September 08, 2010

Researchers debunk ancient comet-strike theory

by Tim Crosby

CARBONDALE, Ill -- They say diamonds are forever. But in this case, they were never there to begin with.

A team of researchers led by a geology professor from Southern Illinois University Carbondale recently punched a hole in a well-known theory involving an ancient alleged comet or meteor strike wiping out a great deal of life on Earth and changing its climate. The “Younger-Dryas Impact Hypothesis” relied on various geological evidence, most of which other scientist have found lacking in recent years. The final bit of evidence left standing was the alleged existence of tiny “nanodiamonds” supposedly created by the intense force of the impact, which the theory holds happened about 12,900 years ago.

But the team, including SIUC geology Professor Nicholas Pinter, the primary investigator on a three-year National Science Foundation-funded study, recently released findings that directly contradict the theory. The researchers found that what supporters of the hypothesis identified as nanodiamonds aren’t actually diamonds at all, but simple carbon that is related to the very common substance graphite.

“The science is now clear that this supposed impact was a non-extent event,” said Pinter, who began working on the project in 2008 and collected samples in California and Arizona as part of it. “You can’t have an event like this that would supposedly affect all of North America and South America” and not have it leave evidence in the geologic record.

The Younger-Dryas Impact Hypothesis aimed to explain a time period following the last ice age, when ongoing warming suddenly stopped and the climate returned to a glacial period. This approximately 1,300-year period, known as Younger Dryas, saw North America experience massive extinctions, including many larger species such as mammoths, mastodons, saber-tooth tigers and others. Humans also were greatly impacted, with tools such as Clovis stone spear tips disappearing.

The hypothesis, which gained popularity a few years ago, holds that a comet or meteor impact was responsible for the climate’s abrupt turnaround and the subsequent effect on life forms. But the evidence for such an event has been under scrutiny the last few years, with much of it discredited, Pinter said. The final evidence its supporters pointed to were carbon spherules that contained nanodiamonds such as Ionsdaleite, a rarely observed hexagonal-shaped diamond formed by the impact.

Pinter’s team, which includes researchers from Washington University in St. Louis and Royal Holloway University of London, used transmission electron microscopy to examine samples from the western United States in search of the objects. Their research identified the objects as graphene and graphene/graphane oxide. They also demonstrated that previous research mislabeled those substances as hexagonal diamonds and cubic diamonds.

Pinter said with the final supporting evidence of the theory now impeached, scientists can return their attention to what really did happen during that time period, and how that resulted in so much change.

“This affected what is now Southern Illinois, too,” Pinter said. “There were mammoths and mastodons and possibly saber tooths roaming this area 15, 000 years ago. The vegetation was completely different, more a spruce-dominated land. But now we can get back to asking important questions about when and why this changed.”

The journal “Proceedings of the National Academy of Sciences” is publishing the findings under the title, “No evidence of nanodiamonds in Younger-Dryas sediments to support an impact event.”