Did a Cosmic Impact Kill the Mammoths?
The Impact Hypothesis
The rise and fall of the theory that cosmic catastrophes altered human prehistory in North America.
Ever since the Alvarez (1980) hypothesis that the end-Cretaceous (Cretaceous-Tertiary or KT) mass extinction was the result of a cosmic impact sixty-five million years ago, the idea of killer asteroids or comets has been frequently discussed. The stunning confirmation of the KT impact initiated a revolution in our thinking about possible external events and their effects on biological evolution. David Raup of the University of Chicago famously proposed that perhaps all major mass extinctions were impact induced. He even published a "kill curve," suggesting that lesser extinctions might be the result of smaller impacts. Unfortunately for those of us who sought a general explanation for mass extinctions, these broader suggestions have not been verified. It seems increasingly likely that cosmic impacts are only one of several catastrophic events that have produced mass extinctions. Still, the discovery that an impact sixty-five million years ago led to the extinction of the dinosaurs remains one of the iconic ideas of late twentieth century science.
The most dramatic recent hypothesis linking extinctions with impacts was proposed in 2007 by a team of twenty-six scientists, led by nuclear chemist Richard Firestone of Lawrence Berkeley National Laboratory, with independent geophysicist Allen West; geologist James Kennett of the University of California, Santa Barbara (a member of the National Academy of Sciences); and archaeologists Douglas Kennett and Jon Erlandson of the University of Oregon. In a widely reported presentation at a joint assembly of the American Geophysical Union (AGU) in Acapulco, Mexico—followed a few months later by a paper in the Proceedings of the National Academy of Sciences (PNAS)—these scientists proposed a cosmic origin for a geologically recent event, the extinction of many large mammals (megafauna) in North America approximately 13,000 years ago. The events they linked were the presence of a dark soil layer that coincided with the extinction of megafauna (including the mammoth and mastodon), the end of the Clovis culture (identified by its large and well-made spear points), and the start of the Younger Dryas (YD) cool period (a millennium pause in the general warming at the end of the last ice age). The in situ bones of extinct megafauna, along with Clovis stone tools, occur below this black mat but not within or above it. At this boundary the team reported finding enriched levels of iridium and other signatures of extraterrestrial material.
In a sweeping conclusion reminiscent of the Alvarez hypothesis, Firestone and his colleagues postulated that these events were tied to one or more cosmic impacts over North America, releasing energy they estimated at about ten million megatons (equivalent to an impacting comet four kilometers in diameter). They suggested that an airburst and/or surface impact by a dense swarm of carbonaceous asteroids or comets set vast areas of the North American continent on fire. This swarm would have exploded above or even into the Laurentide Ice Sheet north of the Great Lakes. Such an airburst would have been a million times larger than the Tunguska impact event of 1908.
While archaeologists pondered the reality of this sharp boundary layer and the new evidence of extraterrestrial materials, a few astronomers and impact experts immediately questioned this scenario. They noted that there was no mechanism to hold such a dense swarm of impactors together in space. To the suggestion that a large comet had broken up just before hitting Earth, they replied that this lacked a physical mechanism. If the comet had shattered when it encountered the atmosphere at an altitude of about one hundred kilometers, the lateral dispersion would be at most tens of kilometers, hardly enough to distribute the effects across North America. An alternate suggestion was that this event was analogous to the 1992 tidal break-up of comet Shoemaker-Levy 9, which resulted in the separate impact of about twenty-three fragments on Jupiter two years later. However, these comet fragments were spread over more than a million kilometers in space, and the impacts were distributed over all longitudes on Jupiter. While it is true that some comets have been seen to spontaneously disintegrate in space, the chances of this happening just before an impact with Earth is negligible—something that might have happened at most once in the past four billion years. There was apparently no way to get a swarm of impactors to target North America alone.
One of Firestone and his colleagues' suggestions that troubled geologists and impact experts was that the same event (or a similar one) might have been responsible for the Carolina Bays geologic formation. The Carolina Bays are several hundred thousand shallow, elliptical depressions of disputed origin along the U.S. eastern seaboard. Firestone suggested that each of these more than 100,000 features was the result of a cosmic impact. Since the well-known Tunguska airburst in Siberia in 1908 did not form a crater, the implication is that these were made by larger objects that reached the ground. But calculation of average impact frequency suggested that only about one super-Tunguska could be expected to hit Earth in the past 13,000 years. The chances of two such extremely unlikely swarm impacts happening within the past few thousand years is worse than negligible.
