The blue graph shows the apparent
percentage (not the absolute number) of marine
animal genera
becoming extinct during any given time interval. It does not represent
all marine species, just those that are readily fossilized. The labels
of the traditional "Big Five" extinction events and the more recently
recognised
End-Capitanian extinction event are clickable hyperlinks; see
Extinction event for more details.
(source and image info)
The K–Pg extinction event was severe, global, rapid, and
selective, eliminating a vast number of species. Based on marine
fossils, it is estimated that 75% or more of all species were made
extinct.
[21]
The event appears to have affected all continents at the same time. Non-avian
dinosaurs, for example, are known from the
Maastrichtian of North America, Europe, Asia, Africa, South America, and Antarctica,
[25]
but are unknown from the Cenozoic anywhere in the world. Similarly,
fossil pollen shows devastation of the plant communities in areas as far
apart as
New Mexico,
Alaska,
China, and
New Zealand.
[19]
Despite the event's severity, there was significant variability in the rate of extinction between and within different
clades. Species that depended on
photosynthesis declined or became extinct as atmospheric particles blocked sunlight and reduced the
solar energy reaching the ground. This plant extinction caused a major reshuffling of the dominant plant groups.
[26] Omnivores,
insectivores, and
carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. No purely
herbivorous or
carnivorous mammals seem to have survived. Rather, the surviving mammals and birds fed on
insects,
worms, and
snails, which in turn fed on
detritus (dead plant and animal matter).
[27][28][29]
In
stream communities,
few animal groups became extinct, because such communities rely less
directly on food from living plants, and more on detritus washed in from
the land, protecting them from extinction.
[30] Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the
water column than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on
primary production from living
phytoplankton, while animals on the
ocean floor always or sometimes feed on detritus.
[27] Coccolithophorids and
mollusks (including
ammonites,
rudists,
freshwater snails, and
mussels), and those organisms whose
food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that
ammonites were the principal food of
mosasaurs, a group of giant marine
reptiles that became extinct at the boundary.
[31] The largest air-breathing survivors of the event,
crocodyliforms and
champsosaurs,
were semi-aquatic and had access to detritus. Modern crocodilians can
live as scavengers and survive for months without food, and their young
are small, grow slowly, and feed largely on invertebrates and dead
organisms for their first few years. These characteristics have been
linked to crocodilian survival at the end of the Cretaceous.
[28]
After the K–Pg extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant
ecological niches.
[27]
MicrobiotaEdit
The
K–Pg boundary represents one of the most dramatic turnovers in the
fossil record for various
calcareous nanoplankton that formed the
calcium deposits for which the Cretaceous is named. The turnover in this group is clearly marked at the species level.
[32][33] Statistical analysis of
marine
losses at this time suggests that the decrease in diversity was caused
more by a sharp increase in extinctions than by a decrease in
speciation.
[34] The K–Pg boundary record of
dinoflagellates is not so well understood, mainly because only
microbial cysts
provide a fossil record, and not all dinoflagellate species have
cyst-forming stages, which likely causes diversity to be underestimated.
[27] Recent studies indicate that there were no major shifts in dinoflagellates through the boundary layer.
[35]
Radiolaria have left a geological record since at least the
Ordovician
times, and their mineral fossil skeletons can be tracked across the
K–Pg boundary. There is no evidence of mass extinction of these
organisms, and there is support for high productivity of these species
in
southern high latitudes as a result of cooling temperatures in the early
Paleocene.
[27] Approximately 46% of
diatom species survived the transition from the
Cretaceous to the Upper Paleocene, a significant turnover in species but not a catastrophic extinction.
[27][36]
The occurrence of
planktonic foraminifera across the K–Pg boundary has been studied since the 1930s.
[37]
Research spurred by the possibility of an impact event at the K–Pg
boundary resulted in numerous publications detailing planktonic
foraminiferal extinction at the boundary;
[27]
however, there is ongoing debate between groups that think the evidence
indicates substantial extinction of these species at the K–Pg boundary,
[38] and those who think the evidence supports multiple extinctions and expansions through the boundary.
[39][40]
Numerous species of
benthic foraminifera became extinct during the event, presumably because they depend on organic debris for nutrients, while
biomass
in the ocean is thought to have decreased. As the marine microbiota
recovered, however, it is thought that increased speciation of benthic
foraminifera resulted from the increase in food sources.
