Nobody doubts that a 15-kilometer-wide asteroid slammed into Earth 65 million years ago. And most scientists agree that the environmental fallout from this impact wiped out the dinosaurs. But convincing evidence connecting other mass extinctions to asteroid impacts has been elusive. Now, a team of researchers has uncovered persuasive evidence in the Moroccan desert of an impact 380 million years ago that may have cut marine biodiversity during the Devonian Period, nearly in half.
About a decade ago, geophysicist Brooks Ellwood of Louisiana State University in Baton Rouge was studying the magnetic properties of 380-million-year-old rocks in the Moroccan desert for another project, when he noticed a strange pattern in the rock layers. Years later while visiting Gubbio, Italy, he found the same magnetic pattern in rocks that record evidence of the dinosaur-killing impact at the Cretaceous-Tertiary (K-T) boundary 65 million years ago: There was a sharp magnetic change in the rock layer at the boundary caused by heavy metals in the impact debris. This led him to wonder if the pattern in the Moroccan rocks could have been caused by an impact as well.
When Ellwood and colleagues returned to Morocco they found several pieces of evidence that are very similar to those found at the K-T boundary: high concentrations of nickel, chromium, cobalt, arsenic, and vanadium; a drastic change in the carbon isotope ratio; tiny bits of material called microspherules and microcrysts that formed in the atmosphere from rapidly cooling impact debris; and quartz containing shock fractures that are typically formed by a huge, catastrophic event such as an impact. The most exciting aspect of the discovery, described in the 13 June issue of Science, is that the apparent impact would have preceded one of the five biggest extinctions ever. Known as the Kacák/otomari event, the Devonian extinction didn't have much of an effect on life on land but had a global impact on marine animals, decimating the reef ecosystems.
"Evidence is beginning to mount for coupling extinction events to extraterrestrial causes," says geochemist Luann Becker of the University of California, Santa Barbara. But the researchers need to show similar evidence for the impact on different continents that would indicate a global effect, she says. "There's still no real smoking gun."