In 2007, R. B. Firestone and colleagues published evidence of an extraterrestrial impact, roughly coincident with the onset of the cold climate event known as the Younger Dryas. This event, around 12,900 years ago, is around about the time of some (but not all) megafaunal extinctions in North America, it is also around the time (but not precisely) of the Clovis culture. The paper argued that the impact event may have "contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America".
Last year, I reported on widespread dissatisfaction with this impact hypothesis. Some critics didn't think that there was any evidence of megafaunal trauma from the impact, some didn't think that the dates matched any "adaptive shifts", and in particular the end of the Clovis culture.
And then others didn't think that there had been an impact at all. These were in some ways the most worrisome, because they directly questioned the supposed evidence in support of an extraterrestrial event -- "microspherules" of magnetic material, clustered in sedimentary contexts at precisely 12,900 years ago in sites across much of North America.
Now, in the Proceedings of the National Academy of Sciences (where Firestone and colleagues originally published their observations), Todd Surovell and colleagues have published a remarkable paper that tests the Firestone impact hypothesis: "An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis." Most critiques attempt to find an alternative explanation for a set of original observations. In this paper, Surovell and colleagues merely attempt to replicate the original observations at multiple sites, and fail -- as their abstract tersely states,
We were unable to reproduce any results of the Firestone et al. study and find no support for Younger Dryas extraterrestrial impact.
Just like that -- it's about as hard-hitting as you're going to see in a scientific research paper.
Of course, this paper only examined one out of a number of observations that Firestone and colleagues had adduced in support of the impact hypothesis. But in the introduction to their paper, Surovell and colleagues reference several other recent studies that re-examined other aspects of the evidence:
A series of critiques of the original Firestone et al. article (1) have been published recently (8-10). Pinter and Ishman (8) argue that the suite of markers used to indicate impact are inconsistent with "any single impactor or any known event." Furthermore, they provide alternative explanations for many of the observed marker peaks. For example, glassy and metallic microspherules are known components of atmospheric dust derived from the constant influx of micrometeorites. An independent evaluation of the charcoal evidence was recently published by Marlon et al. (9). Examining concentrations of charcoal from 35 pollen cores across North America, they found no evidence for large-scale, continent-wide wildfires specifically associated with the onset of the [Younger Dryas].
In the current case, the results are very simple: they went looking for a spike in the number of impact-generated particles coincident with the Younger Dryas. They looked at seven sites with long and continuous records of sedimentation across that interval. They found the supposed impact-generated particles, but not patterned with any kind of spike.
They suggest a different model for the presence and accumulation of the magnetic particles:
Alternatively, it may be that the presence, absence, and relative abundance of magnetic materials, especially the spherules, is due to characteristics of the parent material and depositional environment instead of some sort of continent-wide extraterrestrial process. The characteristics of the local depositional setting before, during, and after 12.9 ka have not been addressed by the proponents of the impact hypothesis. The zones producing the YDB ‘‘impact markers’’ are typically associated with soils (stable surfaces) or shifts in the depositional environment (e.g., alluvial to lacustrine conditions at Blackwater Draw, Lubbock Lake, Murray Springs, and Lake Hind; buried soils in the Carolina Bays and at Lommel, Belgium).
One might imagine atmospheric particles accumulating on stable paleosols over long stretches of time, generating a local spike in the number of such particles in the stratigraphic column. In any event, the data presented here don't bear out the hypothesis of any unusually large impact event.
I'm not a geologist, and I have no special insight into the analyses here, beyond reading the charts. But remember that the impact hypothesis made a tremendous media splash. Maybe more damaging to the scientific side of things, the hypothesis that the Younger Dryas cold period came from an extraterrestrial force, made it seem for a moment less necessary to investigate terrestrial sources of cooling at the terminal Pleistocene. The science will correct itself, but the public perception of the climate changes at the end of the Ice Ages will need quite a bit more nursing to get a more realistic perspective on the story.
Firestone RB and lots of others. 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proc Nat Acad Sci USA, 104:16016-16021. doi:10.1073/pnas.0706977104
Kerr RA. 2008. Experts find no evidence for a mammoth-killer impact. Science 319:1331-1332. doi:10.1126/science.319.5868.1331
Surovell TA, Holliday VT, Gingerich JAM, Ketron C, Haynes CV, Jr, Hilman I, Wagner DP, Johnson E, Claeys P. 2009. An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. Proc Nat Acad Sci USA (early) doi:10.1073/pnas.0907857106