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Impact hypothesis loses its sparkle

Posted on 31/08/2010

Many large species became extinct during the last Ice Age

A team of scientists – including Professor Andrew C Scott from Royal Holloway, University of London – have cast resounding doubt upon the last crucial piece of evidence supporting the Younger Dryas impact hypothesis.

Just 12,900 years ago, gradual warming following the last Ice Age ceased abruptly, and glacial conditions were restored for a 1,300 year interval known as the Younger Dryas. In North America, many large species (such as mammoths, mastodons, saber-tooth tigers, giant short-faced bears) became extinct, and paleoindians, who had just arrived and spread across North and South America underwent a regional change, their Clovis stone spear points disappearing from the archaeological record near the onset of the Younger Dryas.

A controversial theory suggests that a comet or meteor airburst and/or impact event was responsible. In sedimentary deposits dating to the beginning of the Younger Dryas, impact proponents had reported carbon spherules containing tiny nano-scale diamonds, including lonsdaleite, a rare hexagonal (2H) polytype of diamond. Lonsdaleite is particularly interesting because it is often associated with shock pressures related to impacts where it has been found to occur naturally.

In the current PNAS paper, the scientists - led by Dr Tyrone Daulton of Washington University in St Louis and also including Professor Nicholas Pinter of Southern Illinois University - show that the original material reported as diamond appears to have been misinterpreted, and also that what was reported as nanodiamonds are instead forms of carbon related to commonplace graphite.

Dr Daulton commented, “Of all the evidence reported for a Younger Dryas impact event, the presence of 2H hexagonal diamond in Younger Dryas boundary sediments represented the strongest evidence suggesting shock processing.” 

However, a close examination of carbon spherules from the Younger Dryas boundary using transmission electron microscopy by the research team found no nanodiamonds. Instead, graphene- and graphene/graphane- oxide aggregates were found ubiquitous in all specimens examined (including carbon spherules dated from before the Younger Dryas to the present). Importantly, the researchers demonstrated that previous Younger Dryas studies misidentified graphene/graphane- oxides as hexagonal diamond and likely misidentified graphene as cubic diamond.

The Younger Dryas impact hypothesis was in trouble already before this latest finding. Many other lines of evidence – including fullerenes, extraterrestrial forms of helium, purported spikes in radioactivity and iridium, and claims of unique spikes in magnetic meteorite particles – had already been discredited. 

Professor Scott, points out that “we should always have a skeptical attitude to new theories and to test them thoroughly, and if the evidence goes against them they should be abandoned.”

The paper, entitled ‘No evidence of nanodiamonds in Younger-Dryas sediments to support an impact event’, is published in the journal Proceedings of the National Academy of Sciences, doi/10.1073/pnas.1003904107



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