Do nanodiamonds prove an asteroid impact killed off North America's massive mammals 13,000 years ago? It depends on which scientist you ask.
A pair of studies published in the last month offer competing theories about whether an extraterrestrial object killed megafauna like woolly mammoths and sabre-toothed cats, along with the Clovis culture of North American human settlers.
At issue are nanodiamonds, especially a form called lonsdaleite, which form in a high-pressure, high-temperature collision. One study claims to have found them in the Greenland ice sheet, which suggests an asteroid or comet impact might have been the cause of major climatic changes that led to the extinction event.
Researchers from the University of California-Santa Barbara, the University of Oregon, Northern Arizona University and the University of Maine examined ice core samples from Greenland that dated to the Younger Dryas, a millennium-long cool period that began about 12,800 years ago. The team, led by University of Oregon archaeologist Douglas Kennett, reported finding a heavy concentration of nanodiamonds, which are smaller than a virus, in a single layer of "extraterrestrial material" in the ice sheet.
The concentration of nanodiamonds in that layer is up to 5 million times higher than anything else around it, according to the University of Maine. The research was published in the International Glaciological Society's Journal of Glaciology.
But another study published this week says the Oregon researchers are wrong: the nanodiamonds are more like fool's diamonds, and are just aggregates of graphene, graphane and their oxides -- the stuff found in pencils. Like diamonds, these are all made of carbon.
"If you don't pay close attention, you can fool yourself to think something is a diamond when it is not," Tyrone Daulton, a materials scientist at Washington University in St. Louis, said in an interview with Nature News.Daulton's paper, published in the Proceedings of the National Academy of Sciences, includes data from a UK team who double-checked the same site where Kennett et. al took their samples. That team said they found nothing but hardened fungal and fecal matter that looked a lot like balls of carbon.
Daulton and others say the nanodiamonds were the last hope for the space collison theory. Since that theory was first proposed three years ago, scientists have debunked all other possible cometary evidence. Without nanodiamonds, the theory falls, the Wash U group says.
Kennett's team stands by their findings, however, and argues that multiple types of analysis confirmed the presence of nanodiamonds. They say they can't be sure when the nanodiamonds were deposited -- it could have been just before, during or just after the Younger Dryas.
Nanodiamonds or not, the studies leave as many questions as they answer. Most scientists think the Younger Dryas was set off by frigid fresh water flowing from a breached lake into the northern Atlantic, interrupting the ocean's circulation. If scientists can't prove an asteroid was at fault, then what really happened is anyone's guess.
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