New evidence suggests a possible impact cause for the greatest mass extinction of all time, although many scientists remain skeptical that this long-standing mystery has been solved. A NASA news conference was held May 13 to announce the discovery of an impact crater near Australia that might be implicated in the Permian-Triassic or PT extinction event, 251 million years ago.
Identification of the cause of this critical event for the history of life is an important challenge for paleontologists and astrobiologists. As Michael Benton recently wrote (Michael J. Benton: When Life Nearly Died – The Greatest Mass Extinction of All Time
, Thames & Hudson, 2003):
"The end-Permian mass extinction may be less well known than the end-Cretaceous, but it was by far the biggest mass extinction of all time. Perhaps as few as 10 percent of species survived the end of the Permian, whereas 50 percent survived the end of the Cretaceous. Fifty percent extinction was associated with devastating environmental upheaval. But there is an enormous difference between 50 percent survival though the end-Cretaceous and only 10 percent survival through the end-Permian. The key difference … is in the diversity of founders available for the reflowering of life after the catastrophe. The survival of only 10 percent of species means that many major groups of plants, animals, and microbes have probably gone forever."
As was mentioned in the discussion during the NASA news conference, at the time when early paleontologists first recognized the PT boundary, some suggested that all life had been killed on Earth, and that the planet started over with entirely new life forms.
The new research is by geochemist and oceanographer Luann Becker (Dept. of Geology, Univ. of California, Santa Barbara), and her colleagues R. J. Poreda, A. R. Basu, K. O. Pope, T. M. Harrison, C. Nicholson, and R. Iasky. The paper title is "Bedout: A Possible End-Permian Impact Crater Offshore Northwestern Australia".
The paper reports on the identification of a large submarine impact structure off western Australia that is dated at 250.7 +/- 4.3 million years (an argon-argon date from a single plagioclase crystal). This geological rise had originally been thought to be volcanic, but a re-examination of drill cores by this team shows clear evidence of impact materials, including abundant shocked mineral grains. Their preliminary work suggests that this original crater was nearly as large as the Chicxulub crater, which caused the KT extinction 65 million years ago.
Recent work by others has shown that the extinction at the end of the Permian was extremely rapid, like that at the end of the Cretaceous. Less is known generally, however, about the earlier extinction. Some other evidence of an impact at the PT boundary has been found, but so far no significant iridium anomalies such as those that first suggested an impact at the KT boundary.
The interpretation of these great mass extinctions is complicated. Both the KT and the PT event are approximately coincident with large volcanic eruptions, although the time scale for volcanism is millions of years, not the thousands of years (or less) that account for the great dying. Few scientists today think the Deccan volcanism in India 65 million years ago played a major role in the KT extinction, but many have proposed that the even larger Siberian eruptions of 250 million years ago might have been responsible for the PT extinction.
In the case of the KT (end-Cretaceous) extinction, there was a great deal of early evidence of an extraterrestrial event, beginning with the pioneering work of Luis and Walter Alvarez in the late 1970s. However, the discovery of the actual Chicxulub crater was key to wide acceptance of the reality of the impact catastrophe 65 million years ago. Identification of a large crater with age 251 million years might also resolve the PT debate, but a great deal more work needs to be done. This is a frustrating situation for many scientists. Collisions with asteroids and comets have certainly happened, on roughly the same time scale as the big mass extinctions – yet only in the KT example is the association of an impact and an extinction clear.
Doug Irwin of the National Museum of Natural History, Smithsonian Intuition, who also participated in the NASA press conference, stated that the arguments that this might be the PT impact crater are suggestive but not compelling. He noted that while the evidence for an impact at the PT has been growing over the past couple of years, he would still not be able to say that an impact was more likely than a volcanic explanation, or perhaps some other cause.
As reported in the press, Michael Rampino, a professor of Earth and Environmental Sciences at New York University, said of this new paper, "It's another piece of evidence, but it's still equivocal." Peter Ward, a paleontologist and professor of Geological Sciences at the University of Washington in Seattle, said "I'm just not sure if there was an impact." He told The New York Times that he is skeptical of the crater theory until more evidence is gathered. "We want to know what is the minimum size [of a meteor impact] that causes big damage," said Ward. "Plus we want to know how safe is the Earth."
The PT extinction remains one of the big mysteries of the history of life. We need to understand mass extinctions if we are to understand the evolution of life on our planet.
A copy of the NASA Press Release follows.
NASA RELEASE: 04-159, May 13, 2004
EVIDENCE OF METEOR IMPACT FOUND OFF AUSTRALIAN COAST
An impact crater believed to be associated with the "Great Dying," the largest extinction event in the history of life on Earth, appears to be buried off the coast of Australia.
NASA and the National Science Foundation (NSF) funded the major research project headed by Luann Becker, a scientist at the University of California, Santa Barbara (UCSB). Science Express, the electronic publication of the journal Science, published a paper describing the crater today.
Most scientists agree a meteor impact, called Chicxulub, in Mexico's Yucatan Peninsula, accompanied the extinction of the dinosaurs 65 million years ago. But until now, the time of the Great Dying 250 million years ago, when 90 percent of marine and 80 percent of land life perished, lacked evidence and a location for a similar impact event.
Becker and her team found extensive evidence of a 125-mile-wide crater, called Bedout, off the northwestern coast of Australia. They found clues matched up with the Great Dying, the period known as the end-Permian. This was the time period when the Earth was configured as one primary land mass called Pangea and a super ocean called Panthalassa.
During recent research in Antarctica, Becker and her team found meteoric fragments in a thin claystone "breccia" layer, pointing to an end-Permian event. The breccia contains the impact debris that resettled in a layer of sediment at end-Permian time.
They also found "shocked quartz" in this area and in Australia. "Few Earthly circumstances have the power to disfigure quartz, even high temperatures and pressures deep inside the Earth's crust," Becker said.
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 tracer for the impact of a meteor.
Becker discovered oil companies in the early 70's and 80's had drilled two cores into the Bedout structure in search of hydrocarbons. The cores sat untouched for decades. Becker and co-author Robert Poreda went to Australia to examine the cores held by the Geological Survey for Australia in Canberra. "The moment we saw the cores, we thought it looked like an impact breccia," Becker said. Becker's team found evidence of a melt layer formed by an impact in the cores.
In the paper, Becker documented how the Chicxulub cores were very similar to the Bedout cores. When the Australian cores were drilled, scientists did not know exactly what to look for in terms of evidence of impact craters.
Co-author Mark Harrison, from the Australian National University inCanberra, determined a date on material obtained from one of the cores, which indicated an age close to the end-Permian era. While in Australia on a field trip and workshop about Bedout, funded by the NSF, co-author Kevin Pope found large shocked quartz grains in end-Permian sediments, which he thinks formed as a result of the Bedout impact. Seismic and gravity data on Bedout are also consistent with an impact crater.
The Bedout impact crater is also associated in time with extreme volcanism and the break-up of Pangea. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Becker said. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout, I don't think we can call such catastrophes occurring together a coincidence anymore," she added.
For information and images about the research on the Internet, visit: http://beckeraustralia.crustal.ucsb.edu/