Melted glass buried deep within the Earth at sites around the world confirms the theory that a comet or meteor struck the planet nearly 13,000 years ago, triggering the Younger Dryas Ice Age, killing off the mammoths and other megafauna in North America, and perhaps even causing the disappearance of the Clovis culture of early Native Americans. The cause of the Younger Dryas cooling period has been very controversial. Some researchers have proposed an extraterrestrial impact and have produced evidence of the event, but others claim that the results have not been replicated. The new findings, reported this week in the Proceedings of the National Academy of Sciences, appear to provide that needed replication.
The Younger Dryas event began about 12,900 years ago and lasted about 1,300 years. The period is named after the alpine-tundra wildflower, Dryas octopetala, which spread southward during the period. Average temperatures during the period dropped by as much as 20 degrees Fahrenheit in Europe, perhaps a little less in North America. The period marked the end of the mammoths, giant ground sloths and other large creatures that had earlier wandered North America. Artifacts from the Clovis culture, whose members are believed to be among the earliest settlers of this continent, disappeared from the archaeological record. Northern glaciers moved southward and forests turned into tundra. The period is linked to the onset of agriculture in the Middle East, perhaps because hunting and gathering could no longer provide adequate food supplies.
An international team of researchers that included earth scientists James Kennett of UC Santa Barbara examined soil samples dating from the Younger Dryas at 18 sites in North America and Europe. At most of the sites, they found microspherules of silica similar to those that have been linked to other meteor impacts. The microspherules were in the layer dated to the Younger Dryas, but not in the layers above or below it.
At three sites, however -- in Pennsylvania, South Carolina and Syria -- they found melted glass that could only have been formed at very high temperatures. The glass, similar to that observed following the Trinity nuclear test explosion in New Mexico, was formed at temperatures of 3,100 to 3,600 degrees Fahrenheit, the temperatures that would have been reached during an impact event. Chemical studies showed that the material was not cosmic in origin, not man-made and no volcanic.
The evidence of impact, the team said, now covers at least a third of the planet, ranging from California to Western Europe and the Middle East. The fact that melted glass has been found at several locations, they added, indicates that the meteor or comet that struck the Earth broke up during its entry to the atmosphere and struck at multiple locations. The team has yet to identify a limit to the debris field.
The Younger Dryas layer at the archaeological site in Syria where the material was found -- Abu Hureyra in the Euphrates Valley -- has a thick charcoal layer, suggesting a major fire associated with the impact. This region was one of the earliest to feature permanent villages, the team said, and the impact could have destroyed their towns and killed many or most of the residents. Combined with the colder weather, it may have taken centuries for people to recover.