Piecing Together a Permian Impact
Scientists have found a crater that they believe is associated with the Permian-Triassic (P-T) extinction, the largest extinction event in the history of life.
Luann Becker, a scientist at the University of California, Santa Barbara, headed the research project. Science Express, the electronic publication of the journal Science, published a paper describing the crater.
|Reconstruction of the Permian Reef Complex in the Glass Mountains of west Texas, showing the diversity of life before the end-Permian mass extinction. The reef is dominated by brachiopods, bryozoans, crinoids, all groups which suffered heavy extinction. This reconstruction is from the National Museum of Natural History, and the image is copyright by the Smithsonian Institution, 1990. The diorama was constructed by Terry Chase and photographed by Chip Clark.Credit: Chip Clark, Terry Chase, Smithsonian
The 125-mile-wide crater, called Bedout, is buried off the northwestern coast of Australia. Oil companies in the early 1970s and 80s had drilled two cores into the Bedout structure in search of hydrocarbons, but the cores had not been examined for decades. Becker and co-author Robert Poreda, a geochemist with the University of Rochester, New York, went to Australia to examine the cores.
"I think I could safely say that Bob and I were absolutely flabbergasted at this core, because it does look very much like an impact breccia," says Becker.
The scientists say the Bedout crater was caused by a 10-kilometer asteroid, similar to the one that created the Chicxulub crater in Mexico's Yucatan Peninsula. Most scientists believe the Chicxulub impact accompanied the extinction of the dinosaurs 65 million years ago.
The P-T extinction occurred roughly 250 million years ago, and Becker and her team dated the Bedout crater to be 250.7 million years old (+/- 4.3 million years). They hope to further narrow down the dating with additional studies.
A meteor impact had previously been suggested as one possible contributor to the P-T "Great Dying," when 90 percent of marine and 80 percent of land life perished. However, there previously was no compelling evidence for such an impact.
There are several lines of evidence that indicate the Bedout crater was caused by a meteor impact rather than volcanic activity. Poreda says that he's spent 25 years looking at volcanic rocks, and the types of glass found in the Bedout core are always produced by impacts, not volcanism. In addition, Becker and her team found meteorite fragments in a thin breccia layer in Antarctica.
Co-author Kevin Pope of Geo Eco Arc Research found large shocked quartz grains in end-Permian sediments in Australia. He thinks these grains formed as a result of the Bedout impact. Similar quartz grains were also found in Antarctica.
Quartz can be fractured by extreme volcanic activity, but only in one direction. Shocked quartz is fractured in several directions and is therefore believed to be a good indication of a meteor impact.
|Base map of the offshore northwestern Canning basin region showing major tectonic elements (i.e. plateaus, basins, platforms, etc.), the Bedout High with the Lagrange-1 and Bedout-1 exploration wells located on top of the high (red dots) and some additional wells onshore (red, blue dots) where Permian-Triassic sediments may be preserved
Credit: A. Mory, GSWA
Shocked quartz associated with the Chicxulub crater shows that the grain size drops the further you get from the crater. The grain size of the shocked quartz associated with the Bedout crater follows the same distribution pattern as Chicxulub.
Although Chicxulub debris is found around the world, most of the debris from the Bedout impact is concentrated in the Southern Hemisphere. Pope says this may be a function of location. Chicxulub was located relatively close to the equator at the time of impact, and global processes may have more easily distributed that impact material worldwide.
Luis and Walter Alvarez first concluded that a meteor caused the Cretaceous-Tertiary (K-T) extinction, because iridium was found in the K-T boundary layer around the world. Iridium is a rare element on Earth, but it is often found in asteroids and meteorites. There is no iridium associated with the Bedout crater, but the scientists say that not every impact crater will have that element. Pope says it just could be that the Chicxulub meteor was especially rich in iridium.
Some scientists have suggested that volcanic activity was the cause of the P-T extinction, and point to massive flood basalt volcanism that occurred in Siberia 250 million years ago.
But it may be possible that impacts induce volcanism. The K-T extinction also had massive lava outflows occurring at the same time, creating the Deccan Traps in India. Scientists largely dismissed the relationship between K-T impact and volcanism as a coincidence, because there were no explanations or models for how these events could be related. But the virtually simultaneous occurrence of impact and massive volcanism at the P-T boundary may cause scientists to rethink this assumption.
"Perhaps a mantle plume was already starting in Siberia, and it could be that the seismic waves from this event caused the outpouring of lava to dramatically increase," says Pope. "I like to use for a model a glass of champagne - if you tap on the glass all the bubbles fizz up to the top."
Although the scientists think the meteors that impacted during the K-T and P-T were about the same size, the P-T extinction was much more severe than the K-T extinction. The world, of course, was a very different place 250 million years ago than it was 65 million years ago, and that might account for the difference in the degree of extinction. Poreda also notes that the Siberian Traps were several times larger than the Deccan Traps, and the difference in volatiles and gases pumped into the atmosphere could have caused the Permian extinction to be much more severe.
|The painting titled "K/T Hit" by artist Donald E. Davis. This impact occured 65 million years ago, ending the reign of the dinosaurs.
Image Credit: Don Davis
Douglas Erwin, a paleobiologist with the Museum of Natural History in Washington D.C, adds that the Siberian lava came up through the largest coal basin in the world. The vaporized coal would have produced immense amounts of carbon dioxide and sulfur dioxide.
Erwin thinks this recent study is compelling, but he says more work need to be done to see if the Bedout crater was created during the P-T boundary.
"When establishing the correlation of events, we have to be very precise about time," says Erwin. "We do need to establish that this did in fact happen at the same time as the end-Permian mass extinction."
Related Web Pages
Great Impact: Part I
Great Impact: Part II
Impact Hazards Website
NASA/JPL Near Earth Object Program