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Permian–Triassic extinction event
Permian–Triassic extinction event
From Wikipedia, the free encyclopedia
Millions of years ago
Plot of extinction intensity (percentage of genera
that are present in each interval of time but do not exist in the following interval) vs time in the past for marine genera.
Geological periods are annotated (by abbreviation and colour) above.
The Permian–Triassic extinction event is the most significant event for
marine genera, with just over 50% (according to this source) failing to
survive. (source and image info)
) extinction event
, colloquially known as the Great Dying
, the End Permian
or the Great Permian Extinction
occurred about 252 Ma
(million years) ago,
forming the boundary between the Permian
and Triassic geologic periods
, as well as the Paleozoic
eras. It is the Earth's most severe known extinction event
, with up to 96% of all marine species
and 70% of terrestrial vertebrate
species becoming extinct
It is the only known mass extinction of insects
Some 57% of all families
and 83% of all genera
became extinct. Because so much biodiversity
was lost, the recovery of life on Earth took significantly longer than after any other extinction event,
possibly up to 10 million years.
There is evidence for between one and three distinct pulses, or phases, of extinction.
Suggested mechanisms for the latter include one or more large bolide impact events
, massive volcanism
, coal or gas fires and explosions from the Siberian Traps
and a runaway greenhouse effect
triggered by sudden release of methane
from the sea floor due to methane clathrate dissociation
or methane-producing microbes
known as methanogens;
possible contributing gradual changes include sea-level change, increasing anoxia
, increasing aridity
, and a shift in ocean circulation driven by climate change
Dating the extinction
δ13C stabilises at +2 ‰. Calcified green algae return.
Full recovery of woody trees
An approximate timescale of events around the P-Tr boundary.
Axis scale: millions of years ago.
Until 2000, it was thought that rock sequences spanning the
Permian–Triassic boundary were too few and contained too many gaps for
scientists to reliably determine its details.
However, it is now possible to date the extinction with remarkable
precision. U-Pb zircon dates from five volcanic ash beds from the Global
Stratotype Section and Point for the Permian-Triassic boundary at
Meishan, China, establish a high-resolution age model for the extinction
– allowing exploration of the links between global environmental
perturbation, carbon cycle disruption, mass extinction, and recovery at
millennial timescales. The extinction occurred between 251.941 ± 0.037
and 251.880 ± 0.031 Ma, a duration of 60 ± 48 ka.
A large (approximately 0.9%), abrupt global decrease in the ratio
of the stable isotope 13C
to that of 12C
, coincides with this extinction,
and is sometimes used to identify the Permian–Triassic boundary in rocks that are unsuitable for radiometric dating.
Further evidence for environmental change around the P–Tr boundary suggests an 8 °C (14 °F) rise in temperature,
and an increase in CO
levels by 2000 ppm
(by contrast, the concentration immediately before the industrial revolution was 280 ppm
There is also evidence of increased ultraviolet radiation reaching the earth causing the mutation of plant spores.
It has been suggested that the Permian–Triassic boundary is
associated with a sharp increase in the abundance of marine and
, caused by the sharp increase in the amount of dead plants and animals fed upon by the fungi.
For a while this "fungal spike" was used by some paleontologists to
identify the Permian–Triassic boundary in rocks that are unsuitable for
radiometric dating or lack suitable index fossils
but even the proposers of the fungal spike hypothesis pointed out that
"fungal spikes" may have been a repeating phenomenon created by the
post-extinction ecosystem in the earliest Triassic.
The very idea of a fungal spike has been criticized on several grounds, including: Reduviasporonites
, the most common supposed fungal spore, was actually a fossilized alga
the spike did not appear worldwide;
and in many places it did not fall on the Permian–Triassic boundary.
The algae, which were misidentified as fungal spores, may even
represent a transition to a lake-dominated Triassic world rather than an
earliest Triassic zone of death and decay in some terrestrial fossil
Newer chemical evidence agrees better with a fungal origin for Reduviasporonites
, diluting these critiques.
Uncertainty exists regarding the duration of the overall extinction
and about the timing and duration of various groups' extinctions within
the greater process. Some evidence suggests that there were multiple
or that the extinction was spread out over a few million years, with a sharp peak in the last million years of the Permian.
Statistical analyses of some highly fossiliferous strata in Meishan, Zhejiang
Province in South China, suggest that the main extinction was clustered around one peak.
Recent research shows that different groups became extinct at different
times; for example, while difficult to date absolutely, ostracod
extinctions were separated by 670 to 1170 thousand years.
In a well-preserved sequence in east Greenland, the decline of animals is concentrated in a period 10 to 60 thousand years
long, with plants taking several hundred thousand additional years to show the full impact of the event.
