Partly due to the lack of any fossils in these desert deposits it is, in Scotland at least, very difficult to identify a break between the Permian and the Triassic periods. It is therefore not possible to see when the end-Permian extinction took place. The desert deposits of the Permian and Triassic are accordingly collectively regarded as composing the New Red Sandstones (in contrast to the Old Red Sandstones that accumulated in late Silurian and early Devonian times) Thus in Scotland the transition between the Palaeozoic and the Mesozoic are hidden at some level within the New Red Sandstone formations which are mostly preserved in the Hebrides and the south-western Scotland with one small outcrop in the north-east near Elgin.
Volcanism in the Permian was almost entirely related to faulting and consequent pressure relief on the underlying mantle and can be categorised as wholly “intra-plate”. Pangaea was no sooner formed than it began to experience extensional stresses leading eventually to its disintegration in the Mesozoic and early Cainozoic. The magmatism tended to be low key and the volcanoes, although numerous, were small ones which did not give rise to any dramatic landscape features in Scotland.

The Permian-Triassic Extinction - the "Great Dying"

Of the five or so mass extinctions recorded in Earth’s geologic record the one at the end of the Permian period and the start of the Triassic was the most catastrophic. The Permian-Triassic [P-Tr] extinction event was so severe that up to 96% of all marine life and up to 70% of terrestrial vertebrate, insect and plant species became extinct. Sometimes called the “Great Dying” it defines the boundary between the Permian and the Triassic and occurred 251.4 mya [million years ago].

For many years not much was known about the Permian-Triassic extinction but starting in the 1990s studies of what might have happened have stirred great controversy. There have been several proposed mechanisms for the extinction event. These can be easily categorized into catastrophic or gradualistic processes. The former include large or multiple bolide impact events, increased volcanism, sudden releases of methane hydrates from the sea floor and even gamma ray bursts [GRB] from the collapse of super-massive stars in the local region of our galaxy.

Marine Extinction

Marine invertebrates were by far the hardest hit by the P-Tr extinction. In the boundary sections preserving the P-Tr transition, large numbers of species disappeared over only a few meters of sediment or less. In one area of China where the P-Tr boundary has been intensively sampled 280 out of the 329 marine invertebrate genera disappeared.

Systematic collections of fossils from the P-Tr sections worldwide has shown that the extinction wiped out many taxa, including all the remaining trilobites, all the fusulinid and 94% of the nonfusulinid foraminifera genera, graptolites, all of the blastoids, acanthodians, rugose and tabulate corals, 98% of the crinozoans, including all of the inadunates and camerates, 96% of the anthozoans, virtually all the radiolaria, 96% of the brachiopod genera, including all of the Orthids and pruductids, 85% of gastropods, 59% of the bivalves, 79% of the bryozoans, including all of the fenestrates, trepostomes, and cryptostomes, 8 families of ostracods, 90% of gastropods genera and 3 of 16 gastropod families, 97% of the ammonoids and others.

Brachiopods were the dominant shelly benthic animals in late Paleozoic deposits. The diversity and sheer physical volume of brachiopod shells in the Paleozoic geologic record is astounding. Their dominance came to an abrupt end in the late Permian, when ~90% of families and 95% of genera became extinct. Whereas hundreds of species are known from the mid-late Permian, only a few species of brachiopod occur in earliest Triassic deposits. One of these survivors, Lingula, briefly proliferated in the earliest Triassic, and is still alive today. Although very rare today, Lingula has been around since the Cambrian, and is perhaps the oldest extant animal genus.

Bivalves, although present in the Paleozoic as inconspicuous elements, diversify following the P-Tr extinction. It is as if bivalves 'took over' the ecological niches vacated by the brachiopod extinctions. As with other taxa and other extinctions, the earliest Triassic bivalve fauna was cosmopolitan -- early Triassic bivalve faunas are dominated worldwide by the four genera Claraia, Eumorphotis, Unionites and Promylina.

Over 33 genera of ammonoids disappear near the P-Tr boundary.

