Recovery from the Permo-Triassic filter

Following mass extinctions there are three phases to the recovery that follows. These are

• survival
• diversification
• stabilisation.

Survival is when, having survived the extinction event, organisms must be able to adapt to an environment which may have been changed by whatever caused the extinction. This may mean a change in environmental variables such as temperature, salinity and atmospheric gas proportions. With so many organisms having been killed there will also be a drop in the number of breeding pairs within a taxa. If this slows down reproduction to a rate where it is exceeded by the death rate within an order then it will eventually lead to extinction.

There are examples of orders of organism which managed to scrape across the Permo-Triassic boundary, only to become extinct early in the Triassic. These include the Orthocerids and the Conodonts. When some of these species became extinct there would have been a knock-on effect to those organisms that preyed on them higher in the food chain. This becomes a phenomenon known as the "Lilliput effect", where often only the smaller species (which are lower down the food chain) survive through extinctions.

Diversification occurs after the survival phase is complete. Surviving lineages have new opportunities to diversify into niches left vacated by the now extinct predecessors. While this radiation is rapid for all surviving taxa, some orders can diversify faster than others. After a mass extinction the orders which diversify the fastest eventually benefit from being able to fill the empty niches and becoming the dominant order. At the Permo-Triassic boundary this is especially the case with the Ammonoidea.

P/Tr Volcanism

The Siberian Traps were the largest volcanic eruption in Earth history and they occurred right at the same time as the largest extinction event in Earth history. Co-incidence? It is ingrained in everyone from an early age that volcanic eruptions are dangerous to life so the Siberian Traps could indeed hold the key to explaining the Permo-Triassic extinction event.

What are they and how did they form?

The Siberian Traps are a large igneous province which were a 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. Whatever their cause a large amount of anomalously hot material rises to the surface and ponds below the earth's 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. Events like this are known as Flood Basalt eruptions and fortunately are very rare with only 8 have occurred in the last 250 million years.

Where are they and how big are they?

They are centred around the Siberian city of Tura and also encompass Yakutsk, Noril'sk and Irkutsk. Present coverage including associated pyroclastics is just under2 million square kilometres which is an area greater than that of Europe. Estimates of the original volume of the traps range from 1 million cubic km up to 4 million cubic km. According to P. B. Wignall in his 99 paper, "the distribution of lavas suggests that they do not constitute a single continuous province but rather the amalgamation of several subprovinces." The eruptions lasted at full intensity for about a million years which coincides with the extinction. The most accurate dating method available at the moment is Argon-Argon radiometric dating which still contains sufficient uncertainties to conclusively prove the exact timing.

Could they cause or add to a mass extinction event?

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 temperatures the year after it erupted. The largest eruption in historic memory occurred on Iceland in 1783-84 spewing 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 island's crops and foliage and lowering global temps by about 1 degree. If events this size can affect temperatures and large areas then the effects of a large scale flood basalt are incomprehensible.

How could the Siberian Traps cause global mass destruction?

The immediate area would be affected by such things as lava and pyroclastic flows but how does this affect the other side of the world? The real power of the Siberian Traps was the climate altering potential by the emission of ash and gases. The Siberian Traps are recognised as having a large proportion of pyroclastic deposits relative to other flood basalts. This indicates an explosive nature with much ash and gases being pumped into the atmosphere. All of this ash and gas has two main effects that, even though they are opposite to each other, act on differing time scales.

Initially sulfur aerosols and volcanic ash envelop the earth's atmosphere blocking out sunlight and sending surface temperatures plunging. Ash and sulphur aerosols can remain in the upper atmosphere for hundreds to thousands of years which would be enough to cause a significant glaciation. At the end of the Permian period the biggest ever drop in sea level in history occurred. Two scientists named Holser and Magaritz in 1987proposed that such a marine regression could be caused by a large scale glaciation.

The second major effect is the emission of greenhouse gases such as CO2, methane and also water vapour. Green house gases warm the climate by allowing sunlight to pass through, while heat reflected by the Earth itself cannot penetrate the atmosphere so is retained. Greenhouse gases stay in the atmosphere much longer so their climate changing effects can last for millions of years.

A third, minor effect is the destruction of the ozone layer caused by gas emissions. Chlorine and fluorine gases are emitted 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.