An older theory, still supported in some recent papers,
is that there were two major extinction pulses 9.4 million years apart,
separated by a period of extinctions well above the background level,
and that the final extinction killed off only about 80% of marine
species alive at that time while the other losses occurred during the
first pulse or the interval between pulses. According to this theory one
of these extinction pulses occurred at the end of the Guadalupian epoch
of the Permian.
For example, all but one of the surviving dinocephalian
genera died out at the end of the Guadalupian,
as did the Verbeekinidae
, a family of large-size fusuline foraminifera
The impact of the end-Guadalupian extinction on marine organisms
appears to have varied between locations and between taxonomic
groups—brachiopods and corals had severe losses.
suffered the greatest losses during the P–Tr extinction. Evidence of
this was found in samples from south China sections at the P–Tr
boundary. Here, 286 out of 329 marine invertebrate genera disappear
within the final 2 sedimentary zones containing conodonts
from the Permian.
The decrease in diversity was probably caused by a sharp increase in extinctions, rather than a decrease in speciation
The extinction primarily affected organisms with calcium carbonate skeletons, especially those reliant on stable CO2
levels to produce their skeletons.
These organisms were susceptible to the effects of the ocean acidification
that resulted from increased atmospheric CO2
organisms, the extinction event multiplied background extinction rates,
and therefore caused most damage to taxa that had a high background
extinction rate (by implication, taxa with a high turnover).
The extinction rate of marine organisms was catastrophic.
Surviving marine invertebrate groups include: articulate brachiopods
(those with a hinge),
which have suffered a slow decline in numbers since the P–Tr extinction; the Ceratitida
order of ammonites
which very nearly became extinct but later became abundant and diverse.
The groups with the highest survival rates generally had active
control of circulation, elaborate gas exchange mechanisms, and light
calcification; more heavily calcified organisms with simpler breathing
apparatus were the worst hit.
In the case of the brachiopods at least, surviving taxa were generally small, rare members of a diverse community.
which had been in a long-term decline for the 30 million years since
the Roadian (middle Permian), suffered a selective extinction pulse 10
million years before the main event, at the end of the Capitanian
stage. In this preliminary extinction, which greatly reduced disparity,
that is the range of different ecological guilds, environmental factors
were apparently responsible. Diversity and disparity fell further until
the P–Tr boundary; the extinction here was non-selective, consistent
with a catastrophic initiator. During the Triassic, diversity rose
rapidly, but disparity remained low.
The range of morphospace
occupied by the ammonoids, that is the range of possible forms, shape
or structure, became more restricted as the Permian progressed. Just a
few million years into the Triassic, the original range of ammonoid
structures was once again reoccupied, but the parameters were now shared
differently among clades
The Permian had great diversity in insect and other invertebrate
species, including the largest insects ever to have existed. The
end-Permian is the only known mass extinction of insects,
with eight or nine insect orders becoming extinct and ten more greatly reduced in diversity. Palaeodictyopteroids
(insects with piercing and sucking mouthparts) began to decline during
the mid-Permian; these extinctions have been linked to a change in
flora. The greatest decline occurred in the Late Permian and was
probably not directly caused by weather-related floral transitions.
Most fossil insect groups found after the Permian–Triassic boundary
differ significantly from those that lived prior to the P–Tr extinction.
With the exception of the Glosselytrodea
, and Protorthoptera
, Paleozoic insect groups have not been discovered in deposits dating to after the P–Tr boundary. The caloneurodeans
, paleodictyopteroids, protelytropterans
, and protodonates
became extinct by the end of the Permian. In well-documented Late
Triassic deposits, fossils overwhelmingly consist of modern fossil
Plant ecosystem response
The geological record of terrestrial plants is sparse and based mostly on pollen
studies. Interestingly, plants are relatively immune to mass
extinction, with the impact of all the major mass extinctions
"insignificant" at a family level.
Even the reduction observed in species diversity (of 50%) may be mostly due to taphonomic
However, a massive rearrangement of ecosystems does occur, with plant
abundances and distributions changing profoundly and all the forests
the Palaeozoic flora scarcely survived this extinction.
At the P–Tr boundary, the dominant floral groups changed, with many groups of land plants entering abrupt decline, such as Cordaites
) and Glossopteris
genera were replaced post-boundary by lycophytes
—extant lycophytes are recolonizers of disturbed areas.
or pollen studies from East Greenland
of sedimentary rock strata laid down during the extinction period indicate dense gymnosperm woodlands
before the event. At the same time that marine invertebrate macrofauna
declined, these large woodlands died out and were followed by a rise in
diversity of smaller herbaceous
plants including Lycopodiophyta
, both Selaginellales
Later, other groups of gymnosperms again become dominant but again
suffered major die offs. These cyclical flora shifts occurred a few
times over the course of the extinction period and afterwards. These
fluctuations of the dominant flora between woody and herbaceous taxa
indicate chronic environmental stress resulting in a loss of most large
woodland plant species. The successions and extinctions of plant
communities do not coincide with the shift in δ13C
values, but occurred many years after.
