Cosmic Impact 12,800 Years Ago Caused Mass Extinction: Impact Spherules Tell All

About 12,800 years ago, the Earth was beginning to warm, shedding its icy cloak as it emerged from the last ice age. Yet something halted that warming, abruptly slamming our planet back to a near-glacial state. Now, scientists have discovered not only that this switch occurred in one year, but also what might have caused it.

The cause of the cooling has been debated among researchers for years. What deepens the mystery is the fact that the cooling temperatures coincided with the abrupt extinction of the majority of the large animals inhabiting the Americas at the time, including saber-toothed cats, mastadons, mammoths and giant sloths. In addition to these large animals, the prehistoric Clovis culture also vanished.

Now, researchers have documented a wide distribution of microspherules in a layer over 500 million square kilometers on four continents, including North America. This layer, which is the Younger Dryas Boundary (YDB) layer, also contains peak abundances of other exotic materials, including nanodiamonds and other unusual forms of carbon such as fullerenes, as well as melt-glass and iridium. In total, the researchers examined 18 sites across North America, Europe and the Middle East.

So what are microspherules? They're tiny spheres formed by the high temperature melting of rocks and soils that then cooled or quenched rapidly in the atmosphere. This process occurs from enormous heat and pressures in blasts generated from something like an atomic explosion--or a cosmic impact. They can also be created by volcanic activity, lightning strikes and coal seam fires.

In order to find out exactly what was responsible for these spheres, the researchers employed scanning electron microscopy and energy dispersive spectrometry on nearly 700 spherule samples that were collected. They found that the samples where geochemically dissimilar from volcanic materials and that they couldn't have formed naturally from lightning strikes.

"Because requisite formation temperature for the impact spherules are greater than 2,200 degrees Celsius, this finding precludes all but a high temperature cosmic impact as a natural formation mechanism for melted silica and other minerals," said James Kennet, one of the researchers, in a news release.

The findings actually suggest that a huge, cosmic impact occurred during this time period. The impact could have potentially caused a cooling effect on the planet, and could explain the mass extinction that occurred at the time.

"This evidence continues to point to a major cosmic impact as the primary cause for the tragic loss of nearly all of the remarkable American large animals that had survived the stresses of many ice age periods only to be knocked out quite recently by this catastrophic event," said Kennet.

The findings are published in the journal Proceedings of the National Academy of Sciences.