Firestone argued that water or ice could have absorbed the impact, possibly leaving behind no crater.
Boslough disagreed. Even if the comet had plunged into the ice sheet covering much of North America, the crater formed beneath it would still be sizable. "We wouldn't be able to miss that right now — it would be obvious," Boslough said.
The arguments and evidence against the impact were published in the December 2012 American Geophysical Union monograph.
"Extraordinary claims require extraordinary evidence"
Powerful impacts are Boslough's field, but the other 15 scientists working on the paper offered up other sources of counterevidence for the existence of a collision.
"We all independently came to the conclusion that the evidence doesn't support a Younger Dryas impact," Boslough said. [Asteroid Basics: A Space Rock Quiz]
"We all came to this based on our own very narrow piece of the puzzle."
For instance, the initial team studying the event announced the discovery of a carbon-rich black layer, colloquially known as a "black mat," at a number of sites in North America. Containing charcoal, soot and nanodiamonds, such material could be formed by a violent collision.
But this isn't the only possible source.
"The things they call impact markers are not necessarily indicators of high-pressure shocks," Boslough said. "There are other processes that potentially could have formed them."
Speaking of the black mat found in central Mexico, Firestone said, "Boslough is correct that there are other black mats, but these are dated to 12,900 years ago at the time of impact." He points to independent research published this fall that located hundreds to thousands of samples.
However, radiocarbon dating of one of the sites in Gainey, Mich., suggested its samples were contaminated.
Melted rock formations and microscopic diamonds found in a lake in Central Mexico last year were also suggested as evidence for the collision, but Boslough's team disagrees with the age of the sediment layer in the region.
Boslough said the standard for indicating a strong shock occurred is pretty high in the impact community, and the findings by the original team don't meet them. Nor do they offer up any physical models that propose how an impact or airburst would have occurred — and the ones Boslough has run just don't pan out.
"It's really a stretch to claim that there was this large impact event with no crater and no unambiguous shock material, because large impacts are such rare events," Boslough said.
"When somebody is making a claim that something extraordinary happened, something out of the ordinary and with a very low probability, and they have ambiguous evidence, then the default is that it didn't happen," he continued.
"Extraordinary claims require extraordinary evidence."
Firestone stands firm. "All the evidence has now been confirmed by others," he said.
This article originally appeared on LiveScience.com.
"Boslough has no data supporting his arguments, and ignores the counter arguments of Bill Napier."
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