The Permian-Triassic extinction event, known as the Great Death, was the most severe extinction event in Earth’s history, wiping out 90% of species. The event was caused by a supervolcano eruption that formed the Siberian Traps, which caused global cooling and disrupted Earth’s life. The mass release of methane clathrates from the world’s oceans caused catastrophic warming, decreased oxygen levels, and destroyed the ozone layer, leading to the extinction of most marine life. The recovery of life after the event was the slowest ever, taking 5-10 million years.
The Permian-Triassic extinction, known informally as the Great Death, the P-Tr boundary, or “the mother of all mass extinctions,” is believed to be the most severe extinction event in the history of life on Earth. Occurring about 250 million years ago, the Permian-Triassic extinction event was a relatively sudden event, lasting less than 80,000 years, with the most severe pulses lasting just 5,000 years. About 96 percent of marine species and 70 percent of land vertebrate species went extinct, with many important Paleozoic families, such as sea scorpions, trilobites, jawless fishes, and armored fishes going extinct completely. Overall, about 90 percent of species were wiped out, in contrast to the disappearance of only 60 percent of species in the Cretaceous-Tertiary event 65.5 million years ago, which wiped out the dinosaurs.
Slow recovery
The recovery of life after the Permian-Triassic extinction event was the slowest ever, taking 5-10 million years instead of the typical less than 1 million. The few genera that survived went on to become world-wide, probably the least diverse life had ever been since the beginning of the Cambrian. Lystrosaurus, a medium-sized herbivore that is the ancestor of all mammals, made up 90% of all land animals for millions of years after the extinction event. The Permian-Triassic extinction is also the only known mass extinction of insects.
Drastic changes
Plant life has been devastated. Perhaps 95 percent of all land plants have been exterminated. In many areas, river flow patterns changed from meandering to braided, just as they were during the Early Silurian, before the evolution of land plants. There was a brief spike in fungi worldwide, caused by a large increase in the amount of dead organic material relative to the amount of living organic material. This part of the fossil record is powerful evidence that the extinction was relatively brief, rather than occurring as a gradual process that wiped out large numbers of genera over time.
Possible causes
After extensive debate and analysis, scientists have come to a general consensus on what caused the Permian-Triassic extinction event. At first, scientists suspected an asteroid impact, much like the one believed to have killed off the dinosaurs. However, a telltale layer of iridium is missing from the Permian-Triassic boundary, which should be deposited by a large asteroid impact.
Instead, the blame fell on a large and extensive supervolcano eruption that formed the so-called Siberian Traps. The Siberian Traps were formed by lava pumped out about 0.24 cubic miles (1 cubic kilometer) of lava each year for 40,000 to 200,000 years, at least 20 percent of it pyroclastically – ejected violently upward rather than released as a there. Initially, this would have blocked out the sun and caused global cooling, and much of Earth’s life would have been disrupted by thick layers of molten ash deposited over a region about the size of Asia.
The role of methane clathrates
Volcanism alone is not thought to have caused the Permian-Triassic extinction event. One of the biggest clues from the time period strata is an increase in the ratio of the isotope carbon-12 to carbon-13. After many years of scratching their heads over the precise cause of the change, scientists believe that only one event could have caused a change as large as the one measured: the mass release of methane clathrates from the world’s oceans.
Methane clathrates are methane molecules trapped in a matrix of ice crystals, located approximately 0.3-0.6 miles (0.5-1.0 km) below the continental boundaries of the world. Estimates of the amount of methane clathrates in the world’s oceans today range from 3,000 to 20,000 gigatons, and the amount is thought to have been similar before the Permian-Triassic boundary. The eruptions of the Siberian Traps mainly poured their lava into areas composed of shallow seas, which would have caused the mass release of methane. Methane is about 20 times more effective at causing global warming than carbon dioxide and would have been released in large quantities.
High temperatures, little oxygen
The release of methane would have caused the Earth, including the oceans, to warm, further releasing more methane clathrates and accelerating warming. Most of the world’s clathrates may have been released in as little as 5,000 years, causing catastrophic warming. This warming would decrease temperature gradients between the poles, impeding the transfer of nutrients from land to sea, causing massive algal blooms that consume the oceans’ oxygen, and causing widespread anoxia, or reduced oxygen levels.
Without oxygen, most marine life died. Anaerobic green sulfur bacteria thrived, displacing other bacteria and causing large emissions of hydrogen sulfide, destroying the ozone layer and exposing terrestrial life to harmful ultraviolet (UV) rays. Evidence of UV damage has been found in plant fossils from the time.
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