Radiolaria extinction was very nearly complete

Recovery of marine ecosystems following the P-Tr event progressed very slowly. Reefs, which were abundant in the Permian disappear near the boundary and do not reappear until the middle Triassic. At the same time, stromatolites spread into many "normal" marine environments during the early Triassic, for the first time since the Ordovician. Stromatolites make a handy food source for benthic marine grazers, and their presence in early Triassic normal marine deposits was probably facilitated by the extinction of shallow marine fauna that would normally consume them.

Terrestrial Invertebrates

The Permian was a time of great diversity for insects but at the end of this period there was the only known mass extinction of insects. As may as nine insect orders became extinct and ten others suffered severe drops in diversity. Most fossil insect groups, which are found after the Permian-Triassic boundary, differ significantly from those that lived prior to the P-Tr extinction. In the mid to late Triassic deposits fossils overwhelmingly consist of modern fossil insect groups.

Terrestrial Plants

Studies into spores and pollens from Israel, China, the southern Alps and elsewhere show that many Permian pollen types disappear near the P-Tr boundary. Other evidence from Europe, Africa and Asia showing that the dominant Permian conifer taxa were replaced post-extinction by a few groups of surviving lycopsids, especially Pleuromeia. Lycopsids living today often function as pioneer plants, recolonizing disturbed areas.Thus the floral mass extinction selectively removed the dominant large plants of the Late Permian and left small, weedy survivors.

One consequence of the P-Tr floral extinction was a unique early Triassic hiatus in coal formation, dubbed the “coal gap”. Whereas thick coals are widespread from the Carboniferous to the late Permian, and from the middle Triassic onwards, there are no coal seams at all in the early Triassic. Thin coals reappear by the mid-Triassic, followed by coals comparable to those of the late Permian. It has been argued that the ~10Ma coal hiatus reflects the gap between the extinction of peat-forming plants at the P-Tr boundary, and the appearance of new plant groups in the mid-Triassic tolerant of the anaerobic, acidic environments in which peat accumulate.

Another interesting aspect of the P-Tr in continental [and marine] sections is a short-lived abundance of fungal material. While fungi are present throughout the Phanerozoic, there is a very abrupt but short-lived enrichment of spores and fungal material. This "fungi" spike has been reported from marine and terrestrial P-Tr sections in Greenland, the Zechstein Basin, the southern Alps, Israel, Australia, the Karoo Basin in South Africa and other locations. This spike occurs irrespective of depositional environment (marine, lacustrine, fluvial), floral provinciality, and climate zonation. This fungal event can be considered to reflect excessive dieback of arboreous vegetation, effecting destabilization and subsequent collapse of terrestrial ecosystems with concomitant loss of standing biomass.

The Karoo P-Tr boundary is marked by a pronounced change in depositional conditions. It is noted that the Karoo P-Tr boundary is marked by a rapid and apparently basin-wide change from meandering to braided river systems, which is argued to be a result of the rapid die-off of sediment-binding vegetation within the basin. Before the origination of land plants in the Silurian, braided fluvial systems were the norm. Other recent studies have found similar abrupt changes from meandering to braided river deposits during Triassic time in Australia, Antarctica and northeren Europe. That supports the notion of a global die-off of land plants, including extinction of Glossopteris trees and bushes, which resembled modern ginkos. A variety of ferns and early pine trees also went extinct at the same time.

Terrestrial Vertebrates

Of the groups of vertebrates that survived most suffered heavy losses and some very nearly became extinct. In fact 21 tetrapod families [63%] became extinct at or near the P-tr boundary. Some of the survivors did not however last long into the Triassic and of some of those that did later produced diverse and long lasting lineages. There is good evidence to indicate that over two-thirds of terrestrial amphibian, sauropsid [reptile] and therapsid [mammal-like reptiles] families became extinct. Large herbivores suffering the greatest losses.