Extraterrestrial Impacts

Background Information

Large meteorites with the potential to cause major global environmental change strikes the earth on average every 100,000 years, according to NASA. Geologically speaking, 100,000 years is a short time interval making it plausible that every so often a "biggie" meteorite could hit the earth and cause a global catastrophe on the Permian type scale. When a large meteorite collides with the earth it leaves much more evidence than the obvious crater. Meteorite craters are often destroyed by burial or plate tectonics so for older impacts where the crater has long since gone the other lines of evidence assume more importance.

Rare Earth Element Anomalies

Earth elements occur in different proportions in meteorites than in the Earth's outer crust and one of the most important of these for meteorite strikes is Iridium. Iridium is a stable element which is only present in the Earth's crust in extremely tiny amounts but is present in much larger quantities in meteorites. Upon vaporization at impact, Iridium is scattered into the atmosphere and gradually falls onto the Earth's surface over time. Rocks that were at the surface at the time of impact are then enriched in iridium which can be measured today in a laboratory. Even relatively small meteorites can cause changes in iridium levels, producing so-called "iridium spikes", because there is so little originally in the crust.

Comets can also impact the Earth bit unfortunately they contain little or no Iridium.

Shocked quartz

Shocked quartz (or Stishovite) is a high pressure polymorph of quartz produced at or close to the impact site. It is produced under such high pressures that the only way it can form is by cosmic impacts. Finding traces of this in any geological strata is a very good indicator of an impact. Shocked quartz grains can only usually be seen with a scanning electron microscope.

Ejective beds

Much rapid deposition occurs near to an impact site and the ejectile beds can often be preserved in the geological record. These horizons appear similar to others produced by other means so that they are difficult to identify as ejectile beds.

The Cretaceous-Tertiary Impact

Meteorites have become a popularly debated cause for mass extinctions ever since the Chixulub crater was identified in the early 90's. The Chicxulub crater is in the Yukatan peninsular in Mexico and is seen as good evidence for a meteorite which could have killed off the dinosaurs in the K-T extinction event. This has been dated accurately to the K-T extinction event by radiometric techniques and also through stratigraphy analysis.

Since this discovery much work has been focused on meteorite impacts being possible causes of other mass extinctions with many people focusing on the largest, the Permo-Triassic event . Unfortunately the evidence for an impact at the time of the Permian / Triassic boundary is much more ambiguous that that for the Cretaceous-Tertiary boundary. There are many reasons for this with the best one probably being that the Permian was much longer ago with the evidence being exposed for much longer.

Climate Change

Climate is continuously changing, and through studies of ice cores and the rock record we can see that throughout the Earth's history the climate has changed a number of times. Climate change has been linked to most of the main extinctions (late Cambrian, late Ordovician, end Permian, the K-T event and the late Triassic.) Climate change often happens as a result of other factors such as volcanism and impact and can lead to the loss of many species.

Getting HOTTER

During the Permian the continents were in one landmass called Pangea. As all the land was together and was so large, there was a hot dry interior because a majority of the land was away from the sea and experienced little rainfall. It also had great seasonal fluctuations due to the lack of the moderating affects of a large water body. The climate's temperature may have risen due to the increase in volcanic activity. During the end of the Permian the Siberian traps were erupting, releasing vast amounts of different gasses into the atmosphere.

One of these gasses was Carbon dioxide (CO2). This has an insulating affect on the atmosphere making the climate temperature warmer. Increased climate temperature has also been shown to slow the metabolism of creatures, and upset the formation of internal and external carbonate (CaCO3) skeletons. Many marine organisms have carbonate skeletons. If these creatures were unable to form their skeletons they would have either no support for their bodies or no external protection, so would be unable to survive. (A. H. Knoll) A hotter climate of the low latitudes during the Permian lead to a reduction in the area of coal swamps. As this habitat decreased, species that lived there, such as amphibians and some spore bearing plants, became extinct.

Getting COLDER

As well as getting hotter there is evidence that the climate also cooled. Sedimentological evidence for cooling comes from glacial deposits in polar zones, and thick dune sands and evaporites from temperate zones that represent a cool dry environment. As already discussed in the section on Vulcanism some of the volcanic gasses released from the Siberian trap flood basalts could have the opposite affect to the CO2, cooling the climate instead of heating it. Other evidence comes form the reduced presence of carbonate limestones around the end of the Permian. This process would have had the greatest affect in the tropics where most of the Earth's limestone production occurs. Cooling would eliminate the tropical ares and kill tropical species, and if there were less Carbonate producers there would be less carbonate which is what is seen.

Another cooling affect comes from glaciation. Cooling can happen in low latitudes without there being glaciation and in this way just the cooling of the climate would be the cause of extinction by the method mentioned above.

