizon, synchro
nous with the K–Pg boundary.
The ~170 km diameter Chicxulub crater, buried under
more than 1 km of post−impact sedimentary succession
(Hildebrand et al. 1991; see current data in
EarthImpactDatabase/chicxulub.html), has provided strong
Acta Palaeontol. Pol. 57 (4): 681–702, 2012
evidence of a K–Pg boundary impact event, in showing that
an asteroid ~10 km in diameter struck the carbonate− and
evaporate−rich target substratum on Yucatán peninsula in
southern Mexico. Three decades of multidisciplinary stud
ies around the globe have revealed the worldwide distribu
tion of a unique geochemical and mineralogical signature
paired with the synchronous fallout from stratospheric dust.
There are multiple lines of evidence, including high−resolu
tion Ir peaks and a variety of other chemical and physical
features that originated from the impact event, such as
shocked minerals, glassy spherules, Ni−rich spinels, os
mium isotopes (187Os/188Os ratios), microdiamonds, amino
acids, among others (see summaries in Kyte 2002a; Alvarez
2003; Koeberl 2007; French and Koeberl 2010).
Schulte et al. (2010) summarized thirty years of interna
tional research and presented comprehensive support for the
Alvarez impact scenario, as proposed for end−Mesozoic eco
system collapse, and the most amazing demise of non−avian
dinosaurs (see also summary of the kill mechanisms in Claeys
2007 and Kring 2007). Of course, as seen in the subsequent
debate, some controversy still exists, because a combination
of volcanic (Deccan traps) and impact−related, adverse envi
ronmental effects remains more plausible to some authors,
while others subscribe to the multiple impact hypothesis as op−
posed to a single giant impact event (Courtillot and Fluteau
2010; Keller et al. 2010; see also e.g., Tsujita 2001; Ellwood et
al. 2003a; Chatterjee et al. 2006; Jolley et al. 2010; Kidder and
Worsley 2010; Keller 2011).
In the context of the extraterrestrial paradigm, the hot po−
lemics almost instantaneously centred on the issue whether
relatively slow−acting, Earth−bound calamities, such as caused
by massive volcanism, are an alternative (or a supplement) to
the hypervelocity impact−generated killing episodes (see a se−
lection of recent views in Alvarez 2003; Palmer 2003; Morgan
et al. 2004; Glikson 2005; Keller 2005, 2011; MacLeod 2005;
White and Saunders 2005; Twitchet 2006; Claeys 2007;
Kelley 2007; Arens and West 2008; Şengör et al. 2008;
Prothero 2009; Kidder and Worsley 2010). Nevertheless, the
Alvarez theory has rapidly been established as the leading
concept for the K–Pg mass extinction, explaining in addition
not only other Phanerozoic biotic crises, but also introducing
eventually the new catastrophism paradigm in geosciences
(e.g., Berggren and Van Couvering 1984; Alvarez et al. 1989;
Hsü 1989; Marvin 1990; Ager 1993; Glen 1994; Palmer 2003;
Reimold 2007). Many workers have looked for comparable
extraterrestrial evidence at all known extinction horizons and
have frequently claimed compelling multi−disciplinary evi
dence of “impact crises” (see exemplary reviews in McLaren
and Goodfellow 1990 and Rampino and Haggerty 1996a).
The desperate search for a widespread cosmic signature was
most notably a priori reasoned by notion highlight in the popu
lar book of Raup (1991) that no global stress triggers other
than different−magnitude impacts could be responsible for
both “background” and mass extinctions (see also McLaren
and Goodfellow 1990; Raup 1992). This “revolution” in
mainstream geosciences and space sciences was also espe
cially striking when paired with purportedly cyclical collisions
with Earth−crossing asteroids and comets, as manifested by
the “Shiva Hypothesis” of Rampino and Haggerty (1996b),
evolving rapidly into “a unified theory of impact crises and
mass extinctions” of Rampino et al. (1997) and, finally, into
“the galactic theory of mass extinctions” of Rampino (1998).
So far, however, periodic astronomic impact on the Earth bio
sphere as well as periodicity in the terrestrial fossil record have
remained a highly controversial matter (see diverse recent
ideas in Lewis and Dorne 2006; Gillman and Erenler 2008;
Prothero 2009; Bailer−Jones 2011; Feulner 2011; Melott and
Bambach 2011).
In consequence, immediately following the hypothesis that
the K–Pg biotic turnover was triggered by the collision of a gi
ant asteroid with the Earth, the impact as a general prime cause
was comprehensively tested for the “big five” mass extinc
tions of Phanerozoic marine life (Raup and Sepkoski 1982;
see review in Hallam and Wignall 1997; Keller 2005; Alroy
2010). However, the K–Pg boundary clay (= impact ejecta)
remains exceptional in preserving abundant impact proxies
arranged in a clear proximal−distal palaeogeographic trend
(Schulte et al. 2010). This common view is broadly accepted
in “classic” monographic works and textbooks by Walliser
(1996), Hallam and Wignall (1997), Courtillot (1999), Stanley
(1999), and Hallam (2004), and also exemplified in recent
overviews by Lucas (2006), Morrow (2006), Kelley (2007),
Reimold (2007), McCall (2009), Prothero (2009), French and
Koeberl (2010), and Reimold and Jourdan (2012). On the
other hand, the major extinction events, particularly the end−
Permian apocalyptic catastrophe, remain the subject of contin−
uously intensive exploration for extraterrestrial markers de−
spite previous impressive misinterpretations and pitfalls. As
reviewed below, the themes are so attractive to public percep−
tion that even erroneous data and vague explanations invari
ably open doors to the most prestigious journals and mass me
dia (see e.g., Twitchet 2006; cf. “media science”of Officer and
Page 1996), but, in fact, only repeatedly increase information
An updated overview of the impact theory applicability is
the main goal of the present paper, with examples mostly
from the Frasnian–Famennian (F–F), Permian–Triassic
(P–T) and Triassic–Jurassic (T–J) global events, and supple−
mentary Eifelian–Givetian and Paleocene–Eocene data, and
with the emphasis on the critical terrestrial cratering record
[data and images from the Earth Impact Database (EID),, managed by the Uni−
versity of New Brunswick, Canada; some ages corrected af−
ter Jourdan et al. 2012]. The time scale used is mostly after
the International Commission on Stratigraphy Chart 2010
Abbreviations. —CAMP, Central Atlantic Magmatic Prov
ince; E–G, Eifelian–Givetian; EID, Earth Impact Database;
ISC, International Commission on Stratigraphy; KW, Kell
wasser; PDF, planar deformation features; PETM, Paleocene–
Eocene thermal maximum; PGE, platinum group element.
Conclusive and incredible impact
The discovery of Alvarez et al. (1980) has promoted wide
spread speculation as to whether such cataclysmic extrater
restrial events have had a strong impact on the whole history
of life. In fact, key impact indicators have been thoroughly
tested and frequently questioned, such as iridium anomalies
(widened to high levels of all platinum group elements),
spherules and shocked quartz, considered as the original “big
three” of the impact paradigm (Alvarez 2003; see also Rei
mold 2007; French and Koeberl 2010; Koeberl et al. 2012;
Reimold and Jourdan 2012). Thus, diagnostic criteria display
highly evolving histories, and some of them have proved to
be erroneous and have recently been discredited (Table 1),
whilst innovative, increasingly consistent proxies are contin
uously tested, such as He, Os, and Cr isotopes. The value of
other proposed tracers is still unclear, exemplified by the
magnetic susceptibility field method to identify ejecta hori
zons (Ellwood et al. 2003c), as shown by two case stories be
In particular, weak to moderate platinum group enhance
ments are truly non−diagnostic of extraterrestrial sources be
cause they can also derive from a variety of terrestrial origins
(e.g., Evans and Chai 1997). Note that the average Ir value
for the Earth’s crust is less than 0.1 parts per billion (ppb),
whilst K–Pg abundances are at least about two orders of
magnitude greater, with the largest anomaly from Denmark,
reported by Alvarez et al. (1980), as high as 41.6 ppb (see up
dated data in Schulte et al. 2010). Iridium contents signifi
cantly below that of the K–Pg levels have usually been clari
fied by Ir−impoverished projectiles or masking sedimentary
processes (reworking and dilution of cosmic material; e.g.,
Rampino et al. 1997), and may be seriously modified by di
verse post−impact redistribution mechanisms (see K–Pg ano
maly cases in Racki et al. 2011). On the other hand, Kyte
(2002b) emphasized that since the chemostratigraphic pat
tern can be biased during diagenesis, physical tracers such as
spinels are restricted to a thin accretionary event horizon.
The impact cratering signature might also be more or less
Table 1. Interpretative status of selected diagnostic criteria for impact identification in the stratigraphic record, mostly in distal settings (for details
see Hallam and Wignall 1997; Koeberl and Martinez−Ruiz 2003; Simonson and Glass 2004; Claeys 2007; Reimold 2007; French and Koeberl 2010;
see also Racki 1999, Kaiho et al. 2006b, Newton and Bottrell 2007; Algeo et al. 2008; Racki et al. 2011; Koeberl et al. 2012; Glass and Simonson
2012; Reimold and Jourdan 2012).
Diagnostic character Postulated impact or impact−related character Alternative non−impact interpretation
Craters Site of hypervelocity meteorite strike Lacking
Abundant shocked quartz grains with
multiple planar deformation features
A momentary enormous release of energy in the
form of pressure of 10–30 GPa recorded in
unmelted ejecta
Iridium and other platinum group ele
ments (PGEs) in chondritic ratios
An extraterrestrial component, mostly distributed
in stratospheric dust
Microbial concentration, volcano−hydrothermal
activity, post−depositional redistribution, anoxia,
incorporation of ultramafic rocks (but in
non−chondritic PGE pattern)
Glassy spherules
Ballistically ejected droplets of melted target rock
and condensed rock vapor clouds: glassy
(microtektites) or a combination of glass and crys
tals grown in flight (microkrystites)
Meteorite ablation debris (?also volcanic droplets
and artificial contaminants, such as products of
metallurgical processes)
Fullerenes−caged noble gases with
planetary isotopic ratios (3He) Preserved component of impacting body Contamination from natural mantle−derived He
3He signal in sediments Input of extraterrestrial material (especially crucial
for comet showers) Contamination from natural mantle−derived He
Excess siderophile element (Ni, Co)
and Cr, Au, signatures Chemical signature from the projectile (see PGEs) Enrichment in target rocks and re−concentration
by post−impact hydrothermal activity
Ni−rich spinels A component directly derived from the projectile Lacking
Large negative d13C excursions Collapse of primary productivity (the dead
Strangelove Ocean model)
Methane ejections due to hydrate melting or fresh
water diagenesis, disruptions of the global carbon
Large negative d34S excursion
Gigantic release of “light” sulfur from the mantle
or sulfide−rich deposits, or an overturn of a strati
fied H2S−dominated ocean
Abrupt climatic change resulting in a drastic mix
ing event or sulfide−flux events due to
chemo−cline−upward excursion in a super−stagnant
ocean, volcanism
Thick and widely distributed
coarse−grained deposits Impact−induced tsunami waves
Seismically induced tsunami or extremely violent
storm events, fault, volcanism, submarine channel
infill at times of falling sea level
obliterated by large−scale geologic processes, in particular
plate subduction (e.g., Claeys 2007), and therefore the ex
tra−crater cosmic signal could survive solely in the form of
spherule horizons and/or PGEs enhancements in pre−Meso
zoic settings (e.g., Simonson and Glass 2004; Glass and
Simonson 2012). This low preservation potential is particu
larly envisaged for “crater−less”, deep−oceanic impacts
(Dypvik and Jansa 2003; Kent et al. 2003a, b), and a low−Ir
comet shower into the ocean was mooted as an attractive al
ternative explanation for an alleged, but Ir−poor, impact hori
zon (e.g., Jansa 1993; Rampino and Haggerty 1996a). How
ever, the geochemical signature of asteroids and comets, and
many other extraterrestrial indicators overall is similar in
both subaerial and submarine impacts (Dypvik and Jansa
2003; Koeberl 2007); a main exception is near absence of the
shocked quartz grains because of basaltic oceanic crust as a
main target (e.g., Simonson and Glass 2004; Claeys 2007).