[27]
Phytoplankton recovery in the early Paleocene provided the food source
to support large benthic foraminiferal assemblages, which are mainly
detritus-feeding. Ultimate recovery of the benthic populations occurred
over several stages lasting several hundred thousand years into the
early Paleocene.
[41][42]
Marine invertebratesEdit
There is significant variation in the fossil record as to the extinction rate of
marine invertebrates across the K–Pg boundary. The apparent rate is influenced by a lack of fossil records, rather than extinctions.
[27]
Ostracods, a class of small
crustaceans
that were prevalent in the upper Maastrichtian, left fossil deposits in
a variety of locations. A review of these fossils shows that ostracod
diversity was lower in the Paleocene than any other time in the
Cenozoic. Current research cannot ascertain, however, whether the extinctions occurred prior to, or during, the boundary interval.
[43][44]
Approximately 60% of late-Cretaceous
Scleractinia coral
genera failed to cross the K–Pg boundary into the Paleocene. Further
analysis of the coral extinctions shows that approximately 98% of
colonial species, ones that inhabit warm, shallow
tropical waters, became extinct. The solitary corals, which generally do not form reefs and inhabit colder and deeper (below the
photic zone) areas of the ocean were less impacted by the K–Pg boundary. Colonial coral species rely upon
symbiosis with photosynthetic
algae, which collapsed due to the events surrounding the K–Pg boundary;
[45][46]
however, the use of data from coral fossils to support K–Pg extinction
and subsequent Paleocene recovery, must be weighed against the changes
that occurred in coral ecosystems through the K–Pg boundary.
[27]
The numbers of
cephalopod,
echinoderm, and
bivalve genera exhibited significant diminution after the K–Pg boundary.
[27] Most species of
brachiopods, a small
phylum of marine invertebrates, survived the K–Pg extinction event and diversified during the early Paleocene.
Rudist bivalves from the Late Cretaceous of the Omani Mountains, United Arab Emirates. Scale bar is 10 mm
Except for
nautiloids (represented by the modern order
Nautilida) and
coleoids (which had already
diverged into modern
octopodes,
squids, and
cuttlefish) all other species of the
molluscan class Cephalopoda became extinct at the K–Pg boundary. These included the ecologically significant
belemnoids, as well as the
ammonoids,
a group of highly diverse, numerous, and widely distributed shelled
cephalopods. Researchers have pointed out that the reproductive strategy
of the surviving nautiloids, which rely upon few and larger eggs,
played a role in outsurviving their ammonoid counterparts through the
extinction event. The ammonoids utilized a planktonic strategy of
reproduction (numerous eggs and planktonic larvae), which would have
been devastated by the K–Pg extinction event. Additional research has
shown that subsequent to this elimination of ammonoids from the global
biota, nautiloids began an evolutionary radiation into shell shapes and
complexities theretofore known only from ammonoids.
[47][48]
Approximately 35% of echinoderm genera became extinct at the K–Pg boundary, although
taxa
that thrived in low-latitude, shallow-water environments during the
late Cretaceous had the highest extinction rate. Mid-latitude,
deep-water echinoderms were much less affected at the K–Pg boundary. The
pattern of extinction points to habitat loss, specifically the drowning
of
carbonate platforms, the shallow-water reefs in existence at that time, by the extinction event.
[49]
Other invertebrate groups, including
rudists (reef-building clams) and
inoceramids (giant relatives of modern
scallops), also became extinct at the K–Pg boundary.
[50][51]
There are substantial fossil records of
jawed fishes
across the K–Pg boundary, which provide good evidence of extinction
patterns of these classes of marine vertebrates. While the deep sea
realm was able to remain seemingly unaffected, there was an equal loss
between the open marine apex predators and the
durophagous demersal feeders on the continental shelf.
Within
cartilaginous fish, approximately 7 out of the 41 families of
neoselachians (modern
sharks, skates, and rays) disappeared after this event and
batoids (skates and rays) lost nearly all the identifiable species, while more than 90% of
teleost fish (bony fish) families survived.
[52][53]
In the Maastrichtian age, 28
shark
families and 13 batoid families thrived, of which 25 and 9,
respectively, survived the K–T boundary event. Forty-seven of all
neoselachian genera cross the K–T boundary, with 85% being sharks.
Batoids display with 15% a comparably low survival rate.
[52][54]
There is evidence of a mass extinction of
bony fishes at a fossil site immediately above the K–Pg boundary layer on
Seymour Island near
Antarctica, apparently precipitated by the K–Pg extinction event;
[55] however, the marine and freshwater environments of fishes mitigated environmental effects of the extinction event.