Post-event recovery

During the early Triassic up to 8 million years after the event the plant biomass was still insufficient to form coal deposits. This would suggest that there was insufficient food for herbivores. Each major part of the ecosystem was dominated by a relative small number of genera, which appeared virtually worldwide. The herbivorous therapsid Lystrosaurus accounted for about 90% of the early Triassic land vertebrates. Similarly the bivalves Claraia, Eumorphotis, Unionites and Promylina were the predominant genera in the early Triassic. This suggests that the early Triassic ecosystem was not a healthy one as a healthy ecosystem has much larger numbers of genera, each living in a few preferred types of habitats.

The early Triassic ecosystem was therefore full of “disaster taxa’ that took advantage of the devastated ecosystem and enjoyed a temporary population boom and increase in their territory. The Lingula, Stromatolites [which had been confined to marginal environments since the Ordovician], Pleuromeia [a small weedy plant] and Dicrodium [a seed fern] all predominated in the early Triassic.

Before the extinction about 67% of marine animals were anchored to the ocean floor such as brachiopods and sea lilies but after the extinction event only 50% were sessile. There was an increase in more complex mobile species such as sea urchins, crabs and snails. Bivalves were fairly rare before the extinction event but became numerous and diverse in the Triassic and one group, the rudist clams, became the Mesozoic’s main reef-builders.

For a long time after the end Permian extinction fungal species were the dominant form of terrestrial life. Prior to the event they only made up 10% of the remains found in the fossil record but after the event they grew rapidly to make up nearly 100% of the available fossil record. It has however been argued that fungal hyphae are simply better suited for preservation in the environment thus creating  an inaccurate representation of certain species in the fossil record.

After the extinction event the few surviving reptiles became the ancestors of all the later Mesozoic archosaurs and mammals. The most common and widespread animal was called Lystrosaurus. This was a medium sized

Lystrosaurus, one of the main “survivors” of the end Permian extinction,1 metre long about the size of a pig [Illustration by John Sibbick[ [http://www.johnsibbick.com/]

herbivore, which became the ancestor of all the mammals. Lystrosaurus was so abundant and widespread that this genus alone constituted as much as 90% of some earliest tetrapod faunas. The only other large animals were the amphibians, reptilomorphs and the semi-aquatic Proterosuchus, which
resembled a small 1.5 meters unarmored crocodile.

Proterosuchus, another survivor of the P-Tr extinction. The largest land reptile in the early Triassic equivalent in size to today’s Komodo Dragon [Illustration by Arthur Weasley]

What Caused the Extinction?

There are many proposed mechanisms for the extinction event, including both catastrophic and gradualistic processes, similar to those theorized for the Cretaceous – Tertiary [K-T] extinction event.

Many different aspects of the extinction period have been documented:

• Salinity of the sea fell sharply during the Permian, changing oceanic physics to make deep water circulation more difficult

• The atmosphere went from very high oxygen concentration [30%] to very low [15%] during the Permian

• The evidence shows global warming AND glaciations near the P-Tr boundary

• Extreme erosion of the land suggests that ground cover disappeared

• Dead organic matter from the land flooded the seas pulling dissolved oxygen from the water and leaving it anoxic at all levels

• A geomagnetic reversal occurred near the P-Tr boundary

• A series of great volcanic eruptions was building up a gigantic body of basalt called the Siberian Traps

Evidence that the Cretaceous – Tertiary [K-T] extinction was caused by an impact event has naturally led to speculation that an impact event may also have led to other extinction events including the P – Tr. Evidence that an impact event was involved in the P –Tr event has come from shocked quartz found in rocks of the correct age both in Australia and Antarctica. However the origin of the shocked quartz found at Graphite Peak in Antarctica has recently been re-examined, and it has been suggested that the features seen are not due to an impact event, but rather due to plastic deformation consistent with formation in a tectonic environment such as volcanism. 

Other evidence that something from space may have been involved comes from the high levels of complex carbon molecules called buckminsterfullerenes, or Buckyballs with the noble [chemically inert] gases helium or argon trapped inside their cage like structure. Fullerenes, which contain at last 60 carbon atoms and have a structure resembling a football are named after Buckminster Fuller, who invented the geodesic dome. It is known that that these particular Buckyballs are extraterrestrial because the noble gases trapped inside have an unusual ratio of isotopes.