The recovery of gymnosperm forests took 4–5 million years.
deposits are known from the Early Triassic, and those in the Middle Triassic are thin and low-grade.
This "coal gap" has been explained in many ways. It has been suggested
that new, more aggressive fungi, insects and vertebrates evolved, and
killed vast numbers of trees. These decomposers themselves suffered
heavy losses of species during the extinction, and are not considered a
likely cause of the coal gap.
It could simply be that all coal forming plants were rendered extinct
by the P–Tr extinction, and that it took 10 million years for a new
suite of plants to adapt to the moist, acid conditions of peat bogs
On the other hand, abiotic
factors (not caused by organisms), such as decreased rainfall or increased input of clastic sediments
, may also be to blame.
Finally, it is also true that there are very few sediments of any type
known from the Early Triassic, and the lack of coal may simply reflect
this scarcity. This opens the possibility that coal-producing ecosystems
may have responded to the changed conditions by relocating, perhaps to
areas where we have no sedimentary record for the Early Triassic.
For example, in eastern Australia a cold climate had been the norm for a long period of time, with a peat mire
ecosystem adapted to these conditions. Approximately 95% of these peat-producing plants went locally
extinct at the P–Tr boundary;
Interestingly, coal deposits in Australia and Antarctica disappear significantly before
the P–Tr boundary.
There is enough evidence to indicate that over two-thirds of terrestrial labyrinthodont
("reptile") and therapsid
("mammal-like reptile") families
became extinct. Large herbivores suffered the heaviest losses. All Permian anapsid
reptiles died out except the procolophonids
have anapsid skulls but are most often thought to have evolved later, from diapsid ancestors). Pelycosaurs
died out before the end of the Permian. Too few Permian diapsid fossils
have been found to support any conclusion about the effect of the
Permian extinction on diapsids
(the "reptile" group from which lizards, snakes, crocodilians, and dinosaurs
[including birds] evolved).
Even the groups that survived suffered extremely heavy losses of
species, and some terrestrial vertebrate groups very nearly became
extinct at the end-Permian. Some of the surviving groups did not persist
for long past this period, while others that barely survived went on to
produce diverse and long-lasting lineages. Yet it took 30
million years for the terrestrial vertebrate fauna to fully recover both numerically and ecologically.
Possible explanations of these patterns
An analysis of marine fossils from the Permian's final Changhsingian
stage found that marine organisms with low tolerance for hypercapnia
(high concentration of carbon dioxide
) had high extinction rates, while the most tolerant organisms had very slight losses.
The most vulnerable marine organisms were those that produced calcareous hard parts (i.e., from calcium carbonate
) and had low metabolic rates
and weak respiratory systems—notably calcareous sponges
and tabulate corals
, calcite-depositing brachiopods
, and echinoderms
; about 81% of such genera
became extinct. Close relatives without calcareous hard parts suffered only minor losses, for example sea anemones
from which modern corals evolved. Animals with high metabolic rates,
well-developed respiratory systems, and non-calcareous hard parts had
negligible losses—except for conodonts
, in which 33% of genera died out.
This pattern is consistent with what is known about the effects of hypoxia
, a shortage but not a total absence of oxygen
. However, hypoxia cannot have been the only killing mechanism for marine organisms. Nearly all of the continental shelf
waters would have had to become severely hypoxic to account for the
magnitude of the extinction, but such a catastrophe would make it
difficult to explain the very selective pattern of the extinction. Models
of the Late Permian and Early Triassic atmospheres show a significant
but protracted decline in atmospheric oxygen levels, with no
acceleration near the P–Tr boundary. Minimum atmospheric oxygen levels
in the Early Triassic are never less than present day levels—the decline
in oxygen levels does not match the temporal pattern of the extinction.
Marine organisms are more sensitive to changes in CO2
levels than are terrestrial organisms for a variety of reasons. CO2
is 28 times more soluble in water than is oxygen. Marine animals normally function with lower concentrations of CO2
in their bodies than land animals, as the removal of CO2
in air-breathing animals is impeded by the need for the gas to pass
through the respiratory system's membranes (lungs' alveolus, tracheae
, and the like), even when CO2
diffuses more easily than oxygen. In marine organisms, relatively modest but sustained increases in CO2
concentrations hamper the synthesis of proteins
, reduce fertilization rates, and produce deformities in calcareous hard parts.
In addition, an increase in CO2
concentration is inevitably linked to ocean acidification
consistent with the preferential extinction of heavily calcified taxa
and other signals in the rock record that suggest a more acidic ocean.