[56]
Terrestrial invertebratesEdit
Insect damage to the fossilized leaves of
flowering plants
from fourteen sites in North America was used as a proxy for insect
diversity across the K–Pg boundary and analyzed to determine the rate of
extinction. Researchers found that Cretaceous sites, prior to the
extinction event, had rich plant and insect-feeding diversity. During
the early Paleocene, however, flora were relatively diverse with little
predation from insects, even 1.7 million years after the extinction
event.
[57][58]
Terrestrial plantsEdit
There is overwhelming evidence of global disruption of plant communities at the K–Pg boundary.
[19][19][59][60] Extinctions are seen both in studies of fossil pollen, and fossil leaves.
[19]
In North America, the data suggests massive devastation and mass
extinction of plants at the K–Pg boundary sections, although there were
substantial megafloral changes before the boundary.
[19][61]
In North America, approximately 57% of plant species became extinct. In
high southern hemisphere latitudes, such as New Zealand and Antarctica,
the mass die-off of flora caused no significant turnover in species,
but dramatic and short-term changes in the relative abundance of plant
groups.
[57][62] In some regions, the Paleocene recovery of plants began with recolonizations by fern species, represented as a
fern spike in the geologic record; this same pattern of fern recolonization was observed after the
1980 Mount St. Helens eruption.
[63]
Due to the wholesale destruction of plants at the K–Pg boundary, there was a proliferation of
saprotrophic organisms, such as
fungi,
that do not require photosynthesis and use nutrients from decaying
vegetation. The dominance of fungal species lasted only a few years
while the atmosphere cleared and plenty of organic matter to feed on was
present. Once the atmosphere cleared, photosynthetic organisms,
initially ferns and other ground-level plants, returned.
[64] Just two species of fern appear to have dominated the landscape for centuries after the event.
[65]
Polyploidy
appears to have enhanced the ability of flowering plants to survive the
extinction, probably because the additional copies of the genome such
plants possessed, allowed them to more readily adapt to the rapidly
changing environmental conditions that followed the impact.
[66]
AmphibiansEdit
There is limited evidence for extinction of amphibians at the K–Pg
boundary. A study of fossil vertebrates across the K–Pg boundary in
Montana concluded that no species of amphibian became extinct.
[67]
Yet there are several species of Maastrichtian amphibian, not included
as part of this study, which are unknown from the Paleocene. These
include the frog
Theatonius lancensis[68] and the
albanerpetontid Albanerpeton galaktion;
[69]
therefore, some amphibians do seem to have become extinct at the
boundary. The relatively low levels of extinction seen among amphibians
probably reflect the low extinction rates seen in freshwater animals.
[70]
Non-archosaursEdit
Kronosaurus Hunt, a rendering by Dmitry Bogdanov in 2008. Large marine reptiles, including
plesiosaurians such as these, became extinct at the end of the
Cretaceous.
ChoristodereEdit
The
choristoderes (semi-aquatic
archosauromorphs) survived across the K–Pg boundary
[27] but would die out in the early
Miocene.
[71] Studies on
Champsosaurus' palatal teeth suggest that there were dietary changes among the various species across the KT event.
[72]
More than 80% of Cretaceous
turtle
species passed through the K–Pg boundary. Additionally, all six turtle
families in existence at the end of the Cretaceous survived into the
Paleogene and are represented by living species.
[73]
LepidosauriaEdit
The living non-archosaurian reptile taxa,
lepidosaurians (
snakes,
lizards and
tuataras), survived across the K–Pg boundary.
[27] Living lepidosaurs include the tuataras (the only living
rhynchocephalians) and the
squamates.
The rhynchocephalians were a widespread and relatively successful group of lepidosaurians during the early
Mesozoic, but began to decline by the mid-Cretaceous, although they were very successful in the
Late Cretaceous of
South America.
[74] They are represented today by a single genus, located exclusively in
New Zealand.
[75]
The
order Squamata, which is represented today by lizards, snakes and
amphisbaenians (worm lizards), radiated into various ecological niches
during the
Jurassic
and was successful throughout the Cretaceous. They survived through the
K–Pg boundary and are currently the most successful and diverse group
of living reptiles, with more than 6,000 extant species. Many families
of terrestrial squamates became extinct at the boundary, such as
monstersaurians and
polyglyphanodonts, and fossil evidence indicates they suffered very heavy losses in the K–T event, only recovering 10 million years after it.
