Mass Extinction Is Traced to Meteorite
New evidence strongly suggests that the greatest extinction event in Earth’s history -- the so-called Great Dying that wiped out more than 90% of marine species and 80% of those on land -- was caused by a meteorite impact off the coast of what is now Australia, UC Santa Barbara researchers said Thursday.
The remains of a newly discovered 125-mile-wide crater indicate that a 6-mile-wide meteorite struck Earth about 251 million years ago, sending up a plume of smoke and ash that produced freezing temperatures and darkened skies that would have persisted for months or years.
The chill and gloom destroyed plant life around the world, eliminating the food supply for most large animals and fish, which reduced their populations below the minimum necessary to survive.
The elimination of old species and the influx of new ones was so dramatic that researchers consider the time a boundary between two geological periods, the Permian and the Triassic.
Biologists have long speculated about the cause of the abrupt disappearance of species, but the new discovery provides “compelling evidence of an impact at the boundary,” said paleobiologist Douglas H. Erwin of the National Museum of Natural History in Washington.
Researchers already had determined that the extinction of the dinosaurs 65 million years ago was caused by the impact of a similar-sized meteorite off Mexico’s Yucatan Peninsula, forming a massive crater known as Chicxulub.
The new evidence suggests that such impacts may have been relatively frequent not only on Earth but on other planets where life may have evolved, said NASA astrobiologist Michael New. “We have become much more aware that forces outside our planet can be linked to these extinctions,” he said.
The world was a much different place 250 million years ago. All the land masses were joined in one supercontinent called Pangaea, which was surrounded by a single ocean called Panthalassa. Land animals were mammal-like reptiles ranging in size from that of a large dog to a small hippopotamus.
A variety of evidence suggests that this world was shattered by an impact producing the crater now called Bedout (pronounced Beh-doo), geologist Luann Becker and her colleagues reported in the online version of the journal Science.
The most important evidence was found in core samples from the area off northwestern Australia taken by geologists looking for oil.
“We were absolutely flabbergasted when we looked at the core samples ... because they look just like cores from Chicxulub,” Becker said. The material had distinctive features, especially nearly pure silica glass and shocked quartz, that was literally “out of this world,” she said.
Quartz is a very strong mineral that can be fractured only with great difficulty. The high temperatures and pressures associated with extreme volcanic activity can fracture it, but typically in only one direction.
Shocked quartz such as that found in the core samples is fractured in many directions and is thought to be caused only by a massive impact. Its presence in the core samples is “absolutely convincing proof of an impact,” said geochemist Robert Poreda of the University of Rochester, a member of the team.
The team also found fragments of shocked quartz and other evidence of a meteorite impact spread roughly in a large circle around the crater site. The team members conceded that they had much more work to do to prove that Bedout was indeed a crater.