Terrestrial helium is mostly helium-4 and contains only a small amount of helium-3, while extraterrestrial helium, the kind found in these fullerenes, is mostly helium-3. These complex molecules formed in carbon stars under extreme conditions of temperature and pressures and are the only way these extraterrestrial noble gases could be forced inside a fullerene. These noble gas-filled fullerenes were thus formed outside the Solar System and their concentration at the Permian - Triassic boundary means they were delivered to the earth by a comet or asteroid.

The telltale fullerenes containing the helium and argon were extracted from sites in Japan, China and Hungary where sediments at the boundary were well exposed. Fullerenes are found at very low concentrations above and below the boundary layer, but they are found in unusually high concentrations at the time of the extinction. The concept that these complex molecules and their contained noble gases is a controversial theory as attempts to replicate the evidence have proved difficult.

There is also strong evidence that the extinction happened very rapidly, in as few as 8,000 to 10,000 years – a “blink in the eye” in geologic terms. However recent work on Permian rocks in Greenland have suggested that instead of the extinction taking under 10,000 years it may have lasted as long as 80,000 years and have three distinct phases. The extinction appeared to kill land and marine life selectively at different times.

Previously it was thought that any asteroid or comet colliding with the earth would leave strong evidence of the element Iridium, the signal found in the sedimentary layers from the time of the dinosaur extinction. Iridium is found in the Permian – Triassic boundary but not in such high concentrations. This might be explained by different compositions and ultimately the origins of the two impacting bodies.

Several possible craters have been proposed as possible causes of the P –Tr extinction including the Bedout structure off the northwest coast of Australia and the so called Wilkes Land crater in East Antarctica. It should be noted that if the impacting object were a comet then evidence of this would be invisible geologically. Also any impact crater may already have been subducted if the object impacted in the sea. It could also be speculated that extensive lava flooding from the mantle after the crust has been punctured or weakened may mask a crater produced by a very large impacting body.

It may be that a massive impact triggered off large-scale volcanism either directly or by a “contra coup” effect. Such large volcanism was present at the time in the Siberian Traps.

The flood basalt eruption, which produced the Siberian Traps, was one of the largest known volcanic events on Earth and covered 200,000 square kilometers with lava. These eruptions were previously thought to have lasted for millions of years but recent research dates them to 251.2 +/- 0.3 Ma – immediately before the end of the Permian!

An impact event has thus been proposed as the cause of the end P-Tr extinction but the evidence for this, unlike the K-T extinction, is somewhat ambiguous. There is some evidence however that large-scale volcanism may have played a part. Several of the classic P-Tr sections in South China are capped by 3-6 cm thick altered volcanic ash layers containing bypyrimidal quartz, melt spherules, glass shards and isotopic ratios typical of siliciclastic rather than basaltic volcanism. The volume of these ash layers is estimated to be about 1000 cubic kilometers. This material has been dated and the age is shown to be comparable to the inception of the main stage of the Siberian volcanism.

The P-Tr boundary thus occurs roughly at the same time as the extrusion of the largest known Phanerozoic flood basalts, the Siberian Traps.

The Siberian Traps are thought to be the result of a mantle plume. A mantle plume is a giant pulse of heat that rises towards the surface from the core/mantle boundary. Plumes are easily identified but not well understood and they are believed to be part of a cooling mechanism for the core.

The Caloris Basin. Photographed by Mariner 10 in 1974. This is a mosaic of half of the basin as it appears on the terminator of the planet.

Mass Concentration in Antarctica as imaged by the NASA GRACE spacecraft 2006

Whatever the cause of flood basalt eruptions a large amount of anomalously hot material rises to the surface and ponds below the earths crust in a head which can be 1000's of km wide and 100's of km deep. This pond of basalt magma penetrates the crust through fissures pouring gigantic amounts of basalt onto the surface.