What did these temperature changes do?

The cooling methods appear to contradict the evidence for heating. However, if chilling and heating happened repeatedly, with geologically rapid repeated heating and cooling occurring, many species would not have time to adapt to the alternative environment. Many that were not able to live in the new climate would have suffered depletion in numbers and possibly extinction.

A cooler climate on its own would have left warm tropical taxa with nowhere to go that with the correct temperature for them, so they would had to have adapted or die out.

A cooler climate has also been linked to changes in ocean circulation. When there is a cool climate, there are changes in the sea's circulation patterns. Water from the deep oceans which is laden with CO₂ and hydrogen sulphide (H₂S) is bought up to the surface. There is also the possibility that methane (CH₄) could also escape from sea floor sediments when they are disturbed by circulation changes. Not much of these compounds are normally found in surface waters and the presence of them in abnormally large quantities could kill many marine organisms.

The changing climate could also have affected terrestrial species in that it would alter vegetation patterns which would lead to food shortages for smaller animals,which in turn caused food shortages further up the food chain. The cold glaciated seas and the toxic compounds bought up to the shallow seas could suggest why so many more marine species became extinct compared to terrestrial extinctions.

It also has been postulated that changes in salinity can occur due to a hotter climate because of the increase in halite deposits. More halite on land would mean less salt circulating in the oceans. Fluctuations in salinity would alter the marine environment which would lead to the selective extinction or reduction in numbers of families that were not able to cope with such changes.

Why the formation of a super continent at the End-Permian played a part in the mass extinction.

During the Late Permian, tectonic movement of the plates led to the formation of one huge land mass called PANGEA. The formation of this super continent had a number of side affects which could have led to species becoming extinct.

The Weather

With such a large mass of land, the weather was severe. With all the land joined, much of the super continent was inland and away from the cooling affects of the sea. A hot dry interior formed where the land was low lying, and higher altitudes experienced unseasonal weather patterns. These extremes would have forced specialized organisms to move to other areas, or share limited resources in the remaining suitable habitats. The competition for food and land could have led to some species suffering losses in numbers, or even extinction.

Land levels

The land level of the Permian super continent was more low lying than today. Sea levels rose as a result of melting of the Permo-Carboniferous glaciation and, since the land was low, flooding was extreme. The flooding would have reduced the land area, and changed the coastal water levels and environments dramatically. The geologically regular change in habitat over the Permian period would have caused some species to die out if they could not adapt to cope with changes or move to other habitats.

Coastal regions

With the land joined together, the area of coastline around the edge of the super continent would have been reduced. Many researchers believe the reduction of shallow marine environments led to less habitat being available to marine organisms. With less habitat, there would have been competition for space and food. Any species that could not adapt to a different environment would be challenged by other species for the remaining shallow marine habitat. This could be one reason why there has been such a loss of marine species recorded for the Permian.

Land at the poles

The presence of land at the poles leads to glaciation. Parts of Pangea lay over the poles. The Pangean super continent led to many changes in the shape of the land, glaciation patterns and climate, which in turn altered sea level and salinity of the oceans. These affects are often interlinked. The presence of Pangea helped to initiate extreme environments, and along with other evidence, such as volcanism and impact, led to the biggest extinction seen in the history of Earth.

Glaciation

Why glaciation occurs.

As well as global cooling due to changes in the proportion of gases in the atmosphere mentioned in the section on volcanism, there have been other suggestions as to why glaciation occurs and can be cyclical.

The Milankovich cycle.

Land over the poles.

Having land over the poles promotes glaciation. The has almost always been ice at the poles. If there was ice in the sea, the salinity of the sea would limit the extent of the sheet. On land this does not melt the fresh water ice and the sheet can spread until it reaches too warm latitudes. Ice at the poles has a knock on effect: because the ice is not floating in the water, it has actually taken water out of the oceans lowering sea level.

Evidence for glaciation.

Evidence for glaciation can be seen in abundant glacial deposits of Permian age found in Australia, Siberia and in the North Sea. Sea level change due to ice sheets also lead to layers of Shale, Siltstone, Limestone, Sandstone, Marl and Dolomite indicating a regressive period.

Effects of Glaciation.

From geochemical evidence, these ice sheets caused a global drop in temperature. This had the affect of forcing species from normally low latitude habitats to move towards the tropics and the equator, and tropical and equatorial species to die out as they had nowhere to migrate to. Glaciation does not necessarily cause climate cooling in the lower latitudes, but has been linked with it.

Glaciation affects global sea level.

