The Great Dying wiped out 81 percent of marine species in a geological blink
About 252 million years ago, life on Earth came closer to being erased than at any other known time. The Permian-Triassic extinction killed roughly four in five marine species and seven in ten land vertebrates. The main pulse lasted perhaps 60,000 years, and the prime suspect is a colossal outpouring of Siberian lava.
Nicknamed the Great Dying, the event marks the boundary between the Permian and Triassic periods and between the Paleozoic and Mesozoic eras. It is the worst of the Big Five mass extinctions: an estimated 57 percent of biological families, 62 percent of genera, 81 percent of marine species and 70 percent of terrestrial vertebrate species vanished. It was also the greatest known extinction of insects. There may have been as many as three separate pulses, and researchers still argue over whether losses on land happened at the same time as those in the sea.
Precision has improved dramatically. Uranium-lead dating of zircon crystals from five volcanic ash layers at Meishan in China, the reference site for the boundary, now pins the extinction to between about 251.94 and 251.88 million years ago, a window of 60,000 years give or take 48,000. A sharp drop in the ratio of carbon-13 to carbon-12 coincides with it and helps geologists recognise the boundary where rocks cannot be dated directly.
Scientists broadly agree that the flood basalt eruptions that built the Siberian Traps were the main trigger. They released sulphur dioxide and carbon dioxide, driving carbon dioxide from around 400 to 2,500 parts per million; for comparison, the pre-industrial level was 280 and today's is about 426. Temperatures rose by roughly 8 degrees Celsius, and oceans became starved of oxygen and rich in sulphur. Proposed accomplices include burning coal and oil ignited by the magma, methane released from frozen deposits or by microbes, and a possible impact that formed the Araguainha crater.
Some rock layers near the boundary show a surge in fungal remains, possibly feeding on the mass of dead organisms. That fungal spike has been disputed, since the main supposed spore might be an alga and the spike is not global, though newer chemical evidence favours a fungal origin.
Source: Permian–Triassic extinction event
