The Permian–Triassic extinction event, colloquially known as the Great Dying, was an extinction event that occurred around the boundary between the Permian and Triassic geologic periods, and with them the Paleozoic and Mesozoic eras. It is Earth's most severe known extinction event, with the extinction of 57% of biological families, 62% of genera, 81% of marine species, and 70% of terrestrial vertebrate species. It is also the greatest known mass extinction of insects. It is the greatest of the "Big Five" mass extinctions of the Phanerozoic. There is evidence for one to three distinct pulses, or phases, of extinction, with the major pulse of marine extinction occurring in a 60,000-100,000 year interval around 251.902 million years ago (mya) marking the boundary between the Permian and Triassic periods. There is some controversy regarding the chronology of the extinctions on land and whether they are synchronous with the main marine extinction event.
The scientific consensus is that the main cause of the extinction was the flood basalt eruptions that created the Siberian Traps, which released sulfur dioxide and carbon dioxide, resulting in euxinia (oxygen-starved, sulfurous oceans), elevated global temperatures, and acidified oceans. The level of atmospheric carbon dioxide rose from around 400 ppm to 2,500 ppm with approximately 3,900 to 12,000 gigatonnes of carbon being added to the ocean-atmosphere system during this period.
Several other contributing factors have been proposed, including the emission of carbon dioxide from the burning of oil and coal deposits ignited by the eruptions; emissions of methane from the gasification of methane clathrates; emissions of methane by novel methanogenic microorganisms nourished by minerals dispersed in the eruptions; longer and more intense El Niño events; and an extraterrestrial impact that created the Araguainha crater and caused seismic release of methane and the destruction of the ozone layer with increased exposure to solar radiation.
Dating
Previously, it was thought that rock sequences spanning the Permian–Triassic boundary were too few and too incomplete for scientists to reliably determine their details. However, it is now possible to date the extinction with millennial 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 first appearance of the conodont Hindeodus parvus has been used to delineate the Permian-Triassic boundary.
The extinction occurred between and years ago, a duration of 60 ± 48 thousand years. A large, abrupt global decrease in δ13C, the ratio of the stable isotope carbon-13 to that of carbon-12, coincides with this extinction, and is sometimes used to identify the Permian–Triassic boundary and the Permian-Triassic Mass Extinction event (PTME) in rocks that are unsuitable for radiometric dating. The negative carbon isotope excursion's magnitude was 4–7% and lasted for approximately 500,000 years, though estimating its exact value is challenging due to diagenetic alteration of many sedimentary facies spanning the boundary.
Further evidence for environmental change around the Permian-Triassic boundary suggests an rise in temperature, and an increase in levels to (for comparison, the concentration immediately before the Industrial Revolution was , and the amount today is about 426 ppm). There is also evidence of increased ultraviolet radiation reaching Earth, causing mutation in plant spores.
It has been suggested that the Permian–Triassic boundary is associated with a sharp increase in the abundance of marine and terrestrial fungi, caused by the sharp increase in the amount of dead plants and animals fed upon by the fungi. This "fungal spike" has been used by some paleontologists to identify a lithological sequence as being on or very close to the Permian–Triassic boundary in rocks that are unsuitable for radiometric dating or have a lack of suitable index fossils. However, 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 during the earliest Triassic. The very idea of a fungal spike has been criticized on several grounds, including: Reduviasporonites, the most common supposed fungal spore, may be a fossilized alga; the spike did not appear worldwide; and in many places it did not fall on the Permian–Triassic boundary. The Reduviasporonites 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 beds. 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 extinction pulses or that the extinction was long and 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 southeastern China, suggest that the main extinction was clustered around one peak, while a study of the Liangfengya section found evidence of two extinction waves, MEH-1 and MEH-2, which varied in their causes, and a study of the Shangsi section also showed two extinction pulses with different causes. Recent research shows that different groups became extinct at different times; for example, while difficult to date absolutely, ostracod and brachiopod extinctions were separated by around 670,000 to 1.17 million years. Paleoenvironmental analysis of Lopingian strata in the Bowen Basin of Queensland indicates numerous intermittent periods of marine environmental stress from the middle to late Lopingian leading up to the end-Permian extinction proper, supporting aspects of the gradualist hypothesis. The decline in marine species richness and the structural collapse of marine ecosystems may have been decoupled as well, with the former preceding the latter by about 61,000 years according to one study.
