The 'Big Freeze', or more formally the 'Younger Dryas stadial', is a roughly 1300 year time-span — between 12,800 years ago and 11,500 years ago — when the planet suddenly reverted back to near-ice-age conditions, causing species extinctions and the downfall of the Clovis culture, one of the earliest ancient cultures. Many hypotheses have been proposed as to how the Big Freeze started and how the Clovis culture fell, and one of the more interesting ones — a comet impact or explosion — has found more support in a new study.
In 2007, Richard Firestone, a staff scientist at the U.S. Department of Energy's Lawrence Berkeley National Laboratory, proposed the controversial 'Younger Dryas impact hypothesis' or 'Clovis comet hypothesis' — an idea that a comet exploded over the Great Lakes area, causing several major species extinctions (including the mammoth, mastodon, and saber-toothed tiger) and the fall of the Clovis culture. No impact crater has been found to support this hypothesis, possibly due to the presence of glaciers over the Great Lakes during that time period, however Firestone's team discovered microspherules of iron, silica, iridium and nano-diamonds — which can be caused by the impact of a comet or meteoroid — in sediments dating back to a period 12,900 years ago.
Since then, the hypothesis has undergone 5 years of debate, with a major blow being dealt to it by 2009 study led by Todd Surovell from the University of Wyoming, which failed to reproduce Firestone's results at seven different sites attributed to the Clovis.
The new study, published on Monday in Proceedings of the National Academy of Sciences (PNAS), has called Surovell's findings in question though.
"Surovell's work was in vain because he didn't replicate the protocol. We missed it too at first. It seems easy, but unless you follow the protocol rigorously, you will fail to detect these spherules. There are so many factors that can disrupt the process. Where Surovell found no spherules, we found hundreds to thousands," said Malcolm LeCompte, lead author of the new study, who is professor at Elizabeth City State University.
Working from Firestone's sampling and sorting techniques, LeCompte conducted his own independent study at several of the sites, including one studied by Surovell, and found the same microspherules that Firestone reported — tiny ball bearings, about the width of a human hair, with a surface texture that was consistent with being crystalized at high temperature and then rapidly cooled. His examination of them concluded that they were of terrestrial origin, and formed by some massive impact event.
The next stage in the research brought the team to the Topper site — an archaeological dig at what was an active tool-making quarry for the Clovis culture. Along with University of South Carolina archaeologist Al Goodyear, they dug out Clovis artifacts from the site and sampled the soils around and under them.
"If debris was raining down from the atmosphere, the artifacts should have acted as a shield preventing spherules from accumulating in the layer underneath. It turns out it really worked!" Goodyear said. "There were up to 30 times more spherules at and just above the Clovis surface than beneath the artifacts."
The debate will likely continue as to what caused the sudden onset of the Younger Dryas period and the disappearance of the Clovis culture. However, studies such as these explore an interesting area of science where the somewhat outlandish ideas have the greatest support, and some of the greatest discoveries of our time have been due to those ideas.