This case is exemplified by the single identified abyssal
Eltanin asteroid impact into a 4 km deep Antarctic Ocean,
where Pliocene impact−disturbed ejecta−rich sediments con
tain Ir at abundances comparable with those of the K–Pg
anomaly (Gersonde et al. 2002; Kyte 2002a). Hassler and
Simonson (2001) claimed that an association of distinctive
sedimentary features indicating high−energy regimes, as a re−
sult of the impact−triggered tsunami, represents the best data
base on the reworked distal record of large−body oceanic im−
pacts (see reviews of tsunami modelling in Wünnemann et al.
2010 and Gisler et al. 2011). Furthermore, these experimen−
tal and theoretical studies have also improved our under
standing of cratering processes and their preservation in
open−ocean basins (see also Gersonde et al. 2002; Davison
and Collins 2007; Shuvalov et al. 2008). Craters can be pro
duced in the oceanic crust exclusively if the projectile is
large−sized enough compared to the target water depth, al
though their structure and morphology can diverge from the
land counterparts. For example, in the case of vertical impact
events at 20 km/s, these morphologic scars are formed when
the oceanic basin depth is ca. 5–7 times less than the projec
tile size (Gisler et al. 2011). In fact, Davison and Collins
(2007: 1925) found that, “the effect of the Earth’s oceans is
to reduce the number of craters smaller than 1 km in diameter
by about two−thirds, the number of craters >30 km in diame
ter by about one−third, and that for craters larger than >100
km in diameter, the oceans have little effect”.
The “great expectations
As discussed by Tsujita (2001), the single−cause impact hy
pothesis, when applied as a paradigm, can lead to force−fit
ting of subsequent observations and elucidations, appropri
ately referred to as the “great expectations syndrome”. An in
formative tale to the scientific community has been presented
in detail by Pintera et al. (2011); they critically analyzed the
Younger Dryas impact hypothesis to account for the decline
of Pleistocene megafauna and collapse of the Clovis culture.
Twelve main markers, acknowledged originally as signa
tures of a catastrophic bolide strike 12 900 years ago, have
been either (i) largely rejected (e.g., impact structure; mag
netic nodules in bones; elevated levels of radioactivity, irid
ium, and fullerenes) or (ii) suspected as representing terres
trial sources (e.g., carbon and magnetic spherules, byprod
ucts of catastrophic wildfire, nanodiamonds). Furthermore,
most of the alleged impact proxies have hitherto been dem
onstrated to be non−reproducible because the fingerprints
have been misunderstood as single synchronous spikes, al
though they probably resulted from inadequate sampling
methods (for details, see Pintera et al. 2011 and also Pigatia
et al. 2012).
In the context of this quasi−actualistic case study, and
topics raised by Morrow (2006), Twitchet (2006), Claeys
(2007), Reimold (2007) and French and Koeberl (2010),
among others, a refined evaluative approach to proper rec
ognition of extraterrestrial records is proposed. The misun
derstanding and misinterpretation symptoms are in fact ele
ments in the succession of three, partially overlapping, test
ing levels (Fig. 1):
1. Are there real impact proxies?
false impact
2. If so, are ?impact proxies synchronous with the extinciton horizon
3. If so, was the impact of appropriate size to cause global changes?
true impact
Fig. 1. Scheme of the three successive levels in the testing process, encom
passing application of the Alvarez impact theory of mass extinction, and
possible errors resulting from the “great expectations syndrome” (sensu
Tsujita 2001).
1. Factual misidentification: i.e., an erroneous or indefi
nite recognition of the extraterrestrial record in sedimento
logical, physical and geochemical contexts. French and Koe
berl (2010: 157) distinguished three types of errors that are
typically involved in impact studies: (i) incorrect identifica
tion of normal petrographic and mineralogical features (e.g.,
random or non−parallel fracturing in quartz); (ii) the applica
tion of non−diagnostic criteria (e.g., brecciation); and (iii) the
use of new, but unconfirmed, characters (e.g., fullerenes with
trapped He).
2. Correlative misinterpretation: the impact marker identi
fication is convincing, but interpretation of its timing is wrong
or appears to be inaccurate in the light of more constrained
dating, which decisively precludes the granted causes and ef
fects. This prerequisite is crucial especially for meteorite crat
ers. As reviewed by Jourdan et al. (2009, 2012), precise and
accurate radiometric dating of impact structures needs essen
tial qualitative and quantitative improvements, as demon
strated by many misleading ages and/or extensive age uncer
tainties. A more integrated approach is called for, such as the
palaeogeographic method of Schmieder and Buchner (2008).
According to Morrow (2006: 314), “a major challenge of im
pact studies is correlating distal evidence of an event to its
source crater, which often may be undiscovered or may have
been obscured or destroyed by active Earth processes (…)
confidently tying distal effects to a specific crater requires a
sophisticated integration of high−resolution stratigraphic, bio−
stratigraphic, radiometric, petrographic, and geochemical fin−
gerprinting technique”. Thus, identification of precisely dated
ejecta and/or extra−crater sedimentary record (tsunamites) and
their source crater is the most significant aim when trying to
connect an extraterrestrial event with a biotic crisis (see heavy
mineral correlation techniques in Thackrey et al. 2009). This is
precisely the problem raised for the Chicxulub impact crater
by Keller (2005, 2011); however, impact−related shelf margin
collapses, large−scale mass movements and fluidization of
sediments are processes that bias interpretation of surrounding
depositional areas (see Hassler and Simonson 2001; Dypvik
and Jansa 2003; Schieber and Over 2005; Claeys 2007; Kring
2007; Purnell 2009; Schulte et al. 2010).
3. Causal overestimation: the crater (or craters) and/or
other extraterrestrial tracers are approved to be coincident
with the biodiversity decline and other ecosystem collapse
attributes. However, the established impact size and pre
dicted destructive effects were clearly insufficient to trigger a
major, global deterioration of life (see below). Note that
Alvarez et al. (1980) rightly estimated the size of the impact
ing object using exclusively the scale of anomalous iridium
values, and three other independent sets of observations.
Both demands of high correlation precision and impact
magnitude threshold are invalid if many smaller collisions had
cumulative adverse climate−environmental effects over sev
eral million years, leading finally to mass extinction (lag−time
multiple impact hypothesis of McGhee 2001, 2005; see criti
cisms in Keller 2005 and Racki 2005). In the scenario of Poag
et al. (2002), the impact−produced late Eocene warm pulses
would have initially delayed biosphere response by interrupt
ing a long−term cooling trend, which led to the Oligocene
stepped extinctions attributable to threshold climatic condi
Two case histories
Diverse “great expectation” symptoms are outlined below
from four successive mass extinctions, but the mid−Devonian
and Paleocene–Eocene boundary examples represent out
standing introductary case histories.
The Eifelian–Givetian boundary.TheMiddletoLateDe
vonian interval comprises several biotic crises, mostly linked
with climatic and oceanographic changes, and especially an
oxia (see review in Walliser 1996). Impact proxies have been
recognized at the Eifelian–Givetian (E–G) boundary in Mo
rocco by Ellwood et al. (2003a; see Fig. 2). Based on a mag
netic susceptibility study, an ejecta layer has been proposed, as
determined by alleged shocked quartz grains in three sections,
in association with microtectites, an enrichment of chalcophile
elements and a large−scale negative shift in d13C.
The mid−Devonian impact was included in some review
papers (e.g., Simonson and Glass 2004: table 1), and even its
climatic consequences were lastly indicated by Giles (2012).
However, the apparent misidentification of the extraterrestrial
signature, in particular shock metamorphism, was noted by
Racki and Koeberl (2004: 471b), “The images identified by
Ellwood et al. [= Ellwood et al. 2003a] (...) as shocked quartz
grains are not convincing, and the orientation measurements
suffer from an insufficient number of observations” (also
French and Koeberl 2010: 151). Sections across the E–G tran
sition in the Ardennes (Belgium) have not yielded impact evi
dence (Claeys 2004). A succeeding paper by Schmitz et al.
(2006), which even includes some members of the Ellwood et
al. (2003a) group as co−authors, decisively rejected an extra
terrestrial origin of the Moroccan horizon because of low PGE
concentrations (e.g., Ir level of 0.28 ppb), coupled with indica
tions that post−depositional redox fronts shaped the chemo
stratigraphic pattern. This exclusively Earth−bound approach
is confirmed in the most recent study on the Moroccan site
(Ellwood et al. 2011), where the putative ejecta horizon is seen
as a record of a large−scale anoxic/organic carbon burial epi
sode (well known as the Kačák event; Walliser 1996).
The Science paper by Ellwood et al. (2003a) is an example
of perfectly circular arguments, as highlighted by Racki and
Koeberl (2004). The Kačák bio−event is marked by a modest
loss of biodiversity (see Fig. 2), interpreted by Walliser (1996)
as a third−order global event, largely limited to pelagic biota.
Despite this, a mass extinction status for this crisis was surpris
ingly approved by these authors in their article title, simulta
neously with the meteorite impact established at that time. In
essence, a supposed impact event was used to propose a mass
extinction level in the Devonian stratigraphic record.
Paleocene–Eocene boundary.—Another characteristic ex
ample is found in five papers published mostly in the re
nowned Earth and Planetary Science Letters. Kent et al.
(2003a, b) and Cramer and Kent (2005) postulated a comet
impact forcing for the Paleocene–Eocene thermal maximum
(PETM) and well−known negative carbon isotope excursion,
interpreted as result of massive input of isotopically light car
bon from a ~10 km volatile−rich projectile. The impact−pro
moted warming would also probably have initiated a thermal
decomposition of marine methane hydrates, and abruptly ac
celerate climatic changes.