Whether the terrestrial and marine extinctions were synchronous or asynchronous is another point of controversy. Evidence from a well-preserved sequence in east Greenland suggests that the terrestrial and marine extinctions began simultaneously. In this sequence, the decline of animal life is concentrated over approximately 10,000 to 60,000 years, with plants taking an additional several hundred thousand years to show the full impact of the event. Many sedimentary sequences from South China show synchronous terrestrial and marine extinctions. Research in the Sydney Basin of the PTME's duration and course also supports a synchronous occurrence of the terrestrial and marine biotic collapses. Other scientists believe the terrestrial mass extinction began between 60,000 and 370,000 years before the onset of the marine mass extinction. Chemostratigraphic analysis from sections in Finnmark and Trøndelag shows the terrestrial floral turnover occurred before the large negative δ13C shift during the marine extinction. Dating of the boundary between the Dicynodon and Lystrosaurus assemblage zones in the Karoo Basin indicates that the terrestrial extinction occurred earlier than the marine extinction. The Sunjiagou Formation of South China also records a terrestrial ecosystem demise predating the marine crisis. Other research has still found that the terrestrial extinction occurred after the marine extinction in the tropics.
Studies of the timing and causes of the Permian-Triassic extinction are complicated by the often-overlooked Capitanian extinction (also called the Guadalupian extinction), just one of perhaps two mass extinctions in the late Permian that closely preceded the Permian-Triassic event. In short, when the Permian-Triassic starts, it is difficult to know whether the end-Capitanian had finished. Many of the extinctions once dated to the Permian-Triassic boundary have more recently been re-dated to the end-Capitanian. Further, it is unclear whether some species that survived the prior extinction(s) had recovered sufficiently for their final demise during the Permian-Triassic event to be considered separate from the Capitanian event. A minority view holds that the sequence of environmental disasters has effectively constituted a single, prolonged extinction event, though this may depend on which species is considered. This older theory, still supported in some recent papers, proposes that there were two major extinction pulses 9.4 million years apart, separated by a period of extinctions that were less extensive, but still well above the background level, and that the final extinction killed about 80% of marine species alive at that time, whereas 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 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.
Extinction patterns Marine extinctions Genera extinct NotesHyolitha100%In decline for a long time before the extinction event and was rare by the Permian. Arthropoda Ostracods 74% Trilobites 100% In decline since the Devonian; only 5 genera living before the extinction Brachiopoda Brachiopods 96% Orthids and orthotetids died out, and productids would die out shortly after Bryozoa Bryozoans 79% Fenestrates, trepostomes, and cryptostomes died out Cnidaria Anthozoans 96% Tabulate and rugose corals died out Echinodermata Blastoids 100% Crinoids 98% Inadunates and camerates died out Mollusca Ammonites 97% Goniatites and Prolecantids died out Bivalves 59% Gastropods 98% Retaria Foraminiferans 97% Fusulinids died out, but were almost extinct before the catastrophe Radiolarians 99%
Marine organisms Marine invertebrates suffered the greatest losses during the P–Tr extinction, although previous estimates of 90–96% marine species extinction were due to historical conflation with the end-Capitanian mass extinction, which occurred 7–10 million years earlier. Evidence of these losses 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 two 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 that relied on stable CO2 levels to produce them. These organisms were susceptible to the effects of the ocean acidification that resulted from increased atmospheric CO2. Organisms that relied on haemocyanin or haemoglobin for transporting oxygen were more resistant to extinction than those utilizing hemerythrin or oxygen diffusion. There is also evidence that endemism was a strong risk factor influencing a taxon's likelihood of extinction. Bivalve taxa that were endemic and localized to a specific region were more likely to go extinct than cosmopolitan taxa. There was little latitudinal difference in the survival rates of taxa. Organisms that inhabited refugia less affected by global warming experienced lesser or delayed extinctions.