We are now back in the exhibitions of the Natural History Museum of Denmark. This second video in the lecture on mass extinction events will take us on a journey back to the greatest of all mass extinctions in Earth's history. This took place 251 million years ago, at the end of the Permian age. Here around 90% of Earth's species disappeared in just a few tens of thousands of years. Evidence for the event was discovered as far back in 1841, by the English geologist John Philips. Philips compiled an immense piece of database research, nineteenth century-style. He contacted leading colleagues who were specialists on marine invertebrates, and asked them to compile a list of fossil marine species, subdivided by age within the-then established geological periods. Philips then summed up the number of species within each subdivision of each time periods, and also noted which groups of organisms were present and how relatively abundant they were. Similar studies are being made today, with the help of computers and electronic databases of fossil taxa. However, Philips did it by hand, and his results are still relevant. This is a curve of results from a later refined study in the same style also made by Philips. On the vertical axis you have geological time, with the youngest periods at the top and oldest at the bottom. The horizontal axis shows the total number of species around during a given subdivision of time. The further to the right, the curve is, the more species exist at a given time. Philips' results showed three things: First of all, there was an increase in total biodiversity of marine invertebrates - the number of species - with time. The curve goes further towards the right hand side as we move up in time. Secondly, Philips noted that there were remarkable differences in the overall faunal composition through time - which groups of marine invertebrates existed and how relatively common they were. He was able to distinguish three separate intervals of geological time, characterised by a consistent formal composition: the Palaeozoic, the Mesozoic and Cenozoic, respectively. Thirdly and finally, his results showed two marked intervals with low biodiversity. They are marked by dips in the curve. These intervals with a relatively low number of species fell right on the boundaries between the above mentioned intervals: between the Palaeozoic and Mesozoic, and the Mesozoic and Cenozoic, respectively. Today, we know that these intervals are coincident with two of the largest mass extinctions in Earth's history: the Permo-Triassic event and the Cretaceous-Tertiary event. However, in Philips' time Lyell's uniformitarianism and its gradualist offspring was getting firmly entrenched, and Philips' results were not seen as indicative of a major revolution, let alone extinction event in Earth's history. Much later, in the late 1950'es and early 1960'es, German palaeontologist Otto Schindewolf returned to the vast faunal differences between the Permian and Triassic periods. He pointed out that there were great differences between the groups of organisms alive in the two periods both on land and in the sea. Calling his idea neocatastrophism, he invoked cosmic radiation from nearby supernova explosions to explain the large-scale extinction. However, his views did not gain acceptance outside Germany. It was only with the recognition of the Cretaceous-Tertiary mass extinction event from the 1980'es and onwards, that geologists and palaeontologists began to investigate faunal turnovers in other periods, than that at the end of the Permian. In the following, I will look at the Permo-Triassic mass extinction event, as it is generally understood among scientists. However, this is a subject currently undergoing many studies, and it is likely that our understanding will be further enhanced in the recent future. The Permian period lasted between 299 and 251 million years ago. The Permian was very much the age of Pangea; the time when most continents on the Earth's surface were joined up into one giant supercontinent. Similarly the surface was dominated by the huge Panthalassic Ocean to the west of Pangea, and the much smaller Paleo-Tethys to the east. In the Permian seas, including the many shallow seas covering part of the continents, marine biodiversity was at a all time high never previously attained in Earth's history. Especially the shallow tropical seas teemed with life, as complex reef structures built by sponges, bryozoans and corals formed. These reefs formed a complex substrate, where brachiopods, bivalves, snails and other mollusks could thrive, while shelled cephalopods such as nautiloids and ammonoids swam above, alongside sharks and fish. Other animals burrowed into the sea bottom itself. On land, the picture was similar. Organisms had diversified greatly since life had ventured ashore in the Devonian period some 150 million years before. There were now complex ecosystems with multilevel food chains populated by an abundance of reptiles and amphibians on land. The first large plant eaters had evolved. They were the pareiasaurs - distant relatives of modern turtles - who weighed in hundreds of kilos. Hunting them were the equally large saber-toothed gorgonopsians - who were distant reptile-like relatives of modern mammals. But the diverse and complex Permian ecosystems on land and in the sea were about to come crashing down in the worst ecological disaster in Earth's history. The trigger was