Deep−sea benthic foraminifera suffer the concomitant
mass extinction, thought to have caused by corrosive and
warmed (and hence oxygen−depleted) bottom waters. As
supportive evidence for the devastating oceanic impact, Kent
et al. (2003a, b) considered: (i) abnormal abundance of mag
netic nano−sized particles, inferred to have originated from
an impact−generated plume condensate, (ii) a small, but sig
nificant, Ir anomaly (0.14 ppb, in one section only), and (iii)
the extremely rapid onset of the initial d13C shift (see also
Cramer and Kent 2005).
As discussed by Dickens and Francis (2003), all these se
lectively used, indirect markers seem highly unlikely for a
cometary impact across the PETM. In particular, the anoma
Percent of genus extinction
Givet. Frasnian Famennian
50 km
La Moinerie
mass extinction
Devonian craters
of Ellwood et al.
Devonian background
Fig. 2. Crater temporal distribution, with possible record at the F–F boundary (A), plotted against Devonian biodiversity losses in terms of substages (B),
data from Bambach 2006: fig. 1 (used with permission from the Annual Review of Earth and Planetary Sciences, Volume34 © 2006 by Annual Reviews,, re−arranged according to the timescale of Kaufman (2006; see the updated tiiming in Becker et al. 2012; Fig. 4); the recon
structed middle Frasnian Alamo crater is also shown to reveal low biodiversity loss in that time (arrowed), as well as the controversial Woodleigh impact
structure (see Fig. 5) and the biostratigraphically dated Flynn Creek submarine crater (Schieber and Over 2005). Vertical lines correspond to possible tem
poral ranges. Abbreviations: Carb., Carboniferous; Givet., Givetian; Lochk., Lochkovian; Prag., Pragian; Silur, Silurian.
lous content of single−domain magnetite on the coastal shelf
can be explained through a sudden accumulation pulse of
exhumed, bacterially−derived magnetite grains during fine−
grained terrigenous deposition, probably paired with diage
nesis in unsteady redox settings. Furthermore, a sole volcanic
mechanism is strongly favoured by Schmitz et al. (2004)in the
light of comprehensively studied PGE and osmium, helium,
and strontium isotopic records, also due to the proved syn
chroneity of the d13C value decrease with the onset of basaltic
volcanism. The Ir−enriched volcanic ashes (0.22–0.31 ppb) in
the critical interval were related to the major episode of flood
basalt eruptions, in connection with the seafloor spreading
phase in the high−latitude North Atlantic. Consequently,
“there is zero incontrovertible evidence” (Dickens and Francis
2003: 199) for an extraterrestrial trigger of the extraordinary
biogeochemical and climatic perturbations in earliest Ceno
zoic time.
The Late Ordovician mass
The Late Ordovician mass extinction, initiated 445.6 Ma ago,
is essentially free of impact evidence, as seen in negligible Ir
enrichments (Hallam and Wignall 1997: 57) and microsphe−
rules (French and Koeberl 2010: 152). An extra variety of cos−
mic killing stimulus has been postulated by Melott et al.
(2004): gamma ray bursts intensively irradiated the Earth’s
surface to result in ozone depletion and ultimately lead to di
sastrous Late Ordovician global cooling.
Surprisingly, asteroid breakup tracers (small−sized crat
ers, extraterrestrial chromite grains, Os isotopes; also world
wide mass movements at continental shelf margins; Purnell
2009) are conspicuous in their frequency in older Ordovician
intervals, and Schmitz et al. (2008) argued that the asteroid
shower in fact accelerated the Great Ordovician Bio
diversification Event. On the other hand, there are several
impact craters, the largest being 30 km in diameter, dated at
455–450 Ma, i.e., in the eventful Katian prelude of the global
biodiversity change (see Kaljo et al. 2011 and Voldman et al.
2012: fig. 3).
The Late Devonian mass
McLaren (1970) proposed a bolide impact scenario, with giant
tsunamis as the main mass killing agent for Frasnian reef biota,
but this idea was not considered seriously. An exhaustive
search was initiated in the 1980s for evidence of a cosmic ca−
tastrophe as the prime cause of the F–F mass extinction [=
Kellwasser (KW) crisis; see review in McGhee 1996]. Several
Fig. 3. Extraterrestrial elemental proxy Ir, and supplementary Ni, against other geochemical markers in the F–F boundary beds at Kowala, Holy Cross
Mountains (after Racki et al. 2002: fig. 8; used with permission from Elsevier); Ir values from an unpublished report (dated 2004) by Yuichi Hatsukawa and
Mohammad Mahmudy Gharaie; Ni contents from Racka (1999: table 2); for other data see references in Racki et al. (2011).
assumed extraterrestrial proxies (e.g., negative d13C excur−
sions and violent high−energy events) were proved to be in−
conclusive in subsequent studies (Table 1; McGhee 1996;
Hallam and Wignall 1997; Racki 1999). In the Luoxiu section,
southern China, a fourfold increase in Ir values with a 0.24 ppb
spike has been observed to coincide with the F–F boundary
(Wang et al. 1991). In the Kowala succession of Poland, a
newly recognized eightfold Ir enrichment merely reaches a
maximum of 0.08 ppb (Fig. 3). Ir anomalies hitherto reported
precisely at the major crisis boundary are causally linked with
volcano−hydrothermal sources, sedimentary starvation, redox
variations and diagenetic enhancement (Racki 1999; Over
2002; Hatsukawa et al. 2003; see also Ma and Bai 2002;
Gordon et al. 2009; Zeng et al. 2011).
An extreme example of Ir enrichment (4 ppb) is seen in
the western Canadian Long Rapids Formation, but this is at
85 cm below the F–F boundary (Levman and von Bitter
2002). In fact, PGE anomalies and spherules occur either in
the Famennian, postdating the KW crisis by 1.5 Ma, or below
the stage boundary (see summary in McGhee 1996). Simi
larly, four discrete levels of probable microtektites around
the F–F boundary in southern China do not seem to be di
rectly associated with the stepwise KW crisis (with only a
minor microspherule peak near the F–F boundary; Ma
and Bai 2002). What is more, the basal Famennian, Si−rich
microtektites from the Ardennes (Claeys et al. 1992) were
even suspected to reflect sample contaminants (industrial
glass beads; Marini and Casier 1997), but this seems less
probable in the light of a comparative compositional study
(Marini 2003; Glass and Simonson 2012). More recently, the
Os isotopic composition has been shown to lack a significant
meteoritic component in F–F passage beds examined in
western New York (Gordon et al. 2009). However, a suffi−
cient temporal resolution of the chemostratigraphic signature
has been called into question, when this is thought of asproof
against the extremely short−term extraterrestrial event (com−
pare the refined end−Cretaceous Os isotopic fingerprinting in
Robinson et al. 2009 and low−resolution F–F data in Turgeon
et al. 2007).
All Late Devonian craters are well below 100 km in diam−
eter (Fig. 2). The 52.7 km (or 65–75 km; Reimold et al. 2005)
diameter Siljan Ring structure in Sweden, has been repeat
McGhee (2001)
Jourdan et al. (2012)
Reimold et al. (2005)
Keller (2005)
McGhee (1996)
368 ± 1
361 1.1±
378 ± 4
Jourdan et al. (2009)
Reimold et al. (2005)
error range
380.9 ± 4.6
timescale after
Kauffman (2006)
timescale after
Becker et al. (2012)
Fig. 4. Evolving timing of the Siljan Ring (53 km diameter; see Fig. 2), depending on different timescales and improved radiometric dates.
Mory et al.
Uysal et al.
Glikson et al.
Renne et al.
359 4±
F-F 364 8
50 km
age of
age of
Fig. 5. Evolving timing of the multi−ring Woodleigh impact structure, mani
fested in purported causal connection with the P–T and F–F mass extinc
tions, as a reflection of variously dated processes. Age constraints still
range from post−Middle Devonian to pre−Early Jurassic, but the connection
with the D–C global event seems to be most likely (Glikson et al. 2005).
edly seen as the major F–F impact site since Nature paper by
Napier and Clube (1979; see Raup 1992; McGhee 1996), but
it was variously dated between middle Frasnian to Devo
nian–Carboniferous transition (Fig. 4). The more recently
re−examined laser argon date of Siljan melt breccia (377±2
Ma, Reimold et al. 2005), constrained by Jourdan et al.
(2012) to 380.9±4.6 Ma, has doubtfully placed this bolide
strike within error at the F–F boundary in the recalibrated
Devonian numerical timescales (376.1±3.6 Ma, Kaufman
2006; 372.24±1.63 Ma, Becker et al. 2012). In the light of
previous correlative pitfalls, exclusively direct conodont dat
ing of undoubted Siljan impact ejecta will be only decisive,
but neither proximal−distal sedimentary effects nor ejecta
have been firmly identified: the suspected spherule−bearing
level in the Belgian Ardennes distinctly postdates the key
time boundary (Claeys et al. 1992; McGhee 1996), and a
similar finding above the upper KW level has been noted
from Morocco (Ellwood et al. 2003b). On the other hand,
several coarse−grained intercalations are known around the
F–F boundary in Pomerania, ~700 km south from the Swed
ish impact site, but these have been attributed to thelowstand
collapse of a carbonate platform edge (Matyja and Nar
kiewicz 1992; see also review in Racki 1999: 618).
Even more questionable is the Woodleigh multi−ring struc−
ture in Western Australia (Uysal et al. 2001; Fig. 5). Accord−
ing to Renne et al. (2002: 247), its size is poorly constrained
and subject of an ongoing debate (between 40 and 120 km),
while the age “could have been much older than mid−Devo−
nian” (see also Hough et al. 2003; Reimold et al. 2003;
Glikson et al. 2005; Uysal et al. 2005).
Consequently, a multiple impact scenario, complementary
to the ecosystem destabilization due to Earth−derived stresses,
is hypothesized as the only option able to explain the observed
data (see discussion in McGhee 1996; also Sandberg et al.
2002; Alvarez 2003). As a meaningful alternative to the sole
impact hypothesis, McGhee (2001, 2005) speculated that the
stepwise F–F extinctions were triggered by a rapid drop in
global temperature (impact winter) that followed on an anom
alous greenhouse interval caused by several mid−Frasnian im
pacts. However, this climate prognosis is not supported by the
palaeotemperature curve of Joachimski et al. (2009) that
shows a gradual Frasnian warming trend (see also Keller
2005). In fact, such proponents of lag−time biotic response
have failed to provide a trustworthy model for why impacts
should be cumulative over millions of years in their deteriorat
ing effect (Reimold 2007; Prothero 2009).
The impact theme is still referred to in Late Devonian ex
tinction studies (Casier and Lethiers 2002; Sandberg et al.
2002; Ellwood et al. 2003b; McGhee 2005; Du et al. 2008;
Zeng et al. 2011). Thus, Alvarez (2003: 155, table 1) consid
ered this epoch to have been characterized by “(…) substan
tial evidence of impact in that ca. 16 Myr interval” (see also
Glikson 2005). There is overall consensus, however, that the
destructive impact of extraterrestrial catastrophic factors on
the generally stressed Frasnian marine biosphere was un
likely (Racki 1999, 2005; Ma and Bai 2002; Reimold et al.