It is difficult to analyze extinction and survival rates of land
organisms in detail, because few terrestrial fossil beds span the
Permian–Triassic boundary. Triassic insects are very different from
those of the Permian, but a gap in the insect fossil record spans
approximately 15 million years from the late Permian to early Triassic.
The best-known record of vertebrate changes across the Permian–Triassic
boundary occurs in the Karoo
Supergroup of South Africa, but statistical analyses have so far not produced clear conclusions.
However, analysis of the fossil river deposits of the floodplains indicate a shift from meandering
to braided river
patterns, indicating an abrupt drying of the climate.
The climate change may have taken as little as 100,000 years, prompting the extinction of the unique Glossopteris
flora and its herbivores, followed by the carnivorous guild
End-Permian extinctions did not occur at an instantaneous time horizon; particularly, floral extinction was delayed in time
Earlier analyses indicated that life on Earth recovered quickly after
the Permian extinctions, but this was mostly in the form of disaster taxa
, opportunist organisms such as the hardy Lystrosaurus
Research published in 2006 indicates that the specialized animals that
formed complex ecosystems, with high biodiversity, complex food webs and
a variety of niches, took much longer to recover. It is thought that
this long recovery was due to the successive waves of extinction, which
inhibited recovery, and prolonged environmental stress to organisms,
which continued into the Early Triassic. Research indicates that
recovery did not begin until the start of the mid-Triassic, 4 to 6
million years after the extinction;
and some writers estimate that the recovery was not complete until 30 Ma
after the P–Tr extinction, i.e. in the late Triassic
A study published in the journal Science
found that during the Great Extinction the oceans' surface temperatures
reached 40 °C (104 °F), which explains why recovery took so long: it
was simply too hot for life to survive.
Of course, not all of the Earth's surface was 40°, and if it was simply too hot for life to survive then nothing would
have survived. Perhaps anoxia provides another aspect of what delayed the recovery.
During the early Triassic (4 to 6 million years after the P–Tr extinction), the plant biomass was insufficient to form coal
deposits, which implies a limited food mass for herbivores.
River patterns in the Karoo
changed from meandering
, indicating that vegetation there was very sparse for a long time.
Each major segment of the early Triassic ecosystem—plant and animal, marine and terrestrial—was dominated by a small number of genera
, which appeared virtually worldwide, for example: the herbivorous therapsid Lystrosaurus
(which accounted for about 90% of early Triassic land vertebrates) and the bivalves Claraia
. A healthy ecosystem
has a much larger number of genera, each living in a few preferred types of habitat.
Disaster taxa took advantage of the devastated ecosystems and enjoyed
a temporary population boom and increase in their territory. Microconchids
are the dominant component of otherwise impoverished Early Triassic encrusting assemblages. For example: Lingula
, which had been confined to marginal environments since the Ordovician
(a small, weedy plant); Dicroidium
(a seed fern
Changes in marine ecosystems
Sessile filter feeders like this crinoid
were significantly less abundant after the P–Tr extinction.
Prior to the extinction, about two-thirds of marine animals were sessile
and attached to the sea floor but, during the Mesozoic, only about half
of the marine animals were sessile while the rest were free-living.
Analysis of marine fossils from the period indicated a decrease in the
abundance of sessile epifaunal suspension feeders
such as brachiopods
and sea lilies
and an increase in more complex mobile species such as snails
, sea urchins
Before the Permian mass extinction event, both complex and simple
marine ecosystems were equally common; after the recovery from the mass
extinction, the complex communities outnumbered the simple communities
by nearly three to one,
and the increase in predation pressure led to the Mesozoic Marine Revolution
were fairly rare before the P–Tr extinction but became numerous and diverse in the Triassic, and one group, the rudist
clams, became the Mesozoic
main reef-builders. Some researchers think much of this change happened
in the 5 million years between the two major extinction pulses.
("sea lilies") suffered a selective extinction, resulting in a decrease in the variety of their forms. Their ensuing adaptive radiation
was brisk, and resulted in forms possessing flexible arms becoming widespread; motility
, predominantly a response to predation pressure, also became far more prevalent.
was by far the most abundant early Triassic land vertebrate.
, a pig-sized herbivorous dicynodont therapsid
, constituted as much as 90% of some earliest Triassic land vertebrate fauna. Smaller carnivorous cynodont therapsids
also survived, including the ancestors of mammals
. In the Karoo
region of southern Africa
, the therocephalians Tetracynodon
survived, but do not appear to have been abundant in the Triassic.
(which included the ancestors of dinosaurs and crocodilians
were initially rarer than therapsids, but they began to displace
therapsids in the mid-Triassic. In the mid to late Triassic, the dinosaurs
evolved from one group of archosaurs, and went on to dominate terrestrial ecosystems during the Jurassic
This "Triassic Takeover" may have contributed to the evolution of mammals
by forcing the surviving therapsids and their mammaliform
successors to live as small, mainly nocturnal insectivores
; nocturnal life probably forced at least the mammaliforms to develop fur and higher metabolic rates
while losing part of the differential color-sensitive retinal receptors reptilians and birds preserved.
