Shock-shocked quartz from three classic Clovis sites in North America supports the "Younger Dryas" hypothesis that an exploding comet caused widespread fires, a "vulnerable winter," and the extinction of megafauna and the Clovis culture
New findings strengthen the hypothesis that shocked quartz found at ancient Clovis sites indicates that a comet that disintegrated and exploded above the Earth's surface about 13 years ago caused widespread destruction - possibly to the point of extinction of the Ice Age giants and the disappearance of early human communities in North America.
Evidence from major archaeological sites suggests that a major cosmic event may have altered the climate and ecosystems of the Late Pleistocene.
Scientists are now expanding the evidence base supporting the idea that a fragmented comet exploded in Earth's atmosphere nearly 13 years ago. This cosmic event may have contributed to the extinction of mammoths, mastodons, and many other large Ice Age animals, as well as the sudden disappearance of the Clovis culture from the archaeological record of North America.
In a study published in PLOS ONE, Professor Emeritus James Kennett of the Department of Earth Sciences at the University of California, Santa Barbara, and his team report the discovery of “shocked” quartz—sand grains that have been altered under extreme heat and pressure—at three major Clovis sites in the United States: Murray Springs in Arizona, Blackwater Draw in New Mexico, and Arlington Canyon in the Channel Islands of California.
“These three sites are classic key sites for discovering and documenting the extinction of megafauna in North America and the disappearance of the Clovis culture,” Kent said.
Extinction, the Younger Dryas, and the Cosmic Impact Hypothesis
The extinction of the large Ice Age animals and the disappearance of the Clovis technocomplex occurred at the same time as the Younger Dryas began—a sudden cooling episode that interrupted the gradual warming of the Earth after the last Ice Age. This return to near-Ice Age conditions lasted about a thousand years.
Scientists have proposed several possible explanations for this dramatic climate change over the years. Kent and his colleagues suggest that a comet that broke up into several bodies exploded in the atmosphere, releasing intense heat and shock waves over large parts of the Earth.
“In other words, everything went wrong all at once,” Kent said. According to the Younger Dryas impact hypothesis, the series of explosions led to widespread fires and the release of smoke and soot, as well as dust that blocked some of the sun’s radiation and created an “impact winter.” Rapid melting of the ice caps would have further exacerbated the cooling in the affected areas. The force of the impact itself, and the harsh conditions that followed, could have contributed to the decline of megafauna in North and South America and the disappearance of the Clovis culture.
Accumulating evidence of a harmful event
For about twenty years, Kent and supporters of the hypothesis have been collecting increasingly strong evidence. Among other things, a dark layer ("black mat") has been identified in sediments at many sites in North America and Europe - evidence of extensive fires. In addition, a growing list of "impact proxies" has been collected: unusual concentrations of rare elements and minerals characteristic of comets, such as platinum and iridium, as well as mineral forms that indicate extreme temperatures and pressures, such as nanodiamonds, metallic spheres and meltglass formed by the melting, cooling and recrystallization of materials.
Thanks to technological advances, the team is now focusing on another proxy considered the “crème de la crème” of evidence for a cosmic impact: shocked quartz—sand grains that show microscopic deformations caused by extreme heat and pressure. In samples from three archaeological sites—Murray Springs, Blackwater Draw, and Arlington Canyon—the researchers identified quartz grains with characteristic cracks, some of which are filled with molten silica. They used a variety of methods, including electron microscopy and cathodoluminescence, to confirm that the quartz had been shocked at very high temperatures and pressures—far beyond what volcanic activity or ancient human activity could have created.
Airborne explosions and the difficulty of identifying impacts without craters
The presence of shock-struck quartz is especially important in the absence of craters—the classic “smoking gun” of evidence for a cosmic impact. Unlike the asteroid that led to the extinction of the dinosaurs about 65 million years ago and left a giant crater beneath the Yucatan Peninsula, “airbursts”—cosmic impacts that occur above the surface, like the comet proposed here—often leave very few geomorphological signatures.
Using hydrocode modeling, the team demonstrated how low-altitude explosions can create a variety of shock patterns in quartz. While in “classic” craters the evidence is based on parallel arrays of cracks in quartz, in airburst events the variety of orientations, pressures and temperatures around the explosion epicenters creates a wide spectrum of shock patterns. “Some grains will be very highly shocked, and others will be less so – which is exactly what you would expect to see,” explains one of the researchers.
When the shock-affected quartz is combined with other proxies from the same sedimentary layer—a dark carbon-rich layer, nanodiamonds, impact spherules—all at three classic Clovis sites, the researchers suggest that the cumulative package of evidence “supports cosmic impact as a major contributor to megafauna extinctions and the collapse of the Clovis technocomplex at the beginning of the Younger Dryas,” as the paper states.
More of the topic in Hayadan:
