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Comet theory carbonized

Sediment studies rule out impact as cause of ancient cold spell.

The idea that a comet impact triggered a widespread climate chill has taken another hit. Credit: MIKE AGLIOLO/SCIENCE PHOTO LIBRARY

The controversial theory that a comet impact sent Earth into a sudden climate chill nearly 13,000 years ago has been dealt a serious blow, according to scientists who have analysed sediments from the time.

The cool period, known as the Younger Dryas, coincided with the disappearance of the Clovis culture of North American humans and the large mammals they hunted. Most scientists think that the cold snap was triggered by a flood of fresh water from a breaching lake that disrupted the northern Atlantic ocean circulation.

But an alternative theory claims that sediments from that time contain a host of evidence — including carbon spherules and iridium — implicating a massive comet impact as the culprit (see Nature 447, 256–257; 2007). The proposition was attractive, as it claimed to explain both the rapid climate change, and the sudden die-off of humans and animals at the time.

A series of publications has since challenged each piece of cometary evidence, save one — nanodiamonds, supposedly created by the comet's impact shock.

Stony silence

Materials scientist Tyrone Daulton of Washington University in St Louis, Missouri, and his colleagues now say that these nanodiamonds are actually aggregates of the carbon materials graphene, graphane and their oxides1. "I believe the earlier reports are in error," says Daulton. "If you don't pay close attention, you can fool yourself to think something is a diamond when it is not."

The study, published in Proceedings of the National Academy of Sciences (PNAS), "is a very convincing analysis by a world expert", says Peter Heaney, a mineralogist at Pennsylvania State University in University Park, who was not involved in the research.

But the lead author of two earlier comet-impact papers, Douglas Kennett, an archaeologist at the University of Oregon in Eugene, calls the study "fundamentally flawed science". "The claim we misidentified diamonds is false, misleading and incorrect," he adds, although he declined to specify his objections.

Daulton's paper comes hot on the heels of work by Nicholas Pinter, a geoarchaeologist at Southern Illinois University in Carbondale and a co-author on the PNAS study.

Pinter and his colleague Andrew Scott of the Royal Holloway, University of London in Egham went to three of the sites where Kennett's team had found nanodiamonds. As well as providing samples for Daulton's study, they also looked for carbon spherules.

What they found instead was hardened fungal material and faecal matter from arthropods that looked similar to carbon spherules2.

Kennett and his team also dispute this finding, and he says that they will be writing to PNAS to "expose the major flaws in the Daulton paper".


  1. Daulton, T. L., Pinter, N. & Scott, A. C. Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1003904107 (2010).

  2. Scott, A. C. et al. Geophys. Res. Lett. doi:10.1029/2010GL043345 (2010).

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Dalton, R. Comet theory carbonized. Nature (2010).

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