Some temnospondyl amphibians
made a relatively quick recovery, in spite of nearly becoming extinct. Mastodonsaurus
were the main aquatic and semiaquatic predators during most of the Triassic
, some preying on tetrapods
and others on fish.
Land vertebrates took an unusually long time to recover from the P–Tr extinction; Palaeontologist Michael Benton
estimated the recovery was not complete until 30 million years
after the extinction, i.e. not until the Late Triassic, in which dinosaurs, pterosaurs
, crocodiles, archosaurs, amphibians, and mammaliforms were abundant and diverse.
Pinpointing the exact cause or causes of the Permian–Triassic
extinction event is difficult, mostly because the catastrophe occurred
over 250 million years ago, and much of the evidence that would have
pointed to the cause has been destroyed by now or is concealed deep
within the Earth under many layers of rock. The sea floor
is also completely recycled every 200 million years by the ongoing process of plate tectonics
and seafloor spreading
leaving no useful indications beneath the ocean. With the fairly
significant evidence that scientists have accumulated, several
mechanisms have been proposed for the extinction event, including both
catastrophic and gradual processes (similar to those theorized for the Cretaceous–Paleogene extinction event
). The former group includes one or more large bolide impact events
, increased volcanism
, and sudden release of methane from the sea floor, either due to dissociation of methane hydrate
deposits or metabolism of organic carbon deposits by methanogenic
microbes. The latter group includes sea level change, increasing anoxia
, and increasing aridity
. Any hypothesis about the cause must explain the selectivity of the event, which affected organisms with calcium carbonate
skeletons most severely; the long period (4 to 6 million years) before
recovery started, and the minimal extent of biological mineralization
(despite inorganic carbonates being deposited) once the recovery began.
Artist's impression of a major impact event: A collision between Earth and an asteroid
a few kilometres in diameter would release as much energy as several million nuclear weapons detonating.
Evidence that an impact event
may have caused the Cretaceous–Paleogene extinction event
(Cretaceous-Tertiary) has led to speculation that similar impacts may
have been the cause of other extinction events, including the P–Tr
extinction, and thus to a search for evidence of impacts at the times of
other extinctions and for large impact craters
of the appropriate age.
Reported evidence for an impact event from the P–Tr boundary level includes rare grains of shocked quartz
in Australia and Antarctica; fullerenes
trapping extraterrestrial noble gases;
meteorite fragments in Antarctica;
and grains rich in iron, nickel and silicon, which may have been created by an impact.
However, the accuracy of most of these claims has been challenged.
Quartz from Graphite Peak
in Antarctica, for example, once considered "shocked", has been
re-examined by optical and transmission electron microscopy. The
observed features were concluded to be not due to shock, but rather to plastic deformation
, consistent with formation in a tectonic
environment such as volcanism.
An impact crater on the sea floor would be evidence of a possible
cause of the P–Tr extinction, but such a crater would by now have
disappeared. As 70% of the Earth's surface is currently sea, an asteroid
or comet fragment is now perhaps more than twice as likely to hit ocean
as it is to hit land. However, Earth has no ocean-floor crust more than
200 million years old because the "conveyor belt" process of seafloor
spreading and subduction
destroys it within that time. Craters produced by very large impacts may be masked by extensive flood basalting
from below after the crust is punctured or weakened.
Subduction should not, however, be entirely accepted as an explanation
of why no firm evidence can be found: as with the K-T event, an ejecta
blanket stratum rich in siderophilic elements
(such as iridium
) would be expected to be seen in formations from the time.
One attraction of large impact theories is that theoretically they
could trigger other cause-considered extinction-paralleling phenomena,[clarification needed]
such as the Siberian Traps
eruptions (see below) as being either an impact site
or the antipode
of an impact site.
The abruptness of an impact also explains why more species did not rapidly evolve
to survive, as would be expected if the Permian-Triassic event had been slower and less global than a meteorite impact.
Possible impact sites
Several possible impact craters have been proposed as the site of an impact causing the P–Tr extinction, including the Bedout
structure off the northwest coast of Australia
and the hypothesized Wilkes Land crater
of East Antarctica.
In each case, the idea that an impact was responsible has not been
proven and has been widely criticized. In the case of Wilkes Land, the
age of this sub-ice geophysical feature is very uncertain – it may be
later than the Permian–Triassic extinction.
The Araguainha crater
in Brazil has been most recently dated to 254.7 ± 2.5 million years
ago, overlapping with estimates for the Permo-Triassic boundary.