As ice takes up large quantities of water in its formation, during glaciation the sea level would have dropped. As glaciation was fluctuating while Pangea moved between poles, this gave rise to sea level fluctuations. The low lying nature of the Pangean topography meant that quite small fluctuations in sea level would have repeatedly flooded and exposed large areas of coast line. Regression would have drained inland water bodies and river estuaries leading to creatures having to migrate and adapt to new conditions, or die.

The repeated changes in type and size of coastal habitats would have lead to species living there being repeatedly disturbed. A link has been shown between the size of habitat available, and the number of creatures that can live there, so if reduction of habitat occurred the number of species that could live there would be reduced. During prolonged periods of glaciation, along with the limited coast line of Pangea, the low water level would lead to a reduction in shallow seas. As shallow seas are the most productive regions, over-all marine productivity would be reduced leading to a strain on the marine environment. The strain could have been due to less space to live or lack of resources, and lead to extinction. As the shallow shelves are the main areas for primary production there would have been repercussive affects further up the food chain.

Sea level fluctuations are linked to another method of extinction. Changing depths of water change currents and bring up anoxic water from the deep oceans. This de-oxygenates the upper levels of the water column causing death. There is only convincing evidence for this at the end Devonian and end Ordovician periods so it may not be a viable hypothesis for the Permian Mass Extinction.

Opposing Views

It has been suggested that the anoxia, mentioned in the section above, occurred too late to have caused the Permian extinction, and is also insignificant compared to other factors. There is some speculation that there was an unglaciated time at the end of the Paleozoic, due to Pangea moving off the South Pole. Glaciation was apparent in the southern hemisphere from the mid Devonian into the middle Permian because of the landmass over the South Pole, but most of the extinctions occurred at the end of the Permian and into the Triassic when Pangea had moved. This would suggest that glaciation was not a contributing factor to the end Permian extinction.

An explanation for why these different opinions occur is because Pangea was so large and stretched from far up in the northern hemisphere, to low down in the southern hemisphere. As the super continent moved off the south pole it became unglaciated, but the continent encroached on the north pole and ice sheets were formed in the north. This meant that throughout much of the Permian there was fluctuating glaciation between the south and north. In a paper by Hallam and Wignall it is suggested that regression, which is often linked with glaciation, did not occur. Although sea level drop is considered one of the main causes of the extinctions they suggest that evidence from biostratigraphy shows that there was not a regression at all. Although taxa became extinct in end Permian, Salt Range sediments of North Pakistan, there is no discontinuity in deposition. This suggests that regression did not happen at the time of extinction.

Other evidence supporting their hypothesis comes from the Dolomites of northern Italy where no change in deposition can be seen at the end of the Permian.

Biotic recovery following the P/Tr crisis

Delayed recovery? (current dating: Early Triassic = c. 2 millions years )
Recovery occurred in the Middle Triassic? (fast recovery cases (or initial recovery) may have occurred in the Early Triassic?)
Various fossil groups have different recovery rates following the P/Tr extinction
Major factors affecting biotic recovery following the mass extinction

Brachiopoda, or lamp shells, is an ancient group. Fewer than 300 species survive today, but the 30,000 fossil species have been described. Brachiopods are all attached bottom-dwelling marine forms that mostly prefer shallow water. Externally they resemble bivalve mollusks in having two calcareous shell valves secreted by the mantle. However, they have dorsal and ventral valves rather than right and left lateral valves.

Biotic recovery after modern and ancient defaunation events: What are the similarities and differences in the response of the marine ecosystem to biotic crises at different scales? The diagram below shows my model of how the benthic marine ecosystem recovered after the end-Permian mass extinction event. In the immediate aftermath of the event only the very shallowest tiers were occupied; predominantly by deposit feeders (stage 1). The shallow infaunal suspension feeders returned next (2), followed by the higher epifaunal tiers (3). Eventually tiering levels above and below the sediment surface returned to pre-extinction levels (4).

Objectives:
Testing interactions between marine communities and environments during P/Tr mass extinction & subsequent recovery

Methods
1. Marine communities throughout P-Tr transition
Alpha diversity, taxonomic/ecologic dominants, bioturbation extent, tiering, Bamchachian megaguilds & sedimentary structures

Diversity indices (Shannon & Simpson indices and Dominance) were calculated using the paleontological statistical
analysis package PAST (Hammer et al., 2001).

The Shannon index evaluates the diversity of paleo-community, whereas both Simpson index and
Dominance (= 1-Simpson index) evaluate the evenness of the community.