2005; Morrow 2006; Gordon et al. 2009). Still, the final con
clusion by McGhee (1996: 244) is compelling: “It is thus
puzzling, and not a little frustrating, to note at this point the
best evidence yet produced in the worldwide search, i.e.,
microtektite layers, points to impacts that both occur after the
most critical biological intervals of the Frasnian–Famennian
crisis had passed (…). The impacts are not principal killers in
the mass extinction”.
The End−Permian mass extinction
Most current studies and the continuing multi−theme dispute
have focused on the apocalyptic “Mother of all Mass Extinc
tions” at the end of the Paleozoic Era. Merely the extreme
abruptness of ecosystem disruption is a priori seen as suffi
cient to imply an extraterrestrial control (e.g., Jin et al. 2000).
Chapman (2005) argued, “(…) absent countervailing evi
dence or some other equally sudden, energetic modifier of the
ecology (no terrestrial alternative is so sudden or energetic),
presumption must favor the inevitable NEO [near−Earth ob
jects] impacts to explain mass extinctions”.
To date, comprehensive results are still elusive. Misidenti−
fication is ascribed to supposedly unaltered micrometeorites
and an ejecta blanket stratum with shocked quartz, spherules,
enhanced Ir and siderophiles and fullerenes trapping extrater−
restrial noble gases, as recently reviewed in depth by French
and Koeberl (2010; see also Koeberl and Martinez−Ruiz 2003;
Keller 2005; White and Saunders 2005; Coney et al. 2007;
Koeberl 2007; Reimold 2007; Ward 2007: 67–81). Interest−
ingly, firstly identified fullerenes from P–T black claystone in
Japan have been interpreted by Chijiwa et al. (1999: 767) as a
primary terrestrial combustion because the C60 “(...) likely
synthesized within locally anoxic zone in the extensive wild
fires on the supercontinent Pangea and deposited on an anoxic
deep−sea floor of the superocean Panthalassa” (see also Li et
al. 2005; Yabushita and Kawakami 2007).
The first reports from China postulated an Ir excess of up to
8 ppb (see Hallam and Wignall 1997: 131), but sophisticated
analyses in several successions around the globe exhibit con
tents not significantly above crustal values (below 0.2 ppb),
and PGE patterns favouring a basaltic volcanic source (Koe
berl et al. 2004; Coney et al. 2007; Xu et al. 2007; Yabushita
and Kawakami 2007; Brookfield et al. 2010). Osmium isotope
ratios do not have extraterrestrial characteristics, nor does the
3He signature argue for a cometary episode (Koeberl et al.
2004; Farley et al. 2005; Georgiev et al. 2011). No convincing
evidence has been found to corroborate the predicted huge de
livery of isotopically light sulfur from the penetrated mantle,
as a result of an enormous impact of a 30 to 60 km sized aster
oid (or a 15− to 30−km in diameter comet) on the ocean that
produced a ~600 to 1200 km crater (Kaiho et al. 2001, 2006a;
see critical discussion in Koeberl et al. 2002). The interpreta
tion of the large negative d34S anomaly, perhaps induced by an
upwelling of euxinic deep−ocean water masses or chemocline
upward−shift, is still unclear (see Newton et al. 2004; Kaiho et
al. 2006a, b; Newton and Bottrell 2007; Algeo et al. 2008; Luo
et al. 2010).
Several possible impact craters have been postulated for
this mass extinction time (see the recentmost summary in
Barash 2012 and Tohver et al. 2012). The initially proposed
120 km−sized Woodleigh impact structure (Mory et al. 2000)
is now known to be closer in age to the Late Devonian mass
extinction (Fig. 5). The next aspiring site to “a smoking gun”
was the so−called Bedout impact structure, located offshore
northwest of Australia (Becker et al. 2004). However, this
claim was discarded for a range of reasons, including the ab
sence of undisputed shocked grains and impactites (e.g.,
Renne et al. 2004; French and Koeberl 2010), as well as
vague isotopic dating (Jourdan et al. 2009). The more refined
geophysical assessment of the puzzling Bedout High reveals
its genetic link with two rifting episodes roughly perpendicu
lar to each other (Müller et al. 2005).
The Wilkes Land crater of East Antarctica (von Frese et al.
2009) is another highly speculative, giant impact site, based
exclusively on satellite geophysical data. The age interpreta
tion of this major positive free−air gravity anomaly, over a
~500−km diameter sub−ice depression, is very uncertain. The
inevitable correlation with the “Great Dying” is based on com−
monly rejected micrometeorite evidence in Antarctica by
Basu et al. (2003; see French and Koeberl 2010). In addition,
von Frese et al. (2009) stressed coeval antipodal volcanism of
the Siberian Traps and were tempted to associate causally this
collision with the development of the hot spot beneath the
thick cratonic lithosphere that initiated the cataclysmic flood
basalt activity. The exact mechanism of the proposed trigger
remains cryptic. The impact volcanism hypothesis, however,
including computer simulations, is a frequently returning mo
tif since the start of the mass extinction debate (e.g., Öpik
1958; Rogers 1982; see Palmer 2003: 220), as seen in variety
of current views (e.g., Glikson 2005; Jones 2005; White and
Saunders 2005; Chatterjee et al. 2006; French and Koeberl
2010). Even if there is no credible statistical correlation be
tween hypervelocity impacts and extrusive activity (Kelley
2007; see also Tejada et al. 2012), and thus no reason to advo
cate a persistent causative link, this testable model is espe
cially attractive for oceanic igneous intrusions developed on
young thinned crust (Glikson 2005; Jones 2005). Large−body
meteorite and cometary impacts may rather only accelerate the
pulsed, long−term volcanic intensity from active mantle
plumes (Abbott and Isley 2002), because of shock−induced
melting and extra decompression melting of the heated target,
sub−crater mantle (Jones 2005).
Thus, data available are not compatible with the exis
tence of an impact event of an apocalyptic scale at the P–T
mass extinction boundary, and recurrently presented hy
potheses are unverified or premature at best (French and
Koeberl 2010). As pointed out by Erwin (2006: 216), “Im−
pact enthusiasts claim that the simplest explanation is that
an impact triggered the Siberian Flood basalt. That would
certainly be an interesting result, and may be the only way
the Permian will ever succeed in Hollywood, but nothing
we know about either the Siberian volcanism or impacts
provides much support”.
within Rhaetian
ICS 2009
Carnian crisis
Saint Martin
Wells Creek Cloud Creek
50 km
seismites (?)
ejecta Manicouagan
Ramezani et al. (2011)
Fig. 6. The Late Triassic cratering record plotted against extinction events (based on Lucas and Tanner 2008: fig. 8; crater dates modified after Schmieder
and Buchner 2008 and Martin Schmieder personal communication, 2011) and two alternative time scales. Note that the 100 km−sized and precisely dated
Manicouagan crater (214.56±0.05 Ma; see ottawa−−Articles− Manicouagan) is within the age range of the end−Carnian
extinction only in the ICS 2009 geochronologic scheme (see also Lucas et al. 2012). Carbon isotope events compiled from Tanner (2010) and Ruhl and
Kürschner (2011: fig.1). Vertical lines correspond to possible temporal ranges. J., Jurassic.
The End−Triassic mass extinction
Significant progress is noted in the study of the T–J mass ex
tinction boundary. In this case, there are also a series of papers,
published mostly in Science, that advocate the application of
the Alvarez impact theory, based largely on circular reasoning
(Hallam and Wignall 1997; Koeberl and Martinez−Ruiz 2003;
Tanner et al. 2004; White and Saunders 2005; Ward 2007:
93–102). The debate was opened by Olsen et al. (1987), who
argued that the large, 100 km diameter Manicouagan impact
structure in Quebec (Fig. 6), one of the best−preserved and
largest Earth craters (in the “top four” to date; Spray et al.
2010), is broadly correlative with the T–J boundary, and thus
the inescapable contributory candidate for this mass extinction
(e.g., Raup 1992). It was hardly surprising that the extraterres
trial cataclysm concept was strengthened five years later when
quartz grains containing multiple PDFs were described from
northern Italy by Bice et al. (1992: 443), who believed in “at
least three closely spaced impacts at the end of the Triassic”.
This finding has been questioned (Hallam and Wignall 1997:
157); there is no subsequent confirmation, nor have other,
more reliable, data been presented on the ejecta spanning the
T–J boundary (e.g., Mossman et al. 1998).
Radiometric dating of the Manicougan crater (214±1 Ma;
Hodych and Dunning 1992; 214.56±0.05 Ma; Jourdan et al.
2012) has shown that the impact event substantially predated
the mass extinction event by 13 Ma. However, it does not
seem to preclude any correlation with changes in the con−
temporary biosphere (as quoted by Tsujita 2001 and Kring
2003), although the flawed timescale of this epoch (see Fig. 6)
has not allowed a confident correlation. Already Hodych and
Dunning (1992) proposed another viable hypothesis: the
Manicougan impact may possibly have participated in an ear
lier biotic turnover spanning the Carnian–Norian boundary
(also e.g., Rampino et al. 1997), whilst others considered a co
incidence with the end−Norian extinction (e.g., Sephton et al.
2002; Tanner et al. 2004). In actuality, several different−sized
impact structures predate the T–J extinction boundary (Fig. 6),
and the poorly dated, 80 km diameter Puchezh−Katunki struc
ture may be allegedly added to the impact set (Pálfy 2004;
Schmieder and Buchner 2008). Although their ages scatter
considerably, Spray et al. (1998) proposed a multiple impact
event, caused by fragmented comets or asteroids colliding
with the Earth 214 Ma ago, and recorded five impact struc
tures (see critical discussion in Jourdan et al. 2012).
Thus, a causal link with the end−Carnian crisis remains a
possibility (see updated data in Fig. 6; Tanner and Lucas
2004). Importantly, an ejecta blanket horizon with shocked
quartz and spherules in the Upper Triassic of southwest Eng
land (Walkden et al. 2002; Kirkham 2003), dated at 214±2.5
Ma, is within the range of the Manicouagan impact event.
This relationship, approved by heavy mineral correlation
(Thackrey et al. 2009), provides a potential reference to
high−resolution verification of the scenario postulated by
Hodych and Dunning (1992) and Spray et al. (1998). How
ever, there are two basic uncertainties linked with this prom
ising association: (i) the biotic turnover magnitude is disput
able (Hallam and Wignall 1997: 144–147; Brusatte et al.
2010; Hunt et al. 2002; Irmis 2011), and (ii) there is great dis
agreement on the age of the Carnian–Norian boundary, 228
Ma having recently been proposed by Ramezani et al. (2011;
see discussion in Lucas et al. 2012). If this date is correct, the
Manicouagan impact lethal effects may be causally linked,
according to Olsen et al. (2011: 223), with “an abrupt though
modest turnover” in late Norian tetrapod diversity.