Much of the local rock was oil shale
The estimated energy released by the Araguainha impact is insufficient
to be a direct cause of the global mass extinction, but the colossal
local earth tremors would have released huge amounts of oil and gas from
the shattered rock. The resulting sudden global warming
might have precipitated the Permian–Triassic extinction event.
The final stages of the Permian had two flood basalt
events. A small one, the Emeishan Traps
, occurred at the same time as the end-Guadalupian
extinction pulse, in an area close to the equator at the time.
The flood basalt eruptions that produced the Siberian Traps
constituted one of the largest known volcanic events on Earth and
covered over 2,000,000 square kilometres (770,000 sq mi) with lava.
The date of the Siberian Traps eruptions and the extinction event are in good agreement.
The Emeishan and Siberian Traps eruptions may have caused dust clouds and acid aerosols
, which would have blocked out sunlight and thus disrupted photosynthesis both on land and in the photic zone
of the ocean, causing food chains to collapse. The eruptions may also
have caused acid rain when the aerosols washed out of the atmosphere.
That may have killed land plants and molluscs
organisms which had calcium carbonate
shells. The eruptions would also have emitted carbon dioxide
, causing global warming
When all of the dust clouds and aerosols washed out of the atmosphere,
the excess carbon dioxide would have remained and the warming would have
proceeded without any mitigating effects.
The Siberian Traps had unusual features that made them even more
dangerous. Pure flood basalts produce fluid, low-viscosity lava and do
not hurl debris into the atmosphere. It appears, however, that 20% of
the output of the Siberian Traps eruptions was pyroclastic
(consisted of ash and other debris thrown high into the atmosphere), increasing the short-term cooling effect.
The basalt lava erupted or intruded into carbonate
rocks and into sediments that were in the process of forming large coal
beds, both of which would have emitted large amounts of carbon dioxide,
leading to stronger global warming after the dust and aerosols settled.
In January 2011, a team, led by Stephen Grasby of the Geological
Survey of Canada—Calgary, reported evidence that volcanism caused
massive coal beds to ignite, possibly releasing more than 3 trillion
tons of carbon. The team found ash deposits in deep rock layers near
what is now Buchanan Lake
According to their article, "coal ash dispersed by the explosive
Siberian Trap eruption would be expected to have an associated release
of toxic elements in impacted water bodies where fly ash slurries
developed.... Mafic megascale eruptions are long-lived events that would
allow significant build-up of global ash clouds."
In a statement, Grasby said, "In addition to these volcanoes causing
fires through coal, the ash it spewed was highly toxic and was released
in the land and water, potentially contributing to the worst extinction
event in earth history."
In 2013, QY Tang reported the total amounts of important volatiles emitted from the Siberian Traps are 8.5 × 107
Tg CO2, 4.4 × 106
Tg CO, 7.0 × 106
Tg H2S and 6.8 × 107
Tg SO2, the data support a popular notion that the end-Permian mass
extinction on the Earth was caused by the emission of enormous amounts
of volatiles from the Siberian Traps into the atmosphere.
In 2015, evidence and a timeline indicated the extinction was caused by events in the Large Igneous Province
of the Siberian Traps.
Methane hydrate gasification
Scientists have found worldwide evidence of a swift decrease of about 1% in the 13C
/12C isotope ratio
rocks from the end-Permian.
This is the first, largest, and most rapid of a series of negative and positive excursions (decreases and increases in 13
ratio) that continues until the isotope ratio abruptly stabilised in
the middle Triassic, followed soon afterwards by the recovery of
calcifying life forms (organisms that use calcium carbonate
to build hard parts such as shells).
A variety of factors may have contributed to this drop in the 13C
ratio, but most turn out to be insufficient to account fully for the observed amount:
- Gases from volcanic eruptions have a 13C/12C ratio about 0.5 to 0.8% below standard (δ13C
about −0.5 to −0.8%), but an assessment made in 1995 concluded that the
amount required to produce a reduction of about 1.0% worldwide requires
eruptions greater by orders of magnitude than any for which evidence has been found. (However, this analysis addressed only CO2 produced by the magma itself, not from interactions with carbon bearing sediments, as later proposed.)
- A reduction in organic activity would extract 12C more slowly from the environment and leave more of it to be incorporated into sediments, thus reducing the 13C/12C
ratio. Biochemical processes preferentially use the lighter isotopes
since chemical reactions are ultimately driven by electromagnetic forces
between atoms and lighter isotopes respond more quickly to these
forces, but a study of a smaller drop of 0.3 to 0.4% in 13C/12C (δ13C −3 to −4 ‰) at the Paleocene-Eocene Thermal Maximum
(PETM) concluded that even transferring all the organic carbon (in
organisms, soils, and dissolved in the ocean) into sediments would be
insufficient: even such a large burial of material rich in 12C would not have produced the 'smaller' drop in the 13C/12C ratio of the rocks around the PETM.
