Nanodiamonds are dirty-looking, often flawed, and, as the prefix implies, really, really tiny. That doesn’t mean they’re not valuable. In the past few years, these microscopic gems have caused quite a stir in a number of scientific fields, archaeology in particular. Not as artifacts from long-lost cultures, but as evidence of doomsday meteors or comets that wiped out some of the earliest human beings in the Western Hemisphere.
In North America (and, to a lesser extent, other parts of the world), there’s a fine black layer of soot in the ground from around 12,900 years ago (pictured above). “It’s an interval in North American history that’s interesting from a number of perspectives,” says Douglas Kennett, an archeologist at the University of Oregon. “There’s a major extinction of a wide range of animals and a long-standing debate of why it occurred, and not too many satisfying answers.”
Those extinctions seem to include the Clovis people, some of the early settlers in North America, named for an archaeological site in New Mexico. Like mammoths, wild horses, and other megafauna, Clovis people seemed to thrive until 12,900 years ago, then disappeared from the archaeological record just below the layer of soot. It’s clear something happened at that time.
On a microscopic level, the soot is even more intriguing. It contains iridium, an element rare in the earth’s crust but common in meteors, asteroids, and comets. The soot also contains nanodiamonds, fused spheres of carbon that, like their prettier, jewelry-quality cousins, only form in immense heat and under immense pressure. But whereas normal diamonds form deep underground over millions of years, the conditions for growing nanodiamonds are less stringent. A projectile from space often has enough energy to create them as it screams through the atmosphere and scorches the earth’s surface.
Based on the confluence of evidence—nanodiamonds in the soil, and sudden extinctions—Kennett and other scientists (most notably Richard Firestone, a chemist at Lawrence Berkeley National Laboratory, and Allen West, a private geophysicist) suggested last year that a number of giant objects might have crashed into the earth 12,900 years ago. This impact could have caused continent-wide forest fires (hence the layer of soot) and destroyed any number of ecosystems across the wilderness.
More recently, Kennett and others published a paper in Science outlining details about the nanodiamonds (left), which offer a vital clue about the likelihood of an impact. While the die-offs could have other explanations (such as human overhunting), it’s nigh impossible to produce nanodiamonds without an extraterrestrial collision—not to mention producing nanodiamonds at six far-flung places across the United States and Canada, as Kennett and his colleagues reported.
One of the co-authors of the paper was Douglas Kennett’s father, James Kennett, a retired geologist. As luck would have it, James was around during the debates about the most famous impact-extinction in earth’s history, the asteroid (or asteroids) that probably wiped out dinosaurs 65 million years ago. (That theory was first put forth in the late 1970s by another father-son team, Luis and Walter Alvarez.) The initial debates over whether that impact actually occurred were so contentious that James sat them out, preferring not to get involved.
If he continues to push the idea of an impact from 12,900 years ago, he may have no choice but jump into the fray. Some scientists find the Clovis impact theory flawed, and at least one team has already published a paper to refute it. The main evidence against the theory is the lack of a crater. The dinosaur asteroid punched in a 112-mile wide depression just off the Yucatan peninsula in Mexico, but there’s no corresponding crater for a more recent impact.
That doesn’t necessarily doom the theory, Douglas Kennett points out. For one, the object could have struck one of the glaciers that covered much of North America at the time, glaciers that have since receded and melted. Or, the object might not have been one solid rock, but a softer mass that broke up upon entering the earth’s atmosphere. If it broke up into enough pieces, none would have left a significant crater, but their sum total could have been devastating.
The other mark against the impact theory is harder to interpret. Some geologists link widespread fires from around 12,900 years ago to an abrupt change in the earth’s climate at that time. A team from England published a paper in the Proceedings of the National Academy of Science to that effect, arguing that they found little evidence for the flash fires that would have followed from an impact.
Then again, Kennett points out that the famous Tunguska impact of 1908—the mysterious explosion of what was probably a comet over Siberia—did not cause massive forest fires, even though it charred and knocked down 80 million trees, some dozens of miles away. The Tunguska comet left no significant crater, either.
If the impact did happen, it of course happened within the collective memory of human beings. And however unreliable that memory may be about scientific details—Clovis people obviously didn’t know much astrophysics—some scientists have turned to those memories for secondary evidence of an impact. In particular, Firestone and West, in a book called The Cycle of Cosmic Catastrophes, mined the myths and creation stories of American Indians. The authors claim it’s no coincidence that many of those hoary tales refer to “Thunderbirds” reigning fire across the sky, followed by widespread apocalyptic destruction
Sam Kean is associate editor of Search.
