"This is very likely the impact site we've been looking for," says Robert Poreda, professor of earth and environmental sciences at the University of Rochester. "For years we've been observing evidence that a meteor or comet hit the southern hemisphere 251 million years ago, and this structure matches everything we've been expecting."
In 2001, Poreda and Luann Becker, research scientist in geological sciences at UCSB, announced that they had detected in 251-million-year-old strata, specific isotopes of helium and argon trapped inside buckyballs--a cage-like formation of carbon atoms--that could only have come from space. Since they were laid down in this same strata around much of the globe, the implication was that a giant meteor had struck the Earth, vaporized, and settled around the southern hemisphere. This past November, the same three authors--Poreda, Becker, and Asish Basu, professor of earth and environmental sciences at the University of Rochester--published another article in Science that found actual pieces of the meteorite that struck the Earth in the same global strata.
Many experts scoffed at the idea of a giant meteor causing the mass extinction between the Permian and Triassic periods, but Poreda points out that many also scoffed at the idea that a meteor was responsible for a later and lesser extinction at the Cretatious/Tertiary boundary that marks the end of the dinosaurs. Now, the impact theory is largely accepted.
The team knew that the chances of finding the crater, even one from an impact large enough to nearly wipe out life on Earth, would be difficult because the majority of the Earth is covered by ocean. Had the meteor struck there, its telltale crater would have long ago disappeared. As luck would have it, an oil-drilling exploration team in 1970 found a "dome" in the area of Bedout, just off the northwestern coast of Australia. Now covered by 2 miles of sediment, this area was most likely dry land 251 million years ago. Frequently, such domes herald large oil deposits, but in this case the drilling team found only what it labeled as "volcanic rock." The core samples were shelved and forgotten for 25 years, until in 1995 a report in a journal aimed at the oil industry mentioned that the rock might have been formed from a meteor impact.
It wasn't until Becker caught wind of the "volcanic" find in 251 million year old rock that the team members began to think they'd found their smoking gun. Poreda and Becker investigated the core samples first hand. "They were unlike any volcanic rocks I've ever seen," says Poreda. "In a volcanic explosion you may find angular pieces of rock that are broken apart mixed with the volcanic melt. In these samples, though, the rocks were shock melted from an impact.. We left convinced Bedout was our crater."
The clincher was the presence of a feldspar glass in the shape of a feldspar crystal. Such features do not form in volcanic eruptions. Many of the plagioclase samples showed evidence of sustaining an intense shock, meaning the meteor likely hit a bed that contained feldspar crystals, shock-melted their interiors, melting their insides the way a microwave oven might bake a potato's inside while leaving the outer areas cool.
"Once we looked at Bedout with the understanding that it was likely a crater, the geophysics just fell into place," says Poreda.
Geophysical analysis shows the rock strata underlying the dome at Bedout is fractured exactly the way the team expected--showing rock strata older than 251 million years old broken apart, with younger rock above laid down without the fractures. Simulations of a six-mile wide rock striking the area suggest a crater rim should be visible about 60 miles from the central dome, and despite the extreme age of the impact site and the rearrangement of continental plates since then, there is evidence of a rim at that distance. The team has plans to explore the geophysical outlay of the region with more scrutiny.
Coincidentally, the Bedout crater, at 120 miles across, is almost exactly the same size as the Chicxulub crater in the Caribbean that has been identified as the impact site of the meteorite that dealt the dinosaurs their death blow. It's likely that the bodies that struck at each site were of the same size and traveling at similar speeds.
Along with both impacts correlating strongly with two of the greatest extinctions in Earth's history, the team has found that massive lava flows in two different parts of the world have similar corrolations. Basu showed that massive lava flows in India date back precisely to the Chicxalub impact, and recently he also reported that similar giant lava flows in Siberia coincide exactly with the Bedout impact.
"There have been five mass extinctions throughout the Earth's history," says Poreda. "Now we have very strong evidence that massive meteor impacts happened precisely at two of those extinctions."
The research was funded by NASA and the National Science Foundation.
The research is published on Science's pre-print express web site, http://www.