[76] Giant non-archosaurian
aquatic reptiles such as
mosasaurs and
plesiosaurs, which were the top marine predators of their time, became extinct by the end of the Cretaceous.
[77][78] The
ichthyosaurs had disappeared from fossil records before the mass extinction occurred.
ArchosaursEdit
The
archosaur clade includes two surviving groups,
crocodilians and
birds, along with the various extinct groups of non-avian dinosaurs and
pterosaurs.
[79]
CrocodyliformsEdit
Ten families of crocodilians or their close relatives are represented
in the Maastrichtian fossil records, of which five died out prior to
the K–Pg boundary.
[80] Five families have both Maastrichtian and Paleocene fossil representatives. All of the surviving families of
crocodyliforms inhabited freshwater and terrestrial environments—except for the
Dyrosauridae,
which lived in freshwater and marine locations. Approximately 50% of
crocodyliform representatives survived across the K–Pg boundary, the
only apparent trend being that no large crocodiles survived.
[27]
Crocodyliform survivability across the boundary may have resulted from
their aquatic niche and ability to burrow, which reduced susceptibility
to negative environmental effects at the boundary.
[56]
Jouve and colleagues suggested in 2008 that juvenile marine
crocodyliforms lived in freshwater environments as do modern marine
crocodile juveniles, which would have helped them survive where other
marine reptiles became extinct; freshwater environments were not so strongly affected by the K–Pg extinction event as marine environments were.
[81]
PterosaursEdit
One family of pterosaurs,
Azhdarchidae,
was definitely present in the Maastrichtian, and it likely became
extinct at the K–Pg boundary. These large pterosaurs were the last
representatives of a declining group that contained ten families during
the mid-Cretaceous. Several other pterosaur lineages may have been
present during the Maastrichtian, such as the
ornithocheirids,
pteranodontids,
nyctosaurids, as well as, a possible
tapejarid, though they are represented by fragmentary remains that are difficult to assign to any given group.
[82][83]
While this was occurring, modern birds were undergoing diversification;
traditionally it was thought that they replaced archaic birds and
pterosaur groups, possibly due to direct competition, or they simply
filled empty niches,
[56][84][85] but there is no correlation between pterosaur and avian diversities that are conclusive to a competition hypothesis,
[86] and small pterosaurs were present in the Late Cretaceous.
[87] In fact, at least some niches previously held by birds were reclaimed by pterosaurs prior to the K–Pg event.
[88]
Most
paleontologists regard birds as the only surviving dinosaurs (see
Origin of birds). It is thought that all non-avian
theropods became extinct, including then-flourishing groups such as
enantiornithines and
hesperornithiforms.
[89] Several analyses of bird fossils show divergence of species prior to the K–Pg boundary, and that duck, chicken, and
ratite bird relatives coexisted with non-avian dinosaurs.
[90]
Large collections of bird fossils representing a range of different
species provides definitive evidence for the persistence of archaic
birds to within 300,000 years of the K–Pg boundary. The absence of these
birds in the Paleogene is evidence that a mass extinction of archaic
birds took place there. A small fraction of the Cretaceous bird species
survived the impact, giving rise to today's birds.
[15][91] The only bird group known for certain to have survived the K–Pg boundary is the Aves.
[15]
Avians may have been able to survive the extinction as a result of
their abilities to dive, swim, or seek shelter in water and marshlands.
Many species of avians can build burrows, or nest in tree holes or
termite nests, all of which provided shelter from the environmental
effects at the K–Pg boundary. Long-term survival past the boundary was
assured as a result of filling ecological niches left empty by
extinction of non-avian dinosaurs.
[56]
Non-avian dinosaursEdit
Tyrannosaurus was among the dinosaurs living on Earth before the extinction.
Excluding a
few controversial claims,
scientists agree that all non-avian dinosaurs became extinct at the
K–Pg boundary. The dinosaur fossil record has been interpreted to show
both a decline in diversity and no decline in diversity during the last
few million years of the Cretaceous, and it may be that the quality of
the dinosaur fossil record is simply not good enough to permit
researchers to distinguish between the options.
[92]
There is no evidence that late Maastrichtian non-avian dinosaurs could
burrow, swim, or dive, which suggests they were unable to shelter
themselves from the worst parts of any environmental stress that
occurred at the K–Pg boundary. It is possible that small dinosaurs
(other than birds) did survive, but they would have been deprived of
food, as herbivorous dinosaurs would have found plant material scarce
and carnivores would have quickly found prey in short supply.