to be found in present-day Siberia: the eruption of the Siberian trap volcanic field situated in the northernmost Pangea. The eruption phase of this massive volcanic field spans the Permian-Triassic boundary and lasted around 600,000 years in total. It is estimated that between two and three million cubic kilometres of lava was erupted, covering an area of almost 4 million square kilometers. That is comparable to the area currently covered by the combined land masses of West- and Central Europe, the British Isles, Italy, the Iberian Peninsula and the Nordic countries. Individual lava flows from one single eruption of the Siberian traps achieved thicknesses of several hundred metres and some ran into thicknesses more than two kilometers thick. The event that triggered the devastating eruption of the Siberian Traps was the appearance of a mantle plume underneath the area. A mantle plume is a funnel of extremely hot rock rising from the Earth's interior towards the surface. Where it arrives below the lithosphere, it gives cause to extensive volcanism on the surface. Recent studies of minerals in the lavas from the Siberian Traps and modelling indicate that this mantle plume might have behaved differently from others. Instead of halting at the boundary between the lithosphere and the astenosphere, it continued upwards until it stopped below the continental crust itself. Furthermore, it appears that on its way up the plume met and recycled a slab of old subducted oceanic plate. This significantly altered the chemical composition of the melt and the lavas deriving from it, which again was to have devastating side-effects. Recent studies indicate that it was the first eruptive phase of the Siberian traps, which was entirely responsible for the great mass extinction. This phase lasted less than 200,000 years and is called the Gudchikinskaya suite. Using minerals from the lavas, Russian geologists computed that the lavas derived from partially melted and recycled oceanic plate. Along with the molten rock, huge amount of volcanic gasses erupted. This included some 170 trillion tons of carbon dioxide - and surprisingly - some 18 trillion tons of hydrochloric acid. A trillion is 10 to a factor of 12. All in all, the eruptions of the initial phase of the Siberian Traps emitted huge amounts of carbon dioxide - an effective greenhouse gas, as well as hydrochloric acid, which spread into the global atmosphere and came down as acid rain. The combination of these two gases emitted from the volcanoes was an especially deadly one-two combination on the global ecosystems on land and in the sea. Geochemical studies of marine sediments at the Permo-Triassic boundary show a drop in delta carbon 13-values around 4 to 6 parts per million. Such a global shift in carbon isotope composition can be explained by a number of factors. Firstly, the addition of large amounts of carbon 12-isotope from the volcanic carbon dioxide to the global system. Secondly, the carbon dioxide would have acted as a greenhouse gas, increasing global temperatures. This may further have triggered the release of methane hydrates from the sea floor as the sea temperatures increased. The methane is bound by bacteria and gas hydrates in sea floor, and is stable at temperatures under 2 degrees centigrade. However, if the temperatures rise above this threshold, the methane is instead released into the sea and further into the atmosphere. And methane is a five-fold more effective greenhouse gas than carbon dioxide. Supporting evidence for a rapid deep-sea warming comes from another geochemical isotope. Studies of delta oxygen 18 values show a drop of some 6 parts per million at the Permo-Triassic boundary. This represents an increase in deep-sea temperatures of around 6 degrees centigrade. Such a rapid increase in deep-sea temperatures would lead to widespread anoxia (lack of oxygen) at the sea-bottom and this is supported by the fact that the earliest Triassic sediments - deposits from just after the extinction event are laminated dark shales with few or no fossils in them. This indicates also an environment with no or very low oxygen levels. And this global shift in both delta carbon 13 and delta oxygen 18 values along with changes in deposition and environment shown by the dark shales is entirely coincident with the extinction of 90% of the marine species alive at the end of the Permian. Some organisms, however, survived the disaster and interestingly some of these achieved near global distribution in the period immediately after the event. These organisms are called disaster taxa. These taxa - one genus of brachiopod and four bivalves thrived in the earliest Triassic because they had adaptations already present. Similar events happened on land, where most of the hitherto-successful animal and plant taxa disappeared in the extinction event. But there were also disaster taxa on land. By far the most successful of these was the medium-sized herbivore Lystrosaurus, a distant relative of mammals. Immediately after the extinction event, species of Lystrosaurus became widespread in large parts of Pangea. Indeed, in some earliest Triassic deposits, Lystrosaurus makes up 95% of the diversity among fossil backboned animals. Lystrosaurus was not particularly specialized or tough, but the species was probably just lucky to survive the event with a few populations scraping through and afterwards becoming extremely widespread due to lack of competition. On