Another hopeful correlation (Martin Schmieder, personal
communication 2011) concerns the re−dated 23 km diameter
Rochechouart crater (Schmieder et al. 2010; Smith 2011;
see Fig. 6) and 2–4 m thick seismite/tsunamite deposits of
Rhaetian age covering ca. 250 000 km2in England, Ireland,
and France (Simms 2007). In addition, Hori et al. (2007) re
ported possibly impact−related PGEs enrichment in upper
most Rhaetian deep−sea sediments of Panthalassa (Ir peak
at 0.07 ppb), linked to the first phase of radiolarian crisis.
Also Ruhl and Kürschner (2011) demonstrated late Rhaetian
(“precursor”; Fig. 6) disruption of the carbon cycling in ma
rine and continental, preceding the lastly highlighted, cata
strophic commencement of eruptive volcanic activity in the
Central Atlantic Magmatic Province (CAMP; see below).
Vegetation changes prior to the stage boundary additionally
argue against a single cataclysmic episode across the T–J
transition (McElwain et al. 2007).
Discussion of an end−Triassic impact hypothesis was
briefly revived, again in a Science article, by Olsen et al.
(2002), who showed elevated levels of Ir in the eastern USA
at the T–J boundary (ses also “fullerene data” in Perry et al.
2003). The “modest” anomaly of 0.285 ppb is coincident
with a fern bloom in palynological signature. Also in Sci
ence, Ward et al. (2001) argued for a catastrophic productiv
ity collapse at the stage boundary recorded in the d13C value
decrease, synchronized with an abrupt extinction pulse
among radiolarians. The negative carbon isotope anomaly is
better explainable by an input of 12C−rich carbon in effect of
methane release from different sources (see Whiteside et al.
2010; Ruhl and Kürschner 2011), combined with massive
CO2outgassing from the CAMP extrusives (Tanner 2010;
Sobolev et al. 2011) The dramatic end−Triassic radiolarian
diversity collapse has recently been questioned by Kiessling
and Danelian (2011) on the basis of analyzed extinction dy
namics (but see Wignall et al. 2010).
Tanner et al. (2008) also discovered multiple PGE en
hancements in latest Triassic to Jurassic−age strata of eastern
Canada, with a distinctive Ir peak of 0.45 ppb. The authors
found no compelling support for an extraterrestrial source for
the enrichment levels, and instead proposed redox control.
The onset of the CAMP furthermore provides a source for the
fairly small PGEs excess observed, that this non−impact sce
nario is clearly visible also in the marine Os isotope record
and 3He proxy (Farley et al. 2002; Kuroda et al. 2010). In
summary, the evidence linking end−Triassic impact event(s)
and extinction(s) is still much disputed, and refers rather to
pre−extinction biotic events (Fig. 6).
As highlighted by Sephton et al. (2002), Lucas (2006) and
Lucas and Tanner (2008), multiple extinction pulses occurred
throughout at least 20 Ma (Fig. 6). They were partly tied to
biogeochemical and climate perturbations (Sephton et al.
2002; Cleveland et al. 2008; Kidder and Worsley 2010; Calle
garo et al. 2012; Dal Corso et al. 2012), also with a severe im
plication for real biotic magnitude of the T–J boundary event
(Bambach 2006; Lucas and Tanner 2008). On the other hand,
Kiessling et al. (2007: 219–220) concluded from detailed
analyses based on the Paleobiology Database: “The enigmatic
end−Triassic extinction is confirmed to represent a true mass
extinction characterized by both elevated extinction rates and
reduced origination rates”, although probably “(…) gradual
processes added to the diversity decline from the Norian–
Rhaetian to the Rhaetian–Hettangian stage boundaries” (see
also Arens and West 2008; Alroy 2010; Ros and Echevarría
2012). Regardless of the fact whether the Late Triassic biotic
pattern was indeed determined by multiple crises or not, the
carbon isotope data strongly suggest a large−scale ecosystem
turnover only across T–J transition time (e.g., Ward et al.
2001; see Tanner 2010; Fig. 6)
Lessons from the submarine
Alamo impact
The accounted data clearly confirm that a third level of im−
pact−extinction connection testing (Fig. 1) is practically pre−
cluded for all mass extinctions, with the impressive exception
of the K–Pg global event. However, as a kind of in−depth falsi−
fication, it is granted herein that in the results of subsequent re
visions of the stratigraphic timescale, paired with timing re−
evaluation of craters (ascertained by biostratigraphic dating of
ejecta), the lethal relationship could be potentially demon
strated for recently identified impact structures of similar age.
The Alamo impact in south−central Nevada is unique in that
conodonts have provided confident dating of its widely dis
tributed (al least 28 000 km2) proximal ejecta (the Alamo
Breccia): middle Frasnian Palmatolepis punctata Zone (i.e.,
382 Ma; see Fig. 2), which thereby offers an excellent oppor
tunity to examine biotic consequences.
The Alamo impact and its aftermath.—A relatively large
bolide, probably a comet, crashed into the Earth in a carbon
ate shelf slope−to−basin setting, coincidentally like the Chic
xulub impact site. Crater−scaling approximations, based on
excavation depth (>1.7 km), suggest a minimum crater size
of 44 to 65 km in diameter (and a maximum, outer diameter
limit of ~150 km; Morrow et al. 2005; Pinto and Warme
2008). The impacting object penetrated a shelf−slope sedi
mentary succession beneath the 300 m deep seafloor, down
to at least Upper Cambrian strata, which comprise mostly
dolostone and limestone, supplemented by sandstone and si
liceous rocks (see Morrow et al. 2005: fig. 3). As guided by
the simulated environmental catastrophe at the aftermath of
the Chicxulub event (e.g., Hildebrand et al. 1991; Kring
2003, 2005; see below), comparing well with contact meta
morphism around volcanic intrusions in carbonates and or
ganic−rich rocks (Ganino and Arndt 2009; see also Arthur
and Barnes 2006), the thermally shocked pre−Alamo impact
sedimentary suite released voluminous climatically active
gases and potentially lethal volatiles (CO2,CH
4, hydrocar
bons) and vaporized water (see McGhee 2005: 41). Thus, the
highly vulnerable target strata provide a possible association
with sudden environmental traumas, and it is not unreason
able to assume a comparable “kill potential” for other pres
ently known Devonian and Triassic wet−target impact events
in subtropical carbonate shelves.
In contrast, the possible coeval Siljan bolide struck (Fig.
2) a continental region in the south−eastern periphery of
Laurussia (Fennoscandian High), formed in Precambrian
crystalline basement covered by Ordovician conglomerate
and limestone and Silurian shale and sandstone (Reimold et
al. 2005), and likely associated with comparatively low vol
atile fluxes. A similarly two−layered target within shield ter
rains, predominantly Precambrian crystalline rocks with a
thin (< 200 m) cover of Ordovician carbonates and shales
(Spray et al. 2010 ), in the arid intra−supercontinental set−
ting of Pangea, characterized the larger Manicougan impact
(see modelled biotic damage of this “lucky” event in Walk−
den and Parker 2008; Kring 2003). The insufficiently rec−
ognized complex Woodleigh structure in the Carnarvon Ba−
sin is within sandstone− and dolomite−dominated sedimen−
tary strata, overlying a granitoid basement (Uysal et al.
Despite the volatile−prone nature of its target rocks, the
Alamo impact did not produce an ecosystem collapse even in
adjacent shelf regions. As summarized by Morrow et al.
(2009), this unexpected conclusion is based on thorough
analysis of pre− and post−impact assemblages of ostracods,
stromatoporoids, brachiopods, corals and ichnofaunas (e.g.,
Casier et al. 2006). Furthermore, fragile stromatoporoid−
coral reef biotas rapidly recovered directly at top the Alamo
Breccia, which provides “(…) direct local evidence that the
Alamo event apparently had no major, long−lived negative
affects on shallow benthic ecosystems” (Morrow et al. 2009:
107). It is also difficult to decipher the influence this impact
may have had on global climate during a contemporaneous
cooling trend (Pisarzowska and Racki 2012).
Magnitude of impact versus its terrestrial setting.—The
data above on the Chicxulub−like (in general terms of localiza
tion features) impact catastrophe from sensitive reef ecosys
tem correspond to well−constrained, negligible lethal conse
quences of the late Eocene cluster of two very large craters,
Popigai (100 km diameter) and Chesapeake Bay (90 km; see
Poag 1997; Kring 2003). Conversely, the devastating Chicxu
lub impact specifically struck 3−km thick evaporate−rich target
lithologies within the carbonate shelf (e.g., Kring 2005;
Claeys 2007), that propelled into the stratosphere a dense sul−
fate aerosol clouds produced by the interaction of S−rich gases
and water vapor, with significant climatic effects (see model−
lings in Toon et al. 1997 and Pierazzo et al. 2003, among oth−
ers). Simulations show that also ozone destruction due to le−
thal nitric oxide addition resulted from substantial changes in
atmospheric chemistry generated by the shock−heated air, but
likely supplemented by release of chemically activated halo
gens (chlorines, bromines) from the vaporized target rocks.
Therefore, an increase in ultraviolet radiation is expected to
have happened several years after the impact (e.g., Ishida et al.
2007; Kring 2007).
Strikes of a cosmic body into the vast oceanic basins
should be more than twice numerous as continental impacts,
and this critical point was immediately recognized in the K–Pg
debate (Emiliani et al. 1981; Rogers 1982). However, the pos
sible kill mechanisms, in particular a climatic response, are
still poorly understood (Gersonde et al. 2002; Kring 2003;
Dypvik et al. 2004; Gisler et al. 2011). These cataclysmic
events differ in several respects from those in subaerial set
tings (as reviewed already by Croft 1982), and some lethal ef
fects could be buffered by water mass screen (Arens and West
2008). A huge volume of shock−vaporized oceanic water, and
sediment and mantle rocky debris might have been ejected
into the stratosphere (see simulations in Toon et al. 1997; Saito
et al. 2008, Pierazzo et al. 2010 and Gisler et al. 2011), paired
with mega−tsunami waves (Gersonde et al. 2002; Dypvik and
Jansa 2003; Wünnemann et al. 2010). Prolonged residence of
vaporized water in the stratosphere, an important greenhouse
gas, is especially hazardous, and, according to the most recent
calculations of Gisler et al. (2011: 1187): “The vaporized wa−
ter carries away a considerable fraction of the impact energy in
an explosively expanding blast wave which is responsible for
devastating local effects and may affect worldwide climate”.
Furthermore, stored halogens from sea salt included in seawa−
ter vapor can generate deleterious changes in upper atmo−
spheric chemistry. Stress on the global biosphere, attributed to
multi−year ozone layer depletion is suggested by modelling of
Pierazzo et al. (2010) even for medium−size (1 km) asteroid
impacts in the mid−latitude ocean.
