Megabeasts' Sudden Death

The Extinction Debate
by Evan Hadingham

From its first public announcement in May 2007, the hypothesis that a comet devastated North America 12,900 years ago has aroused intense skepticism and debate among scientists. The debate revolves around several key questions:

  • Is there solid, unambiguous evidence for an extraterrestrial impact or airburst at this time?

  • Did the impact cause the dramatic extinction of some 35 types of large animals, or megafauna, in North America, also dated roughly to this period?

  • Or should we blame these extinctions on the Clovis people, the first well-documented prehistoric hunters in North America, who had entered the continent only two or three centuries before?

  • If the Clovis hunters were not to blame, could a sudden, sharp "deep freeze"—an abrupt reversal of the climate back to Ice Age conditions—have been the culprit?

  • Finally, could this sudden chill somehow be connected to the claimed impact?

To dig deeper into the clashing arguments, follow the links highlighted below, all of which connect to sources freely available on the Web and do not require special academic access. We also provide full references to each journal article or book.


The Cosmic-Impact Hypothesis

The impact hypothesis was first publicly presented at the Spring 2007 joint assembly of the American Geophysical Union in Acapulco, Mexico, by its leading proponents: James Kennett of the University of California, Santa Barbara, Richard Firestone of Lawrence Berkeley National Laboratory, geological consultant Allen West, and others:

New insights into extraterrestrial impacts, Younger Dryas cooling, mass extinction, and the Clovis people I

Video of the press conference related to this session, which was shot in Acapulco on May 23, 2007, can be viewed on YouTube:

Younger Dryas (YD) impact AGU press conference

Rex Dalton, a correspondent for the journal Nature, summarized the immediate response to the AGU session:

Blast in the past? [PDF]

A more critical report appeared shortly afterwards in the journal Science:

Mammoth-killer impact gets mixed reception from Earth scientists [PDF]

The impact team's main arguments, based on nearly a dozen different types of evidence at 26 sites from the U.S. West Coast to Belgium, were first published in October 2007 in the Proceedings of the National Academy of Sciences (PNAS):

Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling

A strongly critical report appeared in March 2008 in Science, which cited inconsistencies in many of the team's claimed impact markers, including a failure to replicate finds of elevated iridium, and the argument that the alleged "E.T." materials could have been deposited gradually by meteoritic dust "fallout" from the upper atmosphere rather than by a sudden impact:

Experts find no evidence for a mammoth-killer impact [PDF]

Later in 2008, an interesting exchange of views appeared in the online journal GSA Today, beginning with a highly critical commentary by Nicholas Pinter and Scott Ishman:

Impacts, mega-tsunami, and other extraordinary claims

In response to Pinter and Ishman, the impact hypothesis team posted two comments:

Comment by Firestone and West

Comment by Bunch, Kennett, and Kennett

Since these initial criticisms, the impact hypothesis team has focused mainly on a single type of evidence, microscopic nanodiamonds, which it claims to be a clear signature of a catastrophic event in the atmosphere. The nanodiamond arguments were reported in Science:

Nanodiamonds in the Younger Dryas boundary sediment layer

This Science article was accompanied by another commentary by Richard Kerr:

Did the mammoth slayer leave a diamond calling card?

A press release summarized this Science paper:

Six North American sites hold 12,900-year-old nanodiamond-rich soil

NOVA's program "Megabeasts' Sudden Death" reports on the team's latest round of research on the Greenland ice sheet, carried out mainly by Paul Mayewski of the University of Maine. As shown on the program, the initial results of this fieldwork indicate strikingly high levels of impact-shocked diamonds in the relevant layer of the Greenland ice sheet. This latest work is currently awaiting publication as follows:

Nanodiamonds discovered in the Greenland ice sheet within the Younger Dryas boundary layer

The Megafauna Extinction Debate

For more than four decades, scientists have debated why so many types of large animals, or megafauna, were driven into extinction at the end of the last ice age. For an authoritative and up-to-date introduction to the arguments, see the following essay by archeologist Gary Haynes, who favors the "overkill" theory that the impact of prehistoric human hunting was the crucial factor:

Introduction to the volume In American Megafaunal Extinctions at the End of the Pleistocene

Anthropologist Donald Grayson argues a contrasting viewpoint—that climate change was decisive. His survey compares the evidence from North America with the very different patterns of extinction in Europe and Asia:

Deciphering North American Pleistocene extinctions [PDF]

A spirited debate between Haynes and Grayson and their colleagues can be followed in these three papers:

A requiem for North American overkill [PDF]

A premature burial: comments on Grayson and Meltzer's "Requiem for overkill" [PDF]

North American overkill continued? [PDF]

Countering both the climate and human "overkill" theories, Ross MacPhee, a curator at the American Museum of Natural History, argues that the megafauna could have been wiped out by an infectious "hyperdisease," perhaps spread by contact with colonizing human populations:

What killed the mammoths and other behemoths that once roamed the Americas? This mammalogist thinks it may have been hyperlethal disease

Origins of hyperdisease hypothesis

Taking a broader perspective on the controversy, biologist Anthony Barnosky relates the megafauna extinction debate to the present-day context of global warming and worldwide extinctions:

Megafauna biomass tradeoff as a driver of Quaternary and future extinctions

Among many books covering the megafauna debate, see the following:

Heatstroke: Nature in an Age of Global Warming
by Anthony D. Barnosky
(Chapter 5 specifically)

Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America
by Paul S. Martin

American Megafaunal Extinctions at the End of the Pleistocene
by Gary Haynes, editor

The Early Settlement of North America: The Clovis Era
by Gary Haynes

The Younger Dryas Mystery: What Caused the Climate to Flip?

From around 16,000 years ago, the world began to emerge from the deep freeze of the last ice age. As the global climate warmed up, it suddenly flipped back to glacial conditions for a thousand years or so before the present age of relatively warm conditions finally took hold. The cause of that mysterious flip has been the focus of intense debate ever since ice-core research in Greenland revealed that the cold interval (named the Younger Dryas after an alpine wildflower, Dryas octopetala) both started and ended in a decade or less.

More recent work indicates that the time frame may have been even more abrupt, flipping in and out of the deep freeze in a mere one to three years:

Did you say fast? [PDF]

What could cause such a sudden flip? Columbia University Earth scientist Wally Broecker pioneered the leading contender among Younger Dryas theories during the 1980s. His theory involves a disruption or shutdown of the great Atlantic Ocean conveyor, the northeast-flowing current that brings warm water from the tropics up to the northern latitudes. Broecker proposes that at the end of the Ice Age, Lake Agassiz, a lake over 700 miles wide fed by the melting glaciers and covering much of present-day North Dakota, Minnesota, Manitoba, Ontario, and Saskatchewan, suddenly drained into the St. Lawrence valley in what is now eastern Canada. A huge pulse of freshwater would have interfered with the Atlantic circulation, plunging the Northern Hemisphere into icy conditions. Broecker summarizes the theory in the following paper:

What if the conveyor were to shut down? Reflections on a possible outcome of the great global experiment

A popular account of the theory and its implications appeared in The Atlantic Monthly:

The great climate flip-flop

Although a shutdown of the Atlantic conveyor remains a widely accepted explanation, hard evidence for a flood from Lake Agassiz remains elusive, as Broecker has recently acknowledged:

Was the Younger Dryas triggered by