The session PP33B. Younger Dryas Boundary: Extraterrestrial Impact or Not? II was standing room only.

The question: What caused the thousand plus year Younger Dryas cold snap that likely killed off the mammoths, saber-toothed cats, and other iconic ice-aged creatures ~12,000 years ago?

In one corner were those who believe that impact from a swarm of comets caused the cooling event. In the other: the skeptics.

Wallace Broecker of the Lamont-Doherty Earth Observatory started the talk out with fighting words: “As far as a comet goes, I never believed it.” He claimed that the only evidence for a blast from outer space is the presence of nano-sized diamonds in the layer of sediment from that time period, and those don’t even prove impact.

Allen West of GeoScience Consulting led the pro-impact camp. He and his colleagues rocked the field when they published a paper in 2007 and another in 2009 suggesting the idea. He argued that the nanodiamonds are, in fact, proof of a major bang, since no other situation could produce the anaerobic conditions and the pressures and temperatures to make them.

Still, there’s a nagging problem: if something slammed into Earth that was big enough to set off more than a millennium of cooling, then where’s the crater?

Pete Schultz of Brown Unviersity suggested that the absence of crater evidence was not necessarily evidence of an impact’s absence. Perhaps a thick layer of ice covered the impact site, and the ice absorbed some of the impact.  Then, when the glaciers melted, they washed away all the evidence.

Or maybe it wasn’t a traditional impact at all. Instead, could the high pressures surrounding a meteor hurtling towards earth create microdiamonds?  No way, said H. J. Melosh. Could these air bursts create nanodiamonds?  Maybe, but then they aren’t proof of impact.

In the end, I wasn’t sure that anyone had delivered a knockout piece of evidence to convince the other side.  But it sure was a good show.

-Tia Ghose, UC Santa Cruz Science Communication Graduate Student