As simulated numerically by many authors (e.g., Toon et
al. 1997; Collins et al. 2005; see review in Kring 2007), only
most energetic impacts forming craters much larger than 100
km were capable of causing a catastrophic biodiversity loss on
a planet−wide scale (see discussion of the extinction−impact
curve in Raup 1991, 1992; Jansa 1993; Poag 1997; Rampino
et al. 1997; Kring 2003; Keller 2005; Kelley 2007; Prothero
2009). In this context, the South African Morokweng impact
structure, originally described as one of the largest Earth im
pact sites (with an overestimated size of up to 340 km in diam
eter; Koeberl et al. 1997), could only be reasonably causally
tied to minor biotic events at the “major” Jurassic–Cretaceous
boundary extinction (20% genus extinction; Bambach 2006),
when its correct size (70 km) was established.
Reimold (2007: 28) pointed out that “neither the impact
magnitude threshold, above which global mass extinctions
must be expected, has been constrained, nor do we under
stand exactly why the K/P event (related to the 200 km
Chicxulub impact structure) was of such lethal effect”. On
the other hand, Ellwood et al. (2003c: 539), for example,
- global climate (greenhouse vs. icehouse)
- biosphere structure and biodiversity
- ocean chemistry, level and circulation mode
- plate distribution
extinction risk
- land vs. shallow sea vs. ocean
- target composition, geology and rheology
- surface morfology
- regional climate and biodiversity
- projectile size (= energy release)
- impact angle and speed
- compositional type: asteroid vs. comet
Fig. 7. Major factors that control impact kill potential and extinction risk, relating what, how, where and when the Earth was struck, with emphasis on under
estimated vulnerability variables reflecting impact place (global positioning) and its timing (moment in biosphere history; based, in part, on Walkden and
Parker 2008: fig. 5; see also Kring 2003); arrow thicknesses reflect a predicted influence scale.
noted for the K–Pg boundary event that “the chance of such
extinctions occurring is tied to a unique set of circum
stances”. In fact, a complex shift from exclusively regional to
different−scale global environmental perturbations should be
thoroughly considered in the cause−effect context. This haz
ard gradation was related directly to impacting projectile
characteristics over a variety of kinetic energies (= thermal
shocks) and compositional/structural types (e.g., Toon et al.
1997; Wilde and Quinby−Hunt 1997; Kring 2003; Gisler et
al. 2011), but also indirectly to a set of terrestrial circum
stances: target rocks, geographic and plate−tectonic location,
global climate, biosphere resilience (e.g,. provinciality),
oceanographic setting jointly with ocean chemistry mode
(see Sobolev et al. 2011), among others (Fig. 7). This key im
pact aspect for predicted extinction severity was mentioned
by several authors (e.g., Raup 1992: 87; Poag et al. 1997:
585–586; Toon et al. 1997: 51; Sephton et al. 2002; Ellwood
et al. 2003c; Kent et al. 2003a: 23; Pálfy 2004: 144; Tanner et
al. 2004; Kring 2005; Arthur and Barnes 2006). Surprisingly,
the essential difference of impact cratering process in vola
tile−free (= crystalline rocks) and volatile−rich (= sedimen
tary rocks) target lithologies was quantitatively studied as
early as by Kieffer and Simonds (1980). However, more ho−
listic approach to estimating seemingly unpredictable biotic
effects was developed only by Kring (2003) and Walkden
and Parker (2006, 2008), who doubt that crater size is the
sole reliable proxy for collision “destructive power” (sensu
Raup 1991). Kring (2003: 124) stressed importance of sub−
stantially evolving environmental states and ecosystem
structures, because: “the environmental outcome of an im−
pact event and subsequent biologic effects are a function
of Earth’s ambient conditions, not just the energy of the im−
pact event”. Walkden and Parker (2006, 2008) particularly
scoped on the geographic (surface conditions, shallow geol
ogy, basement) and timing factors in geological history and
world biodiversity evolution, paired with climatic regime.
So, the basic questions for the “kill potential” are where,
what, and when the bolide struck (Walkden and Parker 2006,
2008). Thus, the destructive potential of the largest continen
tal impacts, Manicouagan and Popigai, is far removed from
the threshold for mass extinctions (see Sephton et al. 2002;
Kring 2003, 2005; Tanner et al. 2004). The more realistic
prediction of lethal hazard should contain not only impact
characteristics, but also its terrestrial spatial and chronologic
settings (Fig. 7). Walkden and Parker (2006, 2008) consid
ered two major controlling parameters: crater diameter and a
generalized time/place factor (termed “vulnerability”), even
if the Alamo instance somewhat counters the implied low re
sistance of high ambient biodiversity (see aspects of bio
sphere resilience in Stanley 1990; Raup 1991; Plotnick and
Sepkoski 2001; Kring 2003; Arens and West 2008; Prothero
2009; Alroy 2010).
Two conclusions can be drawn from the above discussion:
1. All currently known impacts, including the Siljan and
Manicougan events, are to be precluded as extraterrestrial
killers in the F–F and T–J mass extinctions.
2. The typically weak negative ecosystem impact of the
currently known impact events contrasts markedly with the
extreme biosphere collapse at the P–T boundary, and, what is
more, the probability of a bolide strike at that time is defi
nitely low. Consequently, other catastrophic high−magnitude
events and cataclysmic processes must have generated these
massive biodiversity losses.
Conclusions, implications,
and perspectives
The spectacular scenario proposed by Alvarez et al. (1980)
for the K–Pg boundary demise of marine and terrestrial eco
systems has triggered an overwhelming interest in the possi
bly devastating role of bolides colliding with the Earth. How
ever, parsimony−driven hypotheses cannot be easily applied
to convoluted geologic records and problems (Tsujita 2001;
see similar earlier views in Glen 1994 and Palmer 2003).
In the context of impact paradigm as a general explanation
of the observed biodiversity losses in the Phanerozoic, the
state−of−the−art situation can be summarized as follows:
·As reviewed above, cases of “great expectations syn−
drome” and circular reasoning bedevil numerous impact
scenarios (see other instructive examples reported by Hal−
lam and Wignall 1997; Tsujita 2001; Koeberl and Marti−
nez−Ruiz 2003; Prothero 2009). With reference to the pro−
posed three successive levels of misunderstanding, which
resulted from straightforward application of the impact
paradigm (Fig. 1), the global events are in actuality still at
the first level of testing, influenced by factual misidentifi−
cation of extraterrestrial signals, such as doubtful Ir en−
richments and shocked minerals.
·Occurrences of large impact structures with an age indistin
guishable from that of mass mortality events are not sub
stantiated (e.g., Kelley 2007). More speculatively, the T–J
boundary is within the error of dating, associated with an 80
km−diameter impact site (Puchezh−Katunki; Pálfy 2004;
Schmieder and Buchner 2008; see Fig. 6, and also Smith
2011), but only the F–F biotic crisis is seen herein in the
context of a possible correlative relationship with the Siljan
crater (?maybe also the debatable Woodleigh structure; Fig.
3). However, even if the discovery of undoubted Siljan im
pact ejecta would provide tight correlation, the crater size
and cratonic/continental setting hit indicates that an extra
terrestrial stimulus for this extinction is unlikely. To discuss
the lethal potential of the impact events in more robust
terms, its geographic and timing vulnerability factors, espe
cially target geology in the context of associated volatile
fluxes, should be rigorously assessed (size versus time and
place; Walkden and Parker 2006, 2008).
·Terrestrial cratering signature is often marked by large tim
ing and size uncertainties, particularly in deeply eroded and
buried impact structures, and the third level of advanced
testing (Fig. 1) applies rather to clustered impacts which
distinctly predate major extinctions with one notable excep
tion, the end−Permian. It is exemplified by the middle Fras
nian Alamo and other impacts (McGhee 2001, 2005; see
also Pisarzowska and Racki 2012) and the Late Triassic
Manicouagan impact (and other impact events; Tanner et al.
2004). As summarized by Alvarez (2003: 158), “Of course,
absence of evidence does not constitute evidence of ab
sence, and it may be that there have been several extinc
tion−causing impacts, with the KT event unique in its abun
dant preservation of impact proxies. One may conclude that
impact as a general cause of extinctions is not supported by
evidence, but has not been falsified”. Despite the continued
debate, the current absence of prime impact signatures at
mass extinction boundaries may indeed be a somewhat pre
mature conclusion, as ca. 90% of crater record is missing
(e.g., Kelley 2007: 929; Stewart 2011, who predicted 228
undiscovered craters larger than 2.5 km). The crucial con
straint is provided by an erratic and at least partly lost oce
anic impact record, characterized by a huge number of un
discovered craters and shock wave marks (Rogers 1982;
Kring 2003; Dypvik et al. 2004; Davison and Collins 2007),
and/or other extra−crater tracers (Gersonde et al. 2002; see
above). Recent estimates of the bombardment rate for Chic−
xulub−sized events, sufficient to form craters with diameters
of ~200 km, confirm only previous predictions (see Shoe−
maker et al. 1990; Jansa 1993; Toon et al. 1997), and are be−
tween 80 to 100 Ma (e.g., Bland 2005; Ivanov 2008; Stew−
art 2011; see also Claeys 2007). This is therefore an essen−
tial aim to reconstruct Earth's impact history properly be−
cause of a clearly underestimated number of identified
high−magnitude events, even if the population of large crat−
ers has a distinctly higher survival potential (see also Trefil
and Raup 1992; Reimold 2007; Bailer−Jones 2011). In addi−
tion, we surely have to consider that more advanced analyti
cal techniques (e.g., for shock effects in different lithologies
and minerals; French and Koeberl 2010; Reimold and Jour
dan 2012) will expose subtle cosmic signals and impact−ex
tinction links untraceable at the present state of knowledge.
·Regardless of these reservations, much more plausible to
me is the diagnosis by Walliser (1996: 238): “because it is
even theoretically impossible to prove the non−existence
of a non−existing impact, I prefer to presume (...) that a less
complicated and a less spectacular solution must not nec
essarily be wrong”. I overall favour to seek a general test
able multi−causal explanation of global−scale violent pro
cesses affecting our planet in the Earth’s system rather
than in space (Pluto school of Ager 1993).
·All major biocrises seem to be marked by overall lesser
catastrophic signatures than the impact−promoted K–Pg
boundary event (as stressed by Şengör et al. 2008 and
Schulte et al. 2010; see contradictory data in Tsujita 2001;
Hallam 2004, Twitchet 2006, and Prothero 2009). There
fore, approved temporal correlations between large igne
ous provinces and biotic crises are becoming a more ac
ceptable alternative for the dilemmas related to the sim
plistic impact catastrophism theory shown above (Wignall
2005; Hough et al. 2006; Courtillot and Olson 2007; Kid
der and Worsley 2010; Rampino 2010; Sobolev et al.
2011; Dal Corso et al. 2012; Greene et al. 2012). This
causal connection is now more obvious for the two−step
Late Permian crisis (e.g., Racki and Wignall 2005; Wig
nall 2005; Kidder and Worsley 2010; Sobolev et al. 2011;
Brand et al. 2012; Payne and Clapham 2012). The grow
ing evidence is better exposed in several recent papers on
the T–J transition, in which use of refined integrative ap
proaches, mostly with leading chemostratigraphy, offers a
reliable time resolution with age differences beyond the
refinement of available data (Deenen et al. 2010; Kuroda
et al. 2010; Schoene et al. 2010; Ruhl and Kürschner 2011;
Schaller et al. 2011; Callegaro et al. 2012; Greene et al.