[56]
The growing consensus about the endothermy of dinosaurs (see
dinosaur physiology)
helps to understand their full extinction in contrast with their close
relatives, the crocodilians. Ectothermic ("cold-blooded") crocodiles
have very limited needs for food (they can survive several months
without eating) while endothermic ("warm-blooded") animals of similar
size need much more food to sustain their faster metabolism. Thus, under
the circumstances of food chain disruption previously mentioned,
non-avian dinosaurs died,
[26]
while some crocodiles survived. In this context, the survival of other
endothermic animals, such as some birds and mammals, could be due, among
other reasons, to their smaller needs for food, related to their small
size at the extinction epoch.
[93]
Whether
the extinction occurred gradually or suddenly has been debated, as both
views have support from the fossil record. A study of 29 fossil sites
in Catalan
Pyrenees
of Europe in 2010 supports the view that dinosaurs there had great
diversity until the asteroid impact, with more than 100 living species.
[94]
More recent research indicates that this figure is obscured by
taphonomical biases, however, and the sparsity of the continental fossil
record. The results of this study, which were based on estimated real
global biodiversity, showed that between 628 and 1,078 non-avian
dinosaur species were alive at the end of the Cretaceous and underwent
sudden extinction after the Cretaceous–Paleogene extinction event.
[95] Alternatively, interpretation based on the fossil-bearing rocks along the
Red Deer River
in Alberta, Canada, supports the gradual extinction of non-avian
dinosaurs; during the last 10 million years of the Cretaceous layers
there, the number of dinosaur species seems to have decreased from about
45 to approximately 12. Other scientists have made the same assessment
following their research.
[96]
Several researchers support the existence of
Paleocene non-avian dinosaurs. Evidence of this existence is based on the discovery of dinosaur remains in the
Hell Creek Formation up to 1.3 m (4 ft 3.2 in) above and 40,000 years later than the K–Pg boundary.
[97] Pollen samples recovered near a fossilized
hadrosaur femur recovered in the
Ojo Alamo Sandstone at the
San Juan River in Colorado, indicate that the animal lived during the Cenozoic, approximately
64.5 Ma
(about 1 million years after the K–Pg extinction event). If their
existence past the K–Pg boundary can be confirmed, these hadrosaurids
would be considered a
dead clade walking.
[98]
Scientific consensus, however, is that these fossils were eroded from
their original locations and then re-buried in much later sediments
(also known as reworked fossils).
[99]
All major Cretaceous mammalian lineages, including
monotremes (egg-laying mammals),
multituberculates,
metatherians,
eutherians,
dryolestoideans,
[100] and
gondwanatheres[101]
survived the K–Pg extinction event, although they suffered losses. In
particular, metatherians largely disappeared from North America, and the
Asian
deltatheroidans became extinct (aside from the lineage leading to
Gurbanodelta).
[102]
In the Hell Creek beds of North America, at least half of the ten known
multituberculate species and all eleven metatherians species, are not
found above the boundary.
[92]
Multituberculates in Europe and North America survived relatively
unscathed and quickly bounced back in the Palaeocene, but Asian forms
were decimated, never again to represent a significant component on
mammalian faunas.
[103]
A recent study indicates that metatherians suffered the heaviest losses
at the K–T event, followed by multituberculates, while eutherians
recovered the quickest.
[104]
Mammalian
species began diversifying approximately 30 million years prior to the
K–Pg boundary. Diversification of mammals stalled across the boundary.
[105]
Current research indicates that mammals did not explosively diversify
across the K–Pg boundary, despite the environment niches made available
by the extinction of dinosaurs.
[106] Several mammalian orders have been interpreted as diversifying immediately after the K–Pg boundary, including Chiroptera (
bats) and Cetartiodactyla (a diverse group that today includes
whales and dolphins and
even-toed ungulates),
[106] although recent research concludes that only
marsupial orders diversified after the K–Pg boundary.
[105]
K–Pg boundary mammalian species were generally small, comparable in size to
rats;
this small size would have helped them find shelter in protected
environments. In addition, it is postulated that some early monotremes,
marsupials, and placentals were semiaquatic or burrowing, as there are
multiple mammalian lineages with such habits today. Any burrowing or
semiaquatic mammal would have had additional protection from K–Pg
boundary environmental stresses.