A warning of the problems with this hypothesis should have been apparent to anyone who read 2006's The Cycle of Cosmic Catastrophes: Flood, Fire, and Famine in the History of Civilization by Richard Firestone and Allen West, with writer and publicist Simon Warwick-Smith. This trade book, which appeared a year before Firestone's AGU presentation, described the YD impact hypothesis as part of a much larger cycle of cosmic events. This book develops Firestone's 2001 suggestion that a cosmic ray catastrophe, probably caused by a supernova, occurred in northeastern North America in the late Pleistocene. He concluded that massive thermal neutron irradiation radically altered the radioactivity of terrestrial materials and "probably figured in the mass extinction of Ice Age fauna." In The Cycle of Cosmic Catastrophes, Firestone links the YD impact to this postulated nearby supernova, which he asserted took place 41,000 years ago and initially devastated most life in Asia. Then 34,000 years ago the shock wave from this supernova initiated another wave of intense cosmic bombardment of Earth. The only evidence for this event is the remarkable claim that mastodon tusks from about that time are pitted with cosmic dust, suggesting that these animals received the direct blast of supernova material striking Earth (unstopped, apparently, by our atmosphere).
In The Cycle of Cosmic Catastrophes, the debris cloud from the supernova
is supposed to have reached Earth about 13,000 years ago. The YD impact
was one manifestation of this blast wave, bathing the planet in radioactivity
and destabilizing the magnetic field. In this book the authors suggest
that the Carolina Bays were created by secondary impacts of ejecta from
the main hit in the North American ice sheet. To produce this much ejecta,
the hit must have been among the most catastrophic events in Earth's
history. They suggested that the YD impact excavated Hudson Bay, making
it larger than the KT impact of sixty-five million years ago, which
is estimated to be a once-in-one-hundred-million-
New Data and Continued Controversy
In January 2009, Doug Kennett published a paper in Science asserting that nanodiamonds provide the strongest evidence for the impact hypothesis, with multiple airbursts and impacts at the onset of the YD cooling. He argued that these nanodiamonds were produced in the moderate shocks associated with comet airbursts. By this time, earlier claims about iridium enrichment and other possible impact markers had been withdrawn. The usual geological evidence of large crater-forming impacts such as the KT, namely shocked quartz, had never been reported at the YD boundary sites. Now the nature and origin of nanodiamonds became the primary issue.
There were a variety of claims and counterclaims concerning the nanodiamonds. Were they produced in the impact, or were they primordial material trapped in the comet when it formed billions of years earlier? Most impact experts agree that nanodiamonds were unlikely to have been formed in the impact. In fact, Mark Boslough of Sandia National Laboratories calculated that the high temperatures and pressures in a large impact would likely destroy existing nanodiamonds. Some note that nanodiamonds are actually ubiquitous on Earth and can even be formed in fires. One scientist joked that perhaps the nanodiamonds were concentrated at human habitation sites where hunters were roasting the meat from mammoths and mastodons. The history of these claims and counterclaims is well documented in articles by Science journalist Richard Kerr published in 2007, 2008, and 2009.
At a meeting of the Geological Society of America (GSA) in October 2009, several presentations argued strongly against the YD impact from a variety of perspectives (see GSA summary in references). One paper claimed that the black mats at the YD boundary were not charcoal from widespread fires but rather peat-rich dark soils formed during a wet period. Another speaker noted that there was no archeological evidence for a sudden decline in the human population of North America at the YD. While one speaks of the end of the Clovis culture, this only means that the style of stone tools changed. We don't know why, although one possibility is a shift to hunting smaller animals. Other scientific teams reported that their efforts in the field to find nanodiamonds or other impact markers at the YD boundary layer were unsuccessful.
Some impact proponents who were not present at the GSA meeting wrote blogs and circulated e-mails accusing these scientists of sloppy fieldwork. They asserted that the boundary layer was very thin and rather spotty in distribution, requiring care to find it—care they implied had not been exercised by their critics. The GSA session resulted in the undercutting of the credibility of the original PNAS and Science papers, but since the two sides did not confront each other directly, nothing was settled.
The American Geophysical Union Symposium
Given the conflicting interpretations concerning a possible YD impact catastrophe, many scientists thought a debate between proponents and critics might help clear the air. The YD impact hypothesis had been discussed for more than two years without any common ground emerging. Indeed, the original team of twenty-six scientists was itself fragmenting, with only Richard Firestone and Allen West still strongly advocating the original multi-comet impact scenario. Mark Boslough of Sandia worked with Allen West to organize a symposium at the 2009 fall meeting of the AGU, with speakers from both sides. While no one expected that public discussion would lead to reconciliation, the organizers hoped this symposium would at least focus on the main issues.
The December 2009 AGU session topic was "Younger Dryas Boundary: Extraterrestrial Impact or Not?" Ten speakers were squeezed into a single two-hour session, including Allen West, impact specialist Peter Schultz of Brown University, and former NASA geoscientist Ted Bunch from among the original PNAS authors. Firestone chose not to attend. There was standing room only at the session, and several hundred others were turned away at the door.
