Permian–Triassic extinction event

The Permian–Triassic extinction event, colloquially known as the Great Dying, was an extinction event that occurred approximately 251.9 million years ago (mya), at 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.

The scientific consensus is that the main cause of the extinction was the flood basalt volcanic 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.

Previously, 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 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 251.941 ± 0.037 and 251.880 ± 0.031 million 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 kyr, 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 8 °C (14 °F) rise in temperature, and an increase in CO
₂ levels to 2,500 ppm (for comparison, the concentration immediately before the Industrial Revolution was 280 ppm, and the amount today is about 422 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 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 showed two extinction pulses with different causes too. 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.