They are centered around the Siberian city of Tura and also encompass Yakutsk, Noril'sk and Irkutsk. Present coverage including associated pyroclastics is just less than 2 million square kilometers, which is an area greater than that of Europe. It is thought that they do not constitute a single continuous province but rather the amalgamation of several sub-provinces.

Volcanism on this scale would release massive amounts of carbon dioxide and sulfur dioxide as well as aerosols that would block a significant amount of sunlight. Initially this would result in cooling. However the sulfur dioxide would be removed from the atmosphere in the form of an acidic rain, and within a few months most of the particulate matter would be gone from the atmosphere. This may have played some part in the extinction on land. However the carbon dioxide would remain and this would result in warming. Another contributory factor to the release of vast amounts of greenhouse gases was the fact that the Siberian lava came up through the largest coal basin in the world. The vaporized coal would have produced immense amounts of carbon dioxide and sulfur dioxide.  

The largest eruption of the 20th century, Mt Pinatubo is tiny compared to the Siberian Traps but caused a 0.5 degree drop in global temps the year after it erupted. The largest eruption in historic memory occurred on Iceland in 1783-84 when a volcano called Laki erupted and spewed out 12 cubic km of lava onto the island (the Siberian Traps erupted about 3 million cu km). The poisonous gases given out are recorded as killing most of the islands crops and foliage and lowering global temps by about 1 degree. Writing in the 18th century, Benjamin Franklin (the then American Ambassador in Paris) desribed 1784 as a year without a summer. Ash from the Laki eruption blacked out the sky and crops failed across Europe.

If events this size can affect temperatures and large areas then the effects of a large scale flood basalt are incomprehensible. Imagine a Laki volcano erupting every year for hundreds of thousands of years!

Warming as a result of volcanic carbon dioxide may also have resulted in the dissociation of methane hydrates, and the development of anoxic conditions in the oceans. Warming promotes anoxia in two ways:

1. The solubility of oxygen in water decreases with increasing water temperature
2. Warming can promote anoxia if the equator-to-pole temperature gradient is weakened, since this would weaken the oceanic circulation

Another minor effect is the destruction of the ozone layer caused by gas emissions. Chlorine and fluorine gases are erupted from almost all volcanic eruptions and these destroy the ozone layer. Without the ozone layer, harmful UV rays can kill organisms therefore contributing to a mass extinction.

Study of other mass extinction intervals shows that several are correlated with both flood basalts and anoxic episodes. Of 11 major flood basalts, 7 coincide with some form of extinction episode. Two of these extinction events, the Toarcian (Jurassic) and latest Paleocene, are similar in many ways to the P-Tr extinction. The Toarcian extinction occurred at the same time as the Karoo-Ferrar basalts in South Africa and Antarctica were extruded about 183 +/- 1 million years ago, and the last Paleocene event occurred at the same time as the extrusion of the Brito-Arctic flood basalts about 55 million years ago. Like the end-Permian, both events were associated with warming, marine anoxia, major carbon isotope excursions, and the preferential extinction of benthic marine organisms. Methane release has also been proposed for both events.

Conclusion

It is probably fair to say that no one single event caused this extinction. Rather, a combination of a number of disadvantageous conditions eventually led to the extinction. The uniting theme for both marine and trrestrial extinctions seems to be global warming, exacerbated by volcanism, methane hydrate release and the relative inefficiency of the global carbon sinks. Whether further research will support the concept that a bolide impact may have aggrevated matters remains to be seen. It may be that volcanism and an impact event is required to tip the Earth into such a critical state.
It is interesting to ponder what might have been if no such extintion had taken place. The earth today would be the same but undoubtedly the animal life alive on it would be different. Some of the organisms thriving in the Permian if they had not been wiped out may have gone on to better things, we will never know.

Could it happen again? We are probably safe enough in the immediate future. There is no massive volcanic areas erupting, there is effective oceanic circulation and there are efficient carbon sinks. This does not mean to say we are safe from space either from a large bolide or even  gamma ray burst or even something else we have not, so far, thought about. It has to be said however safe we feel at the moment the destruction of Homo Sapiens is a near certainty.

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