- Buried sedimentary organic matter has a 13C/12C ratio 2.0 to 2.5% below normal (δ13C
−2.0 to −2.5%). Theoretically, if the sea level fell sharply, shallow
marine sediments would be exposed to oxidization. But 6500–8400 gigatons (1 gigaton = 109 metric tons)
of organic carbon would have to be oxidized and returned to the
ocean-atmosphere system within less than a few hundred thousand years to
reduce the 13C/12C ratio by 1.0%, which is not thought to be a realistic possibility. Moreover, sea levels were rising rather than falling at the time of the extinction.
- Rather than a sudden decline in sea level, intermittent periods of ocean-bottom hyperoxia and anoxia (high-oxygen and low- or zero-oxygen conditions) may have caused the 13C/12C ratio fluctuations in the Early Triassic;
and global anoxia may have been responsible for the end-Permian blip.
The continents of the end-Permian and early Triassic were more clustered
in the tropics than they are now, and large tropical rivers would have
dumped sediment into smaller, partially enclosed ocean basins at low
latitudes. Such conditions favor oxic and anoxic episodes; oxic/anoxic
conditions would result in a rapid release/burial, respectively, of
large amounts of organic carbon, which has a low 13C/12C ratio because biochemical processes use the lighter isotopes more.
That or another organic-based reason may have been responsible for both
that and a late Proterozoic/Cambrian pattern of fluctuating 13C/12C ratios.
Other hypotheses include mass oceanic poisoning releasing vast amounts of CO2
and a long-term reorganisation of the global carbon cycle.
Prior to consideration of the inclusion of roasting carbonate
sediments by volcanism, the only proposed mechanism sufficient to cause a
global 1% reduction in the 13
C ratio was the release of methane
from methane clathrates
Carbon-cycle models confirm that it would have had enough effect to produce the observed reduction.
Methane clathrates, also known as methane hydrates, consist of methane
molecules trapped in cages of water molecules. The methane, produced by methanogens
(microscopic single-celled organisms), has a 13
C ratio about 6.0% below normal (δ13
C −6.0%). At the right combination of pressure and temperature, it gets trapped in clathrates fairly close to the surface of permafrost
and in much larger quantities at continental margins (continental shelves
and the deeper seabed close to them). Oceanic methane hydrates are
usually found buried in sediments where the seawater is at least 300 m
(980 ft) deep. They can be found up to about 2,000 m (6,600 ft) below
the sea floor, but usually only about 1,100 m (3,600 ft) below the sea
The area covered by lava from the Siberian Traps eruptions is about
twice as large as was originally thought, and most of the additional
area was shallow sea at the time. The seabed probably contained methane hydrate
deposits, and the lava caused the deposits to dissociate, releasing vast quantities of methane.
A vast release of methane might cause significant global warming since methane is a very powerful greenhouse gas
Strong evidence suggests the global temperatures increased by about
6 °C (10.8 °F) near the equator and therefore by more at higher
latitudes: a sharp decrease in oxygen isotope ratios (18
the extinction of Glossopteris
and plants that grew in the same areas), which needed a cold climate,
with its replacement by floras typical of lower paleolatitudes.
However, the pattern of isotope shifts expected to result from a
massive release of methane does not match the patterns seen throughout
the early Triassic. Not only would such a cause require the release of
five times as much methane as postulated for the PETM,
but would it also have to be reburied at an unrealistically high rate to account for the rapid increases in the 13
C ratio (episodes of high positive δ13
C) throughout the early Triassic before it was released again several times.
Evidence for widespread ocean anoxia
(severe deficiency of oxygen) and euxinia
(presence of hydrogen sulfide
) is found from the Late Permian to the Early Triassic. Throughout most of the Tethys
Oceans, evidence for anoxia, including fine laminations in sediments, small pyrite framboids
, high uranium/thorium ratios, and biomarkers
for green sulfur bacteria
, appear at the extinction event.
However, in some sites, including Meishan
, China, and eastern Greenland, evidence for anoxia precedes the extinction.
Biomarkers for green sulfur bacteria, such as isorenieratane, the diagenetic
product of isorenieratene
, are widely used as indicators of photic zone
euxinia because green sulfur bacteria require both sunlight and
hydrogen sulfide to survive. Their abundance in sediments from the P-T
boundary indicates hydrogen sulfide was present even in shallow waters.
This spread of toxic, oxygen-depleted water would have been
devastating for marine life, producing widespread die-offs. Models of
ocean chemistry show that anoxia and euxinia would have been closely
associated with hypercapnia
(high levels of carbon dioxide).
