US geologists claim to have identified a suspect in the Earth鈥檚 largest and most mysterious mass extinction 鈥� a large impact crater off the coast of Australia. However, other geologists are not convinced that the 鈥渟moking gun鈥� is even a gun, let alone the lethal weapon.
Luann Becker, of the University of California at Santa Barbara, and colleagues present data in the journal Science that they says reveals a crater rivalling the one blamed for killing the dinosaurs. They say the effects of the newly revealed impact would have devastated the planet 250 million years ago at the end of the Permian period.
Paleontologists have long puzzled over the Permian mass extinction, which killed over 90 per cent of marine species and 50 to 70 per cent of land animals. The discovery of the Chicxulub impact structure in Mexico indicated that a 10-kilometre asteroid wiped out the dinosaurs 65 million years ago and put impacts at the top of the suspect list for the Permian event.
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But key evidence was missing, including the iridium-rich fingerprints of an asteroid in sediments and an impact crater. Many researchers came to suspect the gas and dust from massive volcanic eruptions in Siberia were to blame.
Shocked quartz
Evidence for a Permian impact has grown over the last few years, says Doug Erwin, a Smithsonian Institution paleontologist. Researchers have found 鈥渟hocked鈥� quartz, tiny meteorite fragments and fullerenes in contemporaneous sediments.
But, as with Chicxulub, the first hints of a crater have been revealed by a reexamination of sediment cores drilled by petroleum geologists years earlier. Becker flew to Australia to examine cores from a feature called the Bedout High, which lies in shallow water a couple hundred kilometres off the northwest coast.
She found they were full of shattered rocks called breccia. 鈥淚t looks just like what you would expect an impact core to look like,鈥� she told a NASA press conference on Thursday. In the lab, she found the cores contain glasses that contain higher fractions of silica than volcanic glass, and other evidence that the rocks had been melted by a powerful shock.
Seismic profiles show a central uplift in the Bedout High, and Becker鈥檚 group argue this is the center of a 200-km crater, filled with later sediments. Dating based on argon isotopes puts the age of the rocks at 250.2 million years, right at the time of the extinction.
Bullseye pattern
However, Christian Koeberl of the University of Vienna, Austria, is not convinced. 鈥淭here is no unique and unambiguous evidence for impact,鈥� he told New 杏吧原创. 鈥淭he gravity map of Bedout does not show any circular features鈥� like those at Chicxulub.
Although Becker says she found shock-induced flaws in quartz from the cores, Koeberl says the samples shown in the paper are not shocked quartz, and also notes the date comes from only a single sample.
鈥淚f it is an impact crater, it鈥檚 one of the most peculiar things I鈥檝e ever seen,鈥� Jay Melosh of the University of Arizona, US, told New 杏吧原创. The seismic profiles appear typical of continental margins, he says, and the gravity profile lacks the 鈥渟ignature鈥� bullseye pattern expected.
The evidence for a Permian impact is 鈥渕uddled,鈥� says Melosh, leaving perhaps only one thing clear 鈥� that more work is needed to reveal the true cause of the biggest loss of life ever seen on Earth.
Journal reference: Science (DOI: 10.1126/science.1093925)