[56]
Evidence for impactEdit
Location of Chicxulub crater, Mexico
In 1980, a team of researchers consisting of
Nobel Prize-winning physicist
Luis Alvarez, his son, geologist
Walter Alvarez, and chemists
Frank Asaro and
Helen Michel discovered that
sedimentary layers found all over the world at the Cretaceous–Paleogene boundary contain a
concentration of
iridium many times greater than normal (30, 160, and 20 times in three sections originally studied). Iridium is extremely rare in
Earth's crust because it is a
siderophile element which mostly sank along with
iron into
Earth's core during
planetary differentiation. As iridium remains abundant in most
asteroids and
comets, the Alvarez team suggested that an asteroid struck the Earth at the time of the K–Pg boundary.
[9] There were earlier speculations on the possibility of an
impact event, but this was the first hard evidence.
[126]
This hypothesis was viewed as radical when first proposed, but
additional evidence soon emerged. The boundary clay was found to be full
of minute spherules of rock, crystallized from droplets of molten rock
formed by the impact.
[127] Shocked quartz[c] and other minerals were also identified in the K–Pg boundary.
[128][129] The identification of giant
tsunami beds along the Gulf Coast and the Caribbean provided more evidence,
[130]
and suggested that the impact may have occurred nearby—as did the
discovery that the K–Pg boundary became thicker in the southern United
States, with meter-thick beds of debris occurring in northern New
Mexico.
[19]
Radar topography reveals the 180 km –wide (112 mi) ring of the
Chicxulub Crater.
Further research identified the giant
Chicxulub crater, buried under
Chicxulub on the coast of
Yucatán, as the source of the K–Pg boundary clay. Identified in 1990
[11]
based on work by geophysicist Glen Penfield in 1978, the crater is
oval, with an average diameter of roughly 180 km (110 mi), about the
size calculated by the Alvarez team.
[131]
The discovery of the crater—a prediction of the impact
hypothesis—provided conclusive evidence for a K–Pg impact, and
strengthened the hypothesis that it caused the extinction.
In a 2013 paper,
Paul Renne of the
Berkeley Geochronology Center dated the impact at
66.043±0.011 million years ago, based on
argon–argon dating. He further posits that the mass extinction occurred within 32,000 years of this date.
[3][132]
In 2007, it was proposed that the impactor belonged to the
Baptistina family of asteroids.
[133] This link has been doubted, though not disproved, in part because of a lack of observations of the asteroid and its family.
[134] It was recently discovered that 298 Baptistina does not share the chemical signature of the K–Pg impactor.
[135] Further, a 2011
Wide-field Infrared Survey Explorer
(WISE) study of reflected light from the asteroids of the family
estimated their break-up at 80 Ma, giving them insufficient time to
shift orbits and impact Earth by 66 Ma.
[136]
Effects of impactEdit
In March 2010, an international panel of 41 scientists reviewed 20
years of scientific literature and endorsed the asteroid hypothesis,
specifically the Chicxulub impact, as the cause of the extinction,
ruling out other theories such as massive
volcanism.
They had determined that a 10-to-15-kilometer (6 to 9 mi) asteroid
hurtled into Earth at Chicxulub on Mexico's Yucatán Peninsula. The
collision would have released the same energy as 100
teratonnes of TNT (420
zettajoules)—more than a billion times the energy of the
atomic bombings of Hiroshima and Nagasaki.
[12]
The
Chicxulub impact caused a global catastrophe. Some of the phenomena
were brief occurrences immediately following the impact, but there were
also long-term geochemical and climatic disruptions that devastated the
ecology. Some scientists are, however, of the opinion that the asteroid
impact would not have caused a global extinction if it had impacted
somewhere else that wasn’t rich in deposits of sulfur, hydrocarbon or
organic fossil fuel as present in the Yucatán peninsula. They explain
that fossil fuel deposits caused an explosion so great that tons of soot
were jetted up into the stratosphere and blotted out the sun for an
extended period of time.
[137]
The re-entry of ejecta into Earth's atmosphere would include a brief (hours-long) but intense pulse of
infrared radiation, cooking exposed organisms.
[56]
This is debated, however, with opponents arguing that local ferocious
fires, probably limited to North America, fall short of global
firestorms. This is the "
Cretaceous–Palaeogene firestorm debate". A paper in 2013 by a prominent modeler of
nuclear winter suggested that, based on the amount of soot in the global debris layer, the entire terrestrial
biosphere might have burned, implying a global soot-cloud blocking out the sun and creating an
impact winter effect.