This suggests that poisoning from hydrogen sulfide, anoxia, and
hypercapnia acted together as a killing mechanism. Hypercapnia best
explains the selectivity of the extinction, but anoxia and euxinia
probably contributed to the high mortality of the event. The persistence
of anoxia through the Early Triassic may explain the slow recovery of
marine life after the extinction. Models also show that anoxic events
can cause catastrophic hydrogen sulfide emissions into the atmosphere
The sequence of events leading to anoxic oceans may have been triggered by carbon dioxide emissions from the eruption of the Siberian Traps
In that scenario, warming from the enhanced greenhouse effect would
reduce the solubility of oxygen in seawater, causing the concentration
of oxygen to decline. Increased weathering of the continents due to
warming and the acceleration of the water cycle
would increase the riverine flux of phosphate to the ocean. The
phosphate would have supported greater primary productivity in the
surface oceans. The increase in organic matter production would have
caused more organic matter to sink into the deep ocean, where its
respiration would further decrease oxygen concentrations. Once anoxia
became established, it would have been sustained by a positive feedback loop
because deep water anoxia tends to increase the recycling efficiency of phosphate, leading to even higher productivity.
Hydrogen sulfide emissions
A severe anoxic event
at the end of the Permian would have allowed sulfate-reducing bacteria
to thrive, causing the production of large amounts of hydrogen sulfide
in the anoxic ocean. Upwelling of this water may have released massive hydrogen sulfide
emissions into the atmosphere and would poison terrestrial plants and animals and severely weaken the ozone layer
, exposing much of the life that remained to fatal levels of UV radiation
evidence for anaerobic photosynthesis by Chlorobiaceae
(green sulfur bacteria) from the Late-Permian into the Early Triassic
indicates that hydrogen sulfide did upwell into shallow waters because
these bacteria are restricted to the photic zone and use sulfide as an
The hypothesis has the advantage of explaining the mass extinction of
plants, which would have added to the methane levels and should
otherwise have thrived in an atmosphere with a high level of carbon
dioxide. Fossil spores from the end-Permian further support the theory:
many show deformities that could have been caused by ultraviolet radiation
, which would have been more intense after hydrogen sulfide emissions weakened the ozone layer.
The supercontinent Pangaea
Map of Pangaea
showing where today's continents were at the Permian–Triassic boundary
About halfway through the Permian (in the Kungurian
age of the Permian's Cisuralian epoch
), all the continents joined to form the supercontinent Pangaea
, surrounded by the superocean Panthalassa
, although blocks that are now parts of Asia did not join the supercontinent until very late in the Permian.
The configuration severely decreased the extent of shallow aquatic
environments, the most productive part of the seas, and it exposed
formerly isolated organisms of the rich continental shelves to
competition from invaders. Pangaea's formation would also have altered
both oceanic circulation and atmospheric weather patterns, creating
near the coasts and an arid climate in the vast continental interior.
Marine life suffered very high but not catastrophic rates of
extinction after the formation of Pangaea (see the diagram "Marine genus
biodiversity" at the top of this article), almost as high as in some of
the "Big Five" mass extinctions
The formation of Pangaea seems not to have caused a significant rise in
extinction levels on land, and, in fact, most of the advance of the therapsids
and increase in their diversity seems to have occurred in the late
Permian, after Pangaea was almost complete. Thus, it seems likely that
Pangaea initiated a long period of increased marine extinctions but was
not directly responsible for the "Great Dying" and the end of the
A hypothesis published in 2014 posits that a genus of anaerobic methanogenic archaea
known as Methanosarcina
was responsible for the event.
Three lines of evidence suggest that these microbes acquired a new metabolic pathway via gene transfer
at about that time, enabling them to efficiently metabolize acetate
into methane. That would have led to their exponential reproduction,
allowing them to rapidly consume vast deposits of organic carbon that
had accumulated in the marine sediment. The result would have been a
sharp buildup of methane and carbon dioxide in the Earth's oceans and
atmosphere, in a manner that may be consistent with the 13
isotopic record. Massive volcanism facilitated this process by
releasing large amounts of nickel, a scarce metal which is a cofactor
for an enzymes involved in producing methane.
On the other hand, in the canonical Meishan sections, the Nickel concentration increases somewhat after the δ13
C concentrations have begun to fall.
Combination of causes
Possible causes supported by strong evidence appear to describe a
sequence of catastrophes, each worse than the last: the Siberian Traps
eruptions were bad enough alone, but because they occurred near coal
beds and the continental shelf, they also triggered very large releases
of carbon dioxide and methane.
The resultant global warming may have caused perhaps the most severe
anoxic event in the oceans' history: according to this theory, the
oceans became so anoxic, anaerobic sulfur-reducing organisms dominated
the chemistry of the oceans and caused massive emissions of toxic
However, there may be some weak links in this chain of events: the changes in the 13
C ratio expected to result from a massive release of methane do not match the patterns seen throughout the early Triassic;
and the types of oceanic thermohaline circulation
that may have existed at the end of the Permian are not likely to have supported deep-sea anoxia.
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