land, the volcanic cocktail combining acid rain from hydrochloric acid and carbon dioxide-induced rapid global warming had a disastrous effect on plant life. Geologists had long noticed that sediments from the earliest Triassic were characterized by signs of widespread erosion. In other words, the sediments show that there were no plants or plant roots around to hold on to the soil - it was washed straight out to the sea. And this appears to have been a global feature. There is also a feature known as the "coal gap". Here is a timeline of the Triassic period which comes immediately after the Permian. During the first 7 million years of the Triassic we find the "coal gap". It is a period immediately after the extinction event, where no coal is deposited anywhere in the world. This is significant, since after the Devonian period, where plants were widespread on land, coal has always been deposited someplace in the world. The only exception is during the coal gap time, where the global plant life was too depauperate to allow further deposition of thick layers of coal. It was only 15 million years after the end of the "coal gap", that forests grew abundant enough for the first solid coal layers to be deposited again. One of the reasons for the delayed recovery of plant life may have been continued high temperatures for a couple of million years after the mass extinction event. A recent study from 2012 based on oxygen isotopes, which can be used as a proxy for measuring prehistoric temperatures - indicate that surface temperatures may have remained at between 36 and 40 degrees centigrade in equatorial regions for some time after the event. Modern studies of carbon three-plants the ones reminiscent of the end-Permian and earliest Triassic ones, show that they stop photosynthesizing and instead begin photorespirating at temperatures over 35 degrees centigrade. Photorespiration uses oxygen to grow instead of carbon dioxide, and plants grow vastly slower. Furthermore, an abundance of fungal spores has been reported from Permo-Triassic boundary sediments from several places in the world. Usually, fungal spore remains account for 10% of pollen in a sediment. But right on the boundary they represent almost 100% of all pollen. The fungal spores are thought to represent rapid proliferation of decomposers feasting on the dead vegetation following catastrophic die-back of the normal plant life. In other places, such as deposits in East Greenland, lycopsids and ferns become especially abundant, immediately after the wood-like gymnosperm plants disappear. These weeds were opportunistic pioneers, who thrived in the desolation after the disaster. So plant life was first damaged by acid rain and then prevented from re-establishing itself by elevated temperatures persisting after the extinction event. Interestingly, the first fossil remains of forests after the event turn up in high latitude-regions, before spreading south and north into equatorial latitudes. Other studies again have indicated that the oxygen content in the atmosphere may have dropped to as little as 12 percent in the aftermath of the extinction event. This would have been the result of plants being unable to photosynthesize as well as rotting plant material being decomposed by oxygen consuming bacteria. The elevated temperatures in the equatorial regions after the mass extinction also affected vertebrate life. There is a lack of fossil fish and marine reptiles in deposits that were situated between 30 degrees northern and 40 degrees southern latitude in the earliest part of the Triassic period; even though the same fossils are abundant at higher latitudes So summing up: the worst mass extinction recorded in Earth's history at the Permo-Triassic boundary was caused by massive volcanic eruptions unleashing massive amounts of carbon dioxide and hydrochloric acid upon the global ecosystem. On land this double dose of global warming and acid rain, killed off most of the vegetation, causing widespread large-scale erosion, as well as taking out the bottom of the complex multi-level food chains, which then collapsed. Persistent temperatures above 36 degrees centigrade in a wide equatorial belt prevented plants from re-establishing themselves. This again caused a drop in global oxygen levels, furthered by the side-effects of bacterial decomposition of rotting organic material. The median latitudes of the continents in the earliest Triassic became a "dead zone" with only fungi and hardy weeds surviving. In the sea, acidification caused havoc amongst plankton - also at the bottom of the food chain. Global warming caused a rapid rise in sea temperatures, killing off the large reefs and causing widespread anoxia (lack of oxygen) at the sea bottom. This was further aggravated by the influx of soil and nutrients washed out by the large-scale erosion on land. Mid-latitude seas also remained a dead zone devoid of large vertebrate life, where organic-rich dark shales were deposited. The interesting thing about the Permo-Triassic mass extinction was not the large amount of species, which disappeared or became extinct. Rather, that the recovery of life in all its forms took so long due to the persistence of globally heightened temperatures. In some cases it has been estimated, that global biodiversity was not back at its original levels until almost 100 million years later - the middle of the Jurassic period and the middle of the age of dinosaurs.