The star of the event was Wally Broecker of the Department of Earth and Environmental Sciences at Columbia University. Broecker is one of the most respected environmental scientists in the world. Credited with first describing the ocean current conveyer belt and inventing the term "global warming," his honors include membership in the National Academy of Sciences and award of the Presidential Medal of Science. His presentation was sober and low key, but he made it clear that he was unconvinced by the evidence for an impact or any catastrophic change at the YD boundary. But rather than condemning the hypothesis, he stated simply that the decline in the North American megafauna could be understood as a result of climate change and overhunting—the conventional explanation. Broecker said, "We do not need the impact hypothesis."
Most of the speakers who followed Broecker restated positions that were already on the record. West and his colleagues repeated their evidence of extraterrestrial markers in the black mat at the YD boundary, with emphasis on the presence of nanodiamonds. They suggested several possible impact scenarios, such as oblique impact on the ice sheet, but admitted that there were many uncertainties. Several critics reiterated that the proposed impact is highly unlikely statistically and that an airburst as large as proposed is inconsistent with our understanding of comets and the impact process.
The most interesting new results were presented by Jacquelyn Gill, a graduate student in the Department of Geography at the University of Wisconsin–Madison. She has been studying lake sediments that contain spores of sporomiella (a fungus that occurs in herbivore dung) in the time range around the YD. This fungus is related to the total mass of herbivores and can be used as a proxy for the megafauna population. Her data show a gradual decline, beginning well before the YD marker and extending beyond the end of the Younger Dryas cool period. Indeed, in some isolated locations mammoths and mastodons did not go extinct until much later: there were dwarf wooly mammoths on Wrangle Island in Alaska until about four thousand years ago. Some large North American mammals did not go extinct at all, including the bison, the moose, and the grizzly bear. Gill's results seem consistent with the worldwide evidence that rapid declines in large mammal population accompanied the arrival of early human hunters, presumably as a consequence of overhunting.
Unfortunately, the overcrowded session ran late, and there was no time for discussion or questions. Even when their conclusions were challenged, most of the scientists in the audience chose not to respond. The result was a lost opportunity for real debate. Perhaps not surprisingly, the AGU session received very little press attention. Indeed, following the AGU and GSA meetings, the YD impact hypothesis seems to have retreated into the obscurity of a few e-mail list-serves and blogs, such as "The Cosmic Tusk" where George Howard (one of the original PNAS authors) is presiding over a variety of catastrophist interpretations of Holocene history.
It is instructive to compare the trajectories of the YD and KT impact hypotheses, as there are close parallels. Both research teams were led by nuclear scientists (Luis Alvarez and Richard Firestone) from the University of California, Berkeley. Both challenged the orthodoxy of mass extinctions. Both postulated an environmental catastrophe triggered by a large cosmic impact. Both were published initially in prestigious journals (Science and PNAS). They each presented a grand synthesis, not only identifying evidence of extraterrestrial materials at the extinction boundary but also proposing a broad impact scenario to explain a wide variety of previously unrelated data. And both ideas were initially resisted by the "old guard" of paleontologists and archaeologists.
While each hypothesis encountered initial resistance, the KT impact theory also gained enthusiastic support (see popular accounts by Walter Alvarez and James Powell). The first confirming paper was published within weeks, and soon multiple impact markers had been identified at a number of additional exposures of the KT boundary. Astronomers and geologists praised the paper and provided context by estimating the impact rate for ten-kilometer comets and asteroids. Atmospheric scientists such as Brian Toon and Kevin Zahnle of NASA Ames Research Center calculated the dispersion and lifetime of dust ejected into the stratosphere by the impact. Paleontologists like Peter Ward (University of Washington)—who initially argued for a gradual decline of populations—gathered new field data and used modern statistics to support an abrupt extinction at the KT boundary. Within three years the first of a series of Snowbird Conferences was held, bringing together top scientists to discuss the role of cosmic impacts on the evolution of life. The idea of an impact extinction gained early and continuing currency in the press.
In contrast, efforts by other scientists to confirm the presence of impact markers at the YD boundary have so far been unsuccessful. Astronomers, rather then welcoming the impact idea, have raised serious objections to the proposal by Firestone and colleagues. New data on megafauna extinction, such as the work of Gill, point to a gradual decline. Archaeologists emphasize that changing styles in stone tools do not demonstrate a sudden shift in human populations at the start of the YD but merely a change in technology or hunting style. In the aftermath of the 2009 GSA and AGU meetings, the press seems to have lost interest, and continuing support for the YD impact comes mostly from blogs by catastrophists who have long advocated cosmic intervention in human history.
