Comet Impact Refuted as Cause for Mammoths' Extinction

In the last few years, some scientists have put forward a controversial hypothesis arguing that one or more comets slammed into North America some 12,900 years ago, causing the mass extinction of wooly mammoth elephants and other large mammals.

This image from NASA's Wide-field Infrared Survey Explorer (WISE) features comet 65P/Gunn, taken on April 24, 2010 (just one month after its closest approach to the Sun) in the constellation Capricornus.
It came from outer space? Not so, says a new report. NASA/JPL-Caltech/WISE Team

That theory has come under fire from other scientists, who have since put forward refutations of most of the evidence in support of the killer comet theory - except one - the presence of nano-scale diamond crystals. The comet school pointed to that as evidence of a cataclysm since the crystals could only have been formed under the extreme pressure of a comet impact.

Now an article in the Proceedings of the National Academy of Sciences (PNAS) makes the case against the nanodiamond crystal theory as well. A team of researchers, led by nuclear chemist Richard Firestone from the Lawrence Berkeley National Laboratory in California, examined carbon-rich materials isolated from sediments from more than 15,000 years ago to the present. Their conclusion: No nanodiamonds were found. "Instead, graphene- and graphene/graphane-oxide aggregates are ubiquitous in all specimens examined. We demonstrate that previous studies misidentified graphene/graphane-oxide aggregates as hexagonal diamond and likely misidentified graphene as cubic diamond. Our results cast doubt upon one of the last widely discussed pieces of evidence supporting the YD impact hypothesis.

You can read the online version of report at PNAS. Also, Science offers a good write-up of the latest turn in the debate.