[138]
Aside
from the hypothesized fire and/or impact winter effects, the impact
would have created a dust cloud that blocked sunlight for up to a year,
inhibiting photosynthesis.
[110] The asteroid hit an area of carbonate rock containing a large amount of combustible hydrocarbons and sulphur,
[139] much of which was vaporized, thereby injecting
sulfuric acid aerosols into the
stratosphere, which might have reduced sunlight reaching the Earth's surface by more than 50%, and would have caused acid rain.
[110][140] The resulting acidification of the oceans would kill many organisms that grow shells of
calcium carbonate. At Brazos section, the sea surface temperature dropped as much as 7 °C (13 °F) for decades after the impact.
[141]
It would take at least ten years for such aerosols to dissipate, and
would account for the extinction of plants and phytoplankton, and
subsequently herbivores and their
predators. Creatures whose food chains were based on
detritus would have a reasonable chance of survival, however.
[93][110] Freezing temperatures probably lasted for at least three years.
[142]
If widespread fires occurred, they would have increased the
CO
2 content of the atmosphere and caused a temporary
greenhouse effect
once the dust clouds and aerosol settled, and, this would have
exterminated the most vulnerable organisms that survived the period
immediately after the impact.
[143]
Although
most paleontologists now agree that an asteroid did hit the Earth at
approximately the end of the Cretaceous, there is an ongoing dispute
whether the impact was the sole cause of the extinctions.
[40][144]
The
river bed at the Moody Creek Mine, 7 Mile Creek / Waimatuku, Dunollie,
New Zealand contains evidence of a devastating event on terrestrial
plant communities at the Cretaceous-Paleogene boundary, confirming the
severity and global nature of the event.
[59]
2016 Chicxulub crater drilling projectEdit
In 2016, a scientific drilling project obtained deep rock-
core samples from the
peak ring
around the Chicxulub impact crater. The discoveries confirmed that the
rock comprising the peak ring had been shocked by immense pressure and
melted in just minutes from its usual state into its present form.
Unlike sea-floor deposits, the peak ring was made of granite originating
much deeper in the earth, which had been ejected to the surface by the
impact.
Gypsum is a
sulfate-containing
rock usually present in the shallow seabed of the region; it had been
almost entirely removed, vaporized into the atmosphere. Further, the
event was immediately followed by a megatsunami
[d] sufficient to lay down the largest known layer of sand separated by grain size directly above the peak ring.
These findings strongly support the impact's role in the
extinction event. The impactor was large enough to create a
190-kilometer-wide (120 mi) peak ring, to melt, shock, and eject deep
granite, to create colossal water movements, and to eject an immense
quantity of vaporized rock and sulfates into the atmosphere, where they
would have persisted for several years. This worldwide dispersal of dust
and sulfates would have affected climate catastrophically, led to large
temperature drops, and devastated the food chain.
[145][146]
Although the concurrence of the end-Cretaceous extinctions with the
Chicxulub asteroid impact strongly supports the impact hypothesis, some
scientists continue to support other contributing causes: volcanic
eruptions, climate change, sea level change, and other impact events.
The end-Cretaceous event is the only
mass extinction known to be associated with an impact, and other large impacts, such as the
Manicouagan Reservoir impact, do not coincide with any noticeable extinction events.
[147]
Deccan TrapsEdit
Before 2000, arguments that the
Deccan Traps flood basalts
caused the extinction were usually linked to the view that the
extinction was gradual, as the flood basalt events were thought to have
started around 68 Mya and lasted more than 2 million years. The most
recent evidence shows that the traps erupted over a period of only
800,000 years spanning the K–Pg boundary, and therefore may be
responsible for the extinction and the delayed biotic recovery
thereafter.
[148]
The
Deccan Traps could have caused extinction through several mechanisms,
including the release of dust and sulfuric aerosols into the air, which
might have blocked sunlight and thereby reduced photosynthesis in
plants. In addition, Deccan Trap volcanism might have resulted in carbon
dioxide emissions that increased the greenhouse effect when the dust
and aerosols cleared from the atmosphere.
[149][150]
In
the years when the Deccan Traps hypothesis was linked to a slower
extinction, Luis Alvarez (d. 1988) replied that paleontologists were
being misled by
sparse data.
While his assertion was not initially well-received, later intensive
field studies of fossil beds lent weight to his claim. Eventually, most
paleontologists began to accept the idea that the mass extinctions at
the end of the Cretaceous were largely or at least partly due to a
massive Earth impact. Even Walter Alvarez acknowledged that other major
changes may have contributed to the extinctions.
[151]
Combining
these theories, some geophysical models suggest that the impact
contributed to the Deccan Traps.
These models, combined with high-precision radiometric dating, suggest
that the Chicxulub impact could have triggered some of the largest
Deccan eruptions, as well as eruptions at active volcanoes anywhere on
Earth.
[152][153]
Multiple impact eventEdit
Other crater-like topographic features have also been proposed as
impact craters formed in connection with Cretaceous-Paleogene
extinction. This suggests the possibility of near-simultaneous multiple
impacts, perhaps from a fragmented asteroidal object similar to the
Shoemaker–Levy 9 impact with
Jupiter. In addition to the 180 km (110 mi) Chicxulub crater, there is the 24 km (15 mi)
Boltysh crater in
Ukraine (
65.17±0.64 Ma), the 20 km (12 mi)
Silverpit crater in the
North Sea (
59.5±14.5 Ma) possibly formed by
bolide impact, and the controversial and much larger 600 km (370 mi)
Shiva crater. Any other craters that might have formed in the
Tethys Ocean would have been obscured by the northward tectonic drift of Africa and India.
[154][155][156][157]
Maastrichtian sea-level regressionEdit
There is clear evidence that sea levels fell in the final stage of
the Cretaceous by more than at any other time in the Mesozoic era. In
some Maastrichtian
stage
rock layers from various parts of the world, the later layers are
terrestrial; earlier layers represent shorelines and the earliest layers
represent seabeds. These layers do not show the tilting and distortion
associated with
mountain building, therefore the likeliest explanation is a
regression,
a drop in sea level. There is no direct evidence for the cause of the
regression, but the currently accepted explanation is that the
mid-ocean ridges became less active and sank under their own weight.
[27][158]
A severe regression would have greatly reduced the
continental shelf area, the most species-rich part of the sea, and therefore could have been enough to cause a
marine
mass extinction; however, this change would not have sufficed to cause
the extinction of the ammonites. The regression would also have caused
climate changes, partly by disrupting winds and ocean currents and
partly by reducing the Earth's
albedo and increasing global temperatures.
[111]
Marine regression also resulted in the loss of
epeiric seas, such as the
Western Interior Seaway of North America. The loss of these seas greatly altered habitats, removing
coastal plains
that ten million years before had been host to diverse communities such
as are found in rocks of the Dinosaur Park Formation. Another
consequence was an expansion of
freshwater environments, since continental runoff now had longer distances to travel before reaching
oceans. While this change was favorable to freshwater
vertebrates, those that prefer marine environments, such as sharks, suffered.
[92]
Multiple causesEdit
Proponents of multiple causation view the suggested single causes as
either too small to produce the vast scale of the extinction, or not
likely to produce its observed taxonomic pattern.
[92]
In a review article, J. David Archibald and David E. Fastovsky
discussed a scenario combining three major postulated causes: volcanism,
marine regression,
and extraterrestrial impact. In this scenario, terrestrial and marine
communities were stressed by the changes in, and loss of, habitats.
Dinosaurs, as the largest vertebrates, were the first affected by
environmental changes, and their diversity declined. At the same time,
particulate
materials from volcanism cooled and dried areas of the globe. Then an
impact event occurred, causing collapses in photosynthesis-based food
chains, both in the already-stressed terrestrial food chains and in the
marine food chains.
Recent work led by Sierra Peterson at Seymour Island, Antarctica,
showed two separate extinction events near the Cretaceous-Paleogene
boundary, with one correlating to Deccan Trap volcanism and one
correlated with the Chicxulub impact.
[159]
The team analyzed combined extinction patterns using a new clumped
isotope temperature record from a hiatus-free, expanded K–Pg boundary
section. They documented a 7.8±3.3 °C warming synchronous with the onset
of Deccan Traps volcanism and a second, smaller warming at the time of
meteorite impact. They suggest local warming may have been amplified due
to simultaneous disappearance of continental or sea ice. Intra-shell
variability indicates a possible reduction in seasonality after Deccan
eruptions began, continuing through the meteorite event. Species
extinction at Seymour Island occurred in two pulses that coincide with
the two observed warming events, directly linking the end-Cretaceous
extinction at this site to both volcanic and meteorite events via
climate change.
[159]