2012, among others). The mass extinction started simulta
neously with the initial lava floods of the Central Atlantic
Magmatic Province, a suffocating supergreenhouse effect
due to CO2excess and marine biocalcification crisis (White
side et al. 2010). These works collectively imply that the
volcanic greenhouse (summer) scenario of Wignall (2005)
and (super) greenhouse (= hothouse of Kidder and Wors
ley 2010) crises are emerging as an exclusively Earth−cen
tred paradigm (Ward 2007; Retallack 2009), without ref−
erence to the impact trigger of magmatic activity.
·A particularly great dying episode corresponds to a uni−
quely complex, specific instance in the fossil record (e.g.,
Hoffman 1989; Walliser 1996; Hallam and Wignall 1997;
Palmer 2003; Keller 2005; MacLeod 2005; Prothero 2009;
Alroy 2010; Kidder and Worsley 2010). Mass extinctions
are thought, for example, by Feulner (2011) as a stochastic
combination of both random events and a variety of still
poorly−known periodic forcings against a noisy background
component (see also e.g., the multiplicative multifractal
model of Plotnick and Sepkoski 2001). In the attractive
press−pulse model of Arens and West (2008), mass extinc
tion causes are seen as interaction of long−term ecosystem
stress processes (e.g., sea level and/or climate change) and
geologically rapid, ultimate catastrophic disturbance. Con
sequently, holistic event−stratigraphic approaches to multi−
causal environmental traumas, refined on a case−by−case
basis, are the sole acknowledged way of dealing with these.
I wish to thank Paul Wignall (Leeds University, Leeds, UK) for
thoughtful review of the typescript, and Christian Koeberl (Vienna
University, Austria), Lawrence Tanner (Le Moyne College, Syracuse,
USA), and Martin Schmieder (Stuttgart University, Germany) for
helpful remarks and data. I am deeply grateful to the journal review
ers, Gerta Keller (Princeton University, Princeton, USA) and Peter
Schulte (Erlangen−Nürnberg University, Germany), whose construc
tive comments and suggestions led to significant improvements. Spe
cial thanks are extended to the guest editors of this thematic issue,
Elena A. Jagt−Yazykova (Opole University, Poland) and John W.M.
Jagt (Natuurhistorisch Museum Maastricht, the Netherlands) for in
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... The possibility that the small extinction event at the end-middle Norian (Upper Triassic) is connected with the formation of the Manicouagan impact structure, Canada (Onoue et al., 2016), cannot be excluded, but neither confirmed, based on currently available data. Aside from these cases, numerous inconclusive and nonreplicable studies have been published and were extensively and critically discussed (see e.g., Reimold, 2007;French and Koeberl, 2010;Jourdan et al., 2012;Racki, 2012;Schmieder et al., 2014). ...
... The modelled climatic effects of such eruptions support development of climatic conditions leading to loss of life (e.g., Courtillot and Renne, 2003;Bond and Wignall, 2014;Bond and Grasby, 2017;Rampino and Caldeira, 2018;Rampino et al., 2019). However, it is also recognized that at least on one occasion, at the end-Cretaceous, a large impact event had a devastating effect for the biosphere on Earth, causing the mass extinction at the K-Pg boundary (e.g., Alvarez et al., 1980;Hildebrand et al., 1991;Smit, 1999;Schulte et al., 2010 and references therein; see also discussion in Raup, 1992;Hallam and Wignall, 1997;Kelley, 2007;Reimold, 2007;French and Koeberl, 2010;Racki, 2012;Rampino and Caldeira, 2017). Thus, when the age of an impact event is accurately and precisely established, a fundamental question arises -what is the possible relationship between the impact event and a recorded event in the biosphere at the same period of time, such as a mass extinction? ...
The possibility of a “death from above” cause for biotic crises and extinction events is intriguing, to say the least, but such claims must be supported by reliable and reproducible data, not only impact diagnostic criteria, but also accurate and precise radioisotopic ages of the impact structures/events. To date, only one example of such an impact related global extinction event is confirmed, at the end of the Cretaceous period. Here we present and discuss results of newly obtained ⁴⁰Ar/³⁹Ar data from step heating analysis of impact melt rock samples from the 40 km-in-diameter Puchezh-Katunki impact structure, Russia, which allow us to precisely and accurately date its formation at 195.9 ± 1.0 Ma (2σ; P = 0.10). Based on these new data, we challenge the proposed temporal correlation with as many as five different extinction events (including the end-Triassic mass extinction) that were based on previous age estimations ranging from ∼164 to 203 Ma. Our new age for the formation of the Puchezh-Katunki impact structure allows us to exclude a relationship between this impact event and a known extinction event. We also show that careful sample preparation and methodology can overcome problems with inherited and trapped ⁴⁰Ar, issues that are common when dating impact melt rocks. This is supported by ⁴⁰Ar* diffusion and mixing numerical models showing that the most prominent negative effects in the case of the Puchezh-Katunki impact melt rock samples are caused by hydrothermal alteration and undegassed melt rock domains present in an otherwise homogenized melt rock. Numerical modeling also shows that the ⁴⁰Ar* from high-Ca inherited crystals or clasts is decoupled from the melt rock during step heating experiments allowing to safely recover a plateau age. Finally, our results highlight the importance of improving the database of ages of impact structures and show that caution should be practiced when suggesting connections between specific impact events and extinction events, especially in the case of poorly dated impact structures.
... Late Devonian mass extinction event might have offered significant pressure on microbial community, as discussed above, due hypoxic ocean and atmospheric environments (Berner, 2006;McGhee, 2012). This event, which a bolide impact as a cause controversial (Racki, 2012), shifted the temperature down and decreased oxygen levels, causing major mass extinction of marine life, around 57% of marine genera (Ward, 2006;Wake and Vredenburg, 2008). In (B) data were compiled from (Berner 1999;Huey and Ward 2005;Ward 2006;Glasspool and Scott 2010b). ...
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Evolution of mitochondrial genomes is essential for the adaptation of yeasts to changes in environmental oxygen levels. Although Saccharomyces cerevisiae mitochondrial DNA lacks all complex I genes, respiration is possible because alternative NADH dehydrogenases are encoded by NDE1 and NDI1 nuclear genes. The apparent whole genome duplication (WGD) in the yeast ancestor 100-150 million years ago caused nuclear gene duplications and secondary losses, although its relation to the loss of mitochondrial complex I is unknown. We produced phylogenomic supertrees and a supermatrix tree of 46 mitochondrial genomes, showing that the loss of complex I predates WGD and occurred independently in the S. cerevisiae group and the fission yeast Schizosaccharomyces pombe. The branching patterns did not differ substantially in supertrees and supermatrix phylogenies. We found consistent relations between conserved mitochondrial chromosomal gene order (synteny) in closely related yeasts. Correlation of mitochondrial molecular clock estimates and atmospheric oxygen variation in the Phanerozoic suggests that the Saccharomyces lineage might have lost complex I during hypoxic periods near Permian-Triassic or Triassic-Jurassic mass extinction events, while the Schizosaccharomyces lineage possibly lost complex I during hypoxic environment periods during the Middle Cambrian until the Lower Devonian. The loss of mitochondrial complex I, as a result of low oxygen levels, might not affect yeast metabolism due to a fermentative switch. The return to increased oxygen periods could have favored adaptations to aerobic metabolism. Additionally, we also show that NDE1 and NDI1 phylogenies indicate evolutionary convergence in yeasts in which mitochondrial complex I is absent.
... The Late Devonian does seem to have been a time of increased impact frequency in that from 6 to 10 known terrestrial impact craters are currently thought to be of Late Devonian age. However, the updated radiometric dates of the craters and the newly established age of the Kellwasser Crisis (Figure 1) exclude a causal link between the impact of large bodies and this crisis (Racki, 2012;Schmieder and Kring, 2020). First of all, the well-known Siljan impact crater in Sweden (with an estimated original diameter of 52 km) has been radiometrically dated at 380.9 ± 4.6 Ma bp (Schmieder and Kring, 2020) and is therefore more than 10 Ma older than the Kellwasser Crisis (Figure 1). ...
Full-text available
The Late Devonian mass extinction, which occurred 371.9 million years ago (Ma), is one of the ‘Big Five’ mass extinctions in Earth history. Suggested main proximate causes of the crisis include oceanic anoxia and climate swings. The severe loss of biodiversity affected both marine and terrestrial ecosystems and animal (especially metazoan reefs) and plant communities. Both high- and low-temperature stresses due to rapid climate switches and oxygen deprivation due to anoxia have been implicated as kill mechanisms in the mass extinction. The ultimate trigger for these kill mechanisms is still conjectural, but a crucial role of Eovariscan tectono-volcanic events has recently been highlighted, coupled with episodic expansion of terrestrial vegetation due to greenhouse effect driven by magmatic outgassing. Oceanic anoxia may have been triggered by the land-derived nutrification pulses. Postulated triggers of global cooling include the lag-time volcanic winter but superimposed on atmospheric carbon dioxide downdraw due to biological and tectonic factors and chemical weathering of terrestrial rocks.
... The Late Devonian does seem to have been a time of increased impact frequency in that from 6 to 10 known terrestrial impact craters are currently thought to be of Late Devonian age. However, the updated radiometric dates of the craters and the newly established age of the Kellwasser Crisis (Figure 1) exclude a causal link between the impact of large bodies and this crisis (Racki, 2012;Schmieder and Kring, 2020). First of all, the well-known Siljan impact crater in Sweden (with an estimated original diameter of 52 km) has been radiometrically dated at 380.9 ± 4.6 Ma bp (Schmieder and Kring, 2020) and is therefore more than 10 Ma older than the Kellwasser Crisis (Figure 1). ...
Full-text available
ARTICLE TO WILEY'S "ENCYCLOPEDIA OF LIFE SCIENCES". The Late Devonian mass extinction, which occurred AU:1 371.9 one million years ago (Ma), is one of the 'Big Five' mass extinctions in Earth history. Suggested main proximate causes of the crisis include oceanic anoxia and climate swings. The severe loss of biodiversity affected both marine and terrestrial ecosystems and animal (especially metazoan reefs) and plant communities. Both high-and low-temperature stresses due to rapid climate switches and oxygen deprivation due to anoxia have been implicated as kill mechanisms in the mass extinction. The ultimate trigger for these kill mechanisms is still conjectural, but a crucial role of Eovariscan tectono-volcanic events has recently been highlighted, coupled with episodic expansion of terrestrial vegetation due to greenhouse effect driven by magmatic outgassing. Oceanic anoxia may have been triggered by the land-derived nutrification pulses. Postulated triggers of global cooling include the lag-time volcanic winter but superimposed on atmospheric carbon dioxide downdraw due to biological and tectonic factors and chemical weathering of terrestrial rocks.
... Importantly, this knowledge growth is indivisible from the progress in the understanding of marine (paleo)ecosystems and (paleo)environments, both at the global and local scales. The most important contributions that formed the very fundamentals of what can be called "mass extinction science" were made by Bambach [1], Benton [2,3], Clapham and Renne [4], Elewa and Abdelhady [5], Erwin [6], Hallam [7], Jablonski [8], Holland [9], Melott and Bambach [10], Racki [11], Rampino and Caldeira [12], Raup and Sepkoski [13], Thomas [14], Twitchett [15], and Wignall [16]. Hundreds of other researchers have also contributed substantially and focused on particular catastrophic events, fossil groups, or extinction factors. ...
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Recent eustatic reconstructions allow for reconsidering the relationships between the fifteen Paleozoic–Mesozoic mass extinctions (mid-Cambrian, end-Ordovician, Llandovery/Wenlock, Late Devonian, Devonian/Carboniferous, mid-Carboniferous, end-Guadalupian, end-Permian, two mid-Triassic, end-Triassic, Early Jurassic, Jurassic/Cretaceous, Late Cretaceous, and end-Cretaceous extinctions) and global sea-level changes. The relationships between eustatic rises/falls and period-long eustatic trends are examined. Many eustatic events at the mass extinction intervals were not anomalous. Nonetheless, the majority of the considered mass extinctions coincided with either interruptions or changes in the ongoing eustatic trends. It cannot be excluded that such interruptions and changes could have facilitated or even triggered biodiversity losses in the marine realm.
... However, this new information on the uniqueness of the K/Pg extinction also further isolates it from all other mass extinctions that are clearly MALF related. Detailed radiometric dating shows that no other impacts correspond exactly and instantaneously to a mass extinction (Kidder and Worsley 2010:Figure 1;Racki 2012) and all other mass extinctions were protracted. Nonetheless, for many in the scientific community, media, and the public, this new information on the K/Pg, and the fact that it exterminated the iconic dinosaurs will inevitably divert attention from other less spectacular and more protracted mass extinctions. ...
This volume represents the proceedings of the homonymous international conference on all aspects of impact cratering and planetary science, which was held in October 2019 in Brasília, Brazil. This volume contains a sizable suite of contributions dealing with regional impact records (Australia, Sweden), impact craters and impactites, early Archean impacts and geophysical characteristics of impact structures, shock metamorphic investigations, post-impact hydrothermalism, and structural geology and morphometry of impact structures—on Earth and Mars. These contributions are authored by many of the foremost impact cratering researchers. Many contributions report results from state-of-the-art investigations, for example, several that are based on electron backscatter diffraction studies, and deal with new potential chronometers and shock barometers (e.g., apatite). Established impact cratering workers and newcomers to this field will both appreciate this multifaceted, multidisciplinary collection of impact cratering studies.
Changes in the climate-sensitive carbon cycle during the major and second-order Late Devonian global events were more complex than usually expected, i.e., they were not characterized only by positive δ13Ccarb and δ13Corg excursions up to 5.5%. In fact, the carbon (and oxygen) isotope signatures in lime mudstones are less susceptible to diagenetic bias and therefore several negative trends and spikes in different facies of distant continents are re-interpreted as primary biogeochemical signals. In addition to significant intra- and inter-regional variability resulting from diverse local carbon cycling factors, the initial phase of both Kellwasser Crisis events [Frasnian-Famennian (F-F) mass extinction] and end-Devonian Hangenberg Crisis confirms the predicted causal relationship between large-scale volcanic degassing (mainly in large igneous provinces?) and the response of global carbon cycle in 13C depleted signals. Potential volcanic signals were confirmed in more than 40% of the 73 F-F sites analyzed for C-isotope stratigraphy, as summarized in the global database. Lowered δ13C values characterize also onset of other less known Late Devonian events, exemplified particularly by well-documented Frasnian Middlesex/punctata Event. Thus, consistently recognized worldwide, paired negative δ13C and δ18O excursions, even only in carbonate signatures, can be cautiously regarded as a primary volcano-climatic (greenhouse) signal and additional proxy of the volcanic trigger. In addition, the repeated negative δ13C spike near the F-F boundary correlates with Hg anomalies considered to be the signature of the arc magmatism acme (and alkaline/carbonatite/ kimberlite paroxysm?). However, both the carbon cycle response and time relationships between disturbances in C and Hg cycles during mass extinctions were certainly complex and required high-resolution studies in the most stratigraphically extended series and advanced quantitative modeling.
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In: Stratigraphy & Timescales, Volume 5, 2020. Changes in the climate-sensitive carbon cycle during the major and second-order Late Devonian global events were more complex than usually expected, i.e., they were not characterized only by positive δ¹³Ccarb and δ¹³Corg excursions up to 5.5‰. In fact, the carbon (and oxygen) isotope signatures in lime mudstones are less susceptible to diagenetic bias and therefore several negative trends and spikes in different facies of distant continents are re-interpreted as primary biogeochemical signals. In addition to significant intra- and inter-regional variability resulting from diverse local carbon cycling factors, the initial phase of both Kellwasser Crisis events [Frasnian-Famennian (F-F) mass extinction] and end-Devonian Hangenberg Crisis confirms the predicted causal relationship between large-scale volcanic degassing (mainly in large igneous provinces?) and the response of global carbon cycle in ¹³C depleted signals. Potential volcanic signals were confirmed in more than 40% of the 73 F-F sites analyzed for C-isotope stratigraphy, as summarized in the global database. Lowered δ¹³C values characterize also onset of other less known Late Devonian events, exemplified particularly by well-documented Frasnian Middlesex/punctata Event. Thus, consistently recognized worldwide, paired negative δ¹³C and δ¹⁸O excursions, even only in carbonate signatures, can be cautiously regarded as a primary volcano-climatic (greenhouse) signal and additional proxy of the volcanic trigger. In addition, the repeated negative δ¹³C spike near the F-F boundary correlates with Hg anomalies considered to be the signature of the arc magmatism acme (and alkaline/carbonatite/kimberlite paroxysm?). However, both the carbon cycle response and time relationships between disturbances in C and Hg cycles during mass extinctions were certainly complex and required high-resolution studies in the most stratigraphically extended series and advanced quantitative modeling.
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The hypothesis that ocean acidification was a proximate trigger of the marine end-Triassic mass extinction rests on the assumption that taxa that strongly invest in the secretion of calcium-carbonate skeletons were significantly more affected by the crisis than other taxa. An argument against this hypothesis is the great extinction toll of radiolarians that has been reported from work on local sections. Radiolarians have siliceous tests and thus should be less affected by ocean acidification. We compiled taxonomically vetted occurrences of late Permian and Mesozoic radiolarians and analyzed extinction dynamics of radiolarian genera. Although extinction rates were high at the end of the Triassic, there is no evidence for a mass extinction in radiolarians but rather significantly higher background extinction in the Triassic than in the Jurassic. Although the causes for this decline in background extinction levels remain unclear, the lack of a major evolutionary response to the end-Triassic event, gives support for the hypothesis that ocean acidification was involved in the dramatic extinctions of many calcifying taxa. doi:10.1002/mmng.201000017
Global events are selective with respect to biota, ecosystem and palaeo-geography. Global bio-events are mostly connected with a facies change. They give rise to a regular evolutionary pattern, the E-R sequence, which comprises after the extinction event a generative phase followed by a radiation, i.e. a strong diversification. At least most of the global events are proximately caused by changes of environmental conditions. Thereby changes in sea level, oceanic conditions, and climate are prevailing. To a great extent the questions about ultimate causes and periodicity are still unanswered.
Based on evidence from astronomical observations, impact dynamics, and the geologic record, we explore a general hypothesis linking impacts of large asteroids and comets with mass extinctions of life. The probability of large-body impacts on the Earth derived from the flux of earth-crossing asteroids and comets and the estimated threshold impact size required to cause a global environmental disaster suggest that impacts of objects ≥ a few kilometers in diameter might be sufficient to explain the Phanerozoic record of extinction pulses. A number of extinction boundaries are known to be marked by severe environmental disturbances, including mass mortality and related extinctions (sometimes in steps), impoverished postextinction fauna and flora, and proliferation of stress-tolerant and opportunistic species, followed by gradual ecological recovery and radiation of new taxa. Abrupt negative shifts in δ13C in marine sedimentary rocks at extinction boundaries suggest major biomass losses followed by low-productivity " Strangelove" oceans, and fluctuations in δ18O may be interpreted as evidence of significant climatic oscillations. These biological, isotopic, and geochemical signatures seem to be consistent with the expected after effects of catastrophic impacts. Six of the extinction pulses may be associated with concurrent (in some cases multiple) impact markers (e.g., layers with high iridium, shocked minerals and/or microtektites, and large, dated impact craters). Elevated iridium levels at, or near, other extinction boundaries have characteristics suggesting a terrestrial origin, although they might be explained by collision of relatively low-Ir objects such as comets, and further work is warranted.
Based on evaluation of past results and new research, we have partitioned the distribution of the Alamo Breccia in southeastern Nevada and western Utah into six genetic Realms that provide a working model for the marine Late Devonian Alamo Impact Event. Each Realm exhibits discrete impact processes and stratigraphic products that are enumerated here. The first five form roughly concentric semicircular bands across the Devonian shallow-water carbonate platform. These are: (1) Rim Realm, where a newly defined impact stratigraphy includes both autogenic and allogenic breccias associated with the crater rim; (2) Ring Realm, where breccias are now interpreted to have formed sequentially by seismic shock, passage of the ejecta curtain, tsunami waves or surge, and runoff that accumulated over tilted terrace(s) bounded by syn-Event, ring-forming, listric faults; (3) Runup Realm, where graded breccias were stranded by tsunami surge or waves; (4) Runoff Realm, where sheet-floods carried traces of impact debris across the distal platform beds and channels filled with impact debris; (5) Seismite Realm, where near-surface beds far across the platform were uniquely deformed; and (6) Runout/Resurge Realm, where offshore channels of thick off-platform Alamo Breccia, together with large-scale olistolith(s), signal contemporaneous massive collapse of the platform margin, possibly into the central crater. Five breccia Units characterize the newly interpreted Rim Realm, in ascending order: (1) deformed target rocks, (2) injected dikes and sills, (3) chaotic fallback, (4) smeared fallback, and (5) resurge. This succession is covered by deepwater limestones deposited inside the crater rim, or across a new slope created after platform margin collapse. Unit 1 exhibits shatter-cone-like structures interpreted as impact products. Newly discovered Ordovician and probable older meter-scale clasts in Unit 3 confirm a minimum excavation depth of 1.5 km. Microscopic components in Units 3 and 4 indicate high pressures (>10 GPa), probable quenched carbonate melt, and accreted particles that may be new kinds of impact products. Postimpact tectonics and other factors obscure the full panorama, including the location and character of the missing central crater, but the assemblage of Realms offers a working model to compare with expected impact paradigms.