Even without considering the technical issues at stake, there are two clues that something is amiss with the YD impact hypothesis. First is the 2006 book The Cycle of Cosmic Catastrophes, which formulates the YD hypothesis within the context of catastrophist pseudoscience. If more scientists and science journalists had been aware of this earlier publication when the YD hypothesis was first published in PNAS, it might never have gained traction. Second is the absence of confirming or supporting papers by scientists who were not members of the original team. A good hypothesis naturally accretes confirmation and gets better with time, as did the Alvarez KT impact hypothesis. Firestone's work has not done so.
It seems clear that the YD impact proponents were trying to follow in the footsteps of the Alvarez team, discovering evidence of a sudden extinction event and linking this to an extraterrestrial impact. However, the story isn't working out that way, and the impact they propose seems to be virtually impossible. One parallel that troubles me, however, is that the reaction of the traditionalists—scientists who say that the megafauna were in decline anyway and "we don't need an impact"—rather closely echoes the reaction of many old-guard scientists to the KT impact hypothesis. There also may be philosophical and political overtones that influence the reception given any proposal that deals with early human history. There is a long tradition of catastrophist ideas, going back to the biblical flood and Plato's story of Atlantis. Philosophically, many people prefer the idea that humans have not had much effect on the planet, either 13,000 years ago or today—better to blame thunderbolts from the gods than to accept responsibility for our stewardship of Earth.
I am grateful to Mark Boslough, Clark Chapman, and Alan Harris for many stimulating discussions of the YD impact hypothesis and especially for their insightful and generous suggestions for improving this paper.
Alvarez, L.W., W. Alvarez, F. Asaro, and H.V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208: 1095.
Alvarez, W. 1997. T. Rex and the Crater of Doom. Princeton University Press.
Firestone, R.B., and W. Topping. 2001. Terrestrial evidence of a nuclear catastrophe in paleoindian times. Mammoth Trumpet Magazine (March): 9, published by the Center for the Study of the First Americans.
Firestone, R., A. West, and S. Warwick-Smith. 2006. The Cycle of Cosmic Catastrophes: Flood, Fire, and Famine in the History of Civilization. Bear and Company, Rochester, Vermont.
Firestone, R.B., et al. 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences 104: 1616.
Geological Society of America (GSA) Annual Meeting (October 18–21, 2009), Portland, Oregon. Relevant oral presentations, with quotes from their abstracts. Paquay et al.: No evidence of extraterrestrial geochemical components at the Bølling-Allerød/Younger Dryas transition. ("Our study discredits the YD impact hypothesis.") Surovelle and Holliday: Non-reproducibility of Younger Dryas extraterrestrial impact results. ("We were unable to reproduce any results of the original Firestone et al. study and find no support for Younger Dryas extraterrestrial impact.") Pinter et al.: Extraterrestrial and terrestrial signatures at the onset of the Younger Dryas. ("Many of the purportedly unique markers at the YD boundary layer were found in most or all other sites and horizons analyzed, often at concentrations much higher than at the YD layer itself.") Holliday and Meltzer: Geoarchaeology of the 12.9 ka impact hypothesis. ("Sites purported to provide direct evidence of the 12.9 ka impact are not well constrained to that time. An ET impact is an unnecessary ‘solution' for an archaeological problem that does not exist.")
Gill, J., J.W. Williams, S.T. Jackson, K.B. Lininger, and G.S. Robinson. 2009. Pleistocene megafaunal collapse, novel plant communities, and enlarged fire regimes in North America. Science 326: 1100.
Kennett, D.J., et al. 2009. Nanodiamonds in the Younger Dryas sediment layer. Science 323: 94.
Kerr, R. 2007. Mammoth-killer impact gets mixed reception from Earth scientists. Science 316: 1264.
———. 2008. Experts find no evidence for a mammoth-killer impact. Science 319: 1331.
———. 2009. Did the mammoth slayer leave a diamond calling card? Science 323: 326.
Powell, J. 1998. Night Comes to the Cretaceous: Dinosaur Extinction and the Transformation of Modern Geology. Freeman.
Plait, P. 2008. Death from the Skies: These Are the Ways the World Will End. Viking Press.
Raup, D.M. 1991. Extinction: Bad Genes or Bad Luck? W.W. Norton.
Signor, P.W., and J.H. Lipps. 1982. Sampling bias, gradual extinction patterns, and catastrophes in the fossil record. In: Geological Implications of Impacts of Large Asteroids and Comets on the Earth, L.T. Silver and P.H. Schultz, editors. Geological Society of America Special Publication 190: 291.
Toon, O.B., K. Zahnle, D. Morrison, R. Turco, and C. Covey. 1997. Environmental perturbations caused by the impacts of asteroids and comets. Reviews of Geophysics 35: 41.
Ward, P.D., W.J. Kennedy, K.G. MacLeod, and J.F. Mount. 1991. Ammonite and inoceramid bivalve extinction patterns in Cretaceous/Tertiary boundary sections of the Biscay region (southwestern France, northern Spain). Geology 19: 1181.