Having already posted two articles which call into question the findings of Richard Firestone et al, in their 2007 paper Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling,  I decided to contact Dr. Firestone to try and add some balance to the current debate; it’s all very well reporting on new research which appears to flatly contradict previously published work, but I think it very important to give the original research team an opportunity to voice their opinion of the new findings. Because there were differences in the various methodologies employed by the various teams of researchers, it should be no surprise that the findings were at variance with one another. However, it is unusual for subsequent research to completely contradict previous research conducted only a couple of years beforehand, so clearly there is need for further clarification from both sides.

As such I am extremely grateful to François Paquay, L. Tyler Faith and Todd Surovell for forwarding me their papers, and more recently to Dr. Richard Firestone for taking the time to so quickly reply to my request via email for some comment of his own. As we shall see, he strongly disagrees with the new findings and the way in which the research was conducted, emphatically standing by his original conclusions. It will be for readers here to decide for themselves the merits of the competing claims in this fascinating conundrum that addresses the mystery surrounding one of the greatest mammalian and avifaunal extinction events in recent geologic time, a mere 10,000-12,000 years ago, in North America.

I should add here, and partly in response to Terry Toohill who pointed out in a recent comment that I had thus far failed to mention Australia in my extinction posts, that I’m currently reading through papers regarding the Pleistocene extinctions in Australia, and I hope to publish something here in the near future. The debate there has by tradition been confined to climate change and human overkill, but as we shall see, recent research there indicates the true story may not be that simple – for example, a large decrease in mammal species diversity has been detected between 120kya and 90 kya, long before humans were on the continent in any significant numbers, more of which in due course.

I’m going to begin with the second of the recent comet papers, An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis, in this instance authored by Todd Surovell et al, and for which this is the abstract:

Based on elevated concentrations of a set of “impact markers” at the onset of the Younger Dryas stadial from sedimentary contexts across North America, Firestone, Kennett, West, and others have argued that 12.9 ka the Earth experienced an impact by an extraterrestrial body, an event that had devastating ecological consequences for humans, plants, and animals in the New World [Firestone RB, et al. (2007) Proc. Natl. Acad. Sci.USA 104:16016–16021]. Herein, we report the results of an independent analysis of magnetic minerals and microspherules from seven sites of similar age, including two examined by Firestone et al. We were unable to reproduce any results of the Firestone et al. study and find no support for Younger Dryas extraterrestrial impact.

As we saw, the paper by Paquay et al stated that the researchers were unable to find sufficiently elevated levels of iridium, one of the prime marker candidates for a cometary or asteroidal impact, also noting a lacking abundance of other indicators that would be expected to be present both on land and in seawater. The story is taken up by Surovell et al, who  analysed 7 sites, also revisiting two of the sites in the original Firestone study, namely Topper and Blackwater Draw, in order to evaluate the claims for an extraterrestrial impact event. As Surovell et al note:

The primary evidence for impact is the apparent presence of a suite of markers that occur in increased concentrations in sediments dating to ca. 12.9  1.0 ka from sites across North America including 10 Clovis-age archaeological sites and 15 Carolina Bays on the Atlantic Coastal Plain (1–5). Clovis occupation features date within a narrow time range between 13.3 and 12.8 ka; some are buried by organic-rich sediments or soils, commonly termed ‘‘black mats’’ (6, 7). Markers found in YD black mats and contemporaneous sedimentary contexts include magnetic microspherules, magnetic grains, iridium and nickel, charcoal, soot and polycyclic hydrocarbons, carbon spherules, fullerenes and ET helium, glass-like carbon, and nanodiamonds. Some markers are more widespread than others.

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 YD.

This same paper was reviewed back in October by John Hawks, and this is part of what he had to say:

…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…it’s about as hard-hitting as you’re going to see in a scientific research paper.

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.

By way of a quick recap, here’s the abstract of Firestone et al‘s original paper, which is freely accessible.

A carbon-rich black layer, dating to ≈12.9 ka, has been previously identified at ≈50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ≅12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period.

Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain.

We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.

As mentioned at the top of this post, I though it would be good to hear a response from Richard Firestone – the critics have been damning, so of course we want to hear how well or how badly this has gone down with the original proponents of the cometary impact theory. What follows is a direct quote via email from Dr. Firestone, on behalf of himself and fellow authors:

Surovell et al failed to confirm the YD Impact data because their sampling methods were fatally flawed.  We discovered that the impact layer was very narrow, often millimeters thick, and used painstaking microstratigraphy to find it.  Although the impact layer initially was deposited just before the formation of the black mat, the magnetic and organic fractions containing the most compelling information often moved about due to turbation of the sediment following deposition.

Surovell et al did no microstratigraphy and analyzed substantially wider sediment layers that included mostly normal sediment.  Thus their results greatly diluted the impact layer contribution to their samples.  Also, Surovell et al chose to recognize only magnetic microspherules that are round and shiny.  It is well known that meteoritic dust is generally not perfectly round, often pitted and dull, and generally weathered.  Despite all of these failures the distributions of magnetic grains and microsph erules shown by Surovell et al at all of their sites do show increases at the time of the YD impact which are consistent with our own results after correction for the sampling dilution effects.  Surovell et al’s conclusions are clearly influenced by their own bias and not by a fair analysis of their own data.

Paquay et al claim to find no evidence for PGE elements in there sediment samples.  We too found much weaker evidence for iridium in the sediment samples than in the magnetic fractions because the iridium is only found in the magnetic material which is a small fraction by weight of the total sediment even in the best cases where microstratigraphy was applied.  As was the case with Surovell et al, Paquay et al took very broad sediment samples diluting their results.  They used a shotgun approach to sample selection with no assurance that they had even sampled the YD Impact layer.  This is especially important because the thin impact layer often disappears from point to point due to the effects of wind, rain, and time.

Despite these experimental problems Paquay et al do report high concentrations of PGE elements at a site where we reported them and then dismiss them as normal despite the fact that their results were higher than were seen at some K/T sites.  Before we publishe d our PNAS paper, Sharma et al (Dartmouth) independently found convincing evidence for large Osmium anomalies in Pacific Ocean sediments dated to about 12,000 years ago.  His paper was rejected by PNAS because “no impact occurred at that time”.  Sharma has since found additional evidence for PGE’s in the YD Impact layer which are reported at the SF AGU meeting this year.   Weaker evidence of excess Iridium associated with the YD layer in Greenland ice has also been found by Paul Mayewski.

To summarize, the results published by Surovell et al and Paquay et al are both fatally flawed by their sampling techniques that failed utilize the microstratigraphy necessary to identify the YD impact layer.  Both authors thus analyzed highly diluted samples, published data showing smaller peaks in magnetic grains, microspherules, and PGE’s consistent with our own results, and then persisted on describing their results as within normal variations.  We find these results biased towards a specific conclusion and inconsistent with a broad range of impact markers found at dozens of sites across North America and into Europe.  Attempts by others to independently report results confirming our research have systematically been rejected by reviewers primary on the basis that no impact occurred.

Clearly there is a significant divergence between what the different researchers were able or unable to find, and the way in which what has been found may have been interpreted – whether one body of research or another was sufficiently detailed or painstaking is for others with more knowledge of the technologies involved than I possess to assess.

I’m not sure where the debate goes from here – as such as I have no personal interest in whether a comet hit or not, but I’d definitely like to know one way or the other. As noted elsewhere, a cometary impact might go some way to explaining the rather alarming speed with which the Younger Dryas appeared and disappeared. If however, the causes of that and the huge extinction event were indeed terrestrial, we have very great cause to be concerned for our own wayward climate in the present, especially with regard to the fact that we appear to be in the midst of the Sixth Great Extinction.

Conversely, if an extraterrestrial impact is proven, there is little or nothing we can do even with our modern technology to avoid another event in the future. With our densely populated urban societies dependent agriculture, we are even more vulnerable now than in the Late Pleistocene to events which could mark our own extinction.

On a final note, a paper published this morning at PLoS ONE, Quantifying the Extent of North American Mammal Extinction Relative to the Pre-Anthropogenic Baseline makes for some related further reading – regardless of past events, it is abundantly clear that we modern humans have damaged this planet and reduced its biodiversity to potentially dangerous levels. The planet of course will continue to exist far into the future, cometary impacts, greenhouse warming and glaciations notwithstanding  – whether we and the current suite of life are destined to be part of that future is by no means certain.

My thanks once again to all those who were kind enough to forward papers and comment to this brief series of posts on North American Pleistocene extinctions – the mystery would appear to be far from solved to the satisfaction of all, and more will doubtless be written here in the future, such as it may be.


Quantifying the Extent of North American Mammal Extinction Relative to the Pre-Anthropogenic Baseline, Carrasco et al PLoS ONE 4(12): e8331. doi:10.1371/journal.pone.0008331 Open Access

Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling, R. B. Firestone et al, Published online before print September 27, 2007, doi: 10.1073/pnas.0706977104    PNAS  October 9, 2007   vol. 104  no. 41  16016-16021  Open Access

Absence of Geochemical Evidence for an Impact Event at the Bølling Allerød/Younger Dryas Transition, by François Paquay et al, Published online before print December 10, 2009, doi: 10.1073/pnas.0908874106

An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis, Todd A. Surovell et al, Published online before print October 12, 2009, doi: 10.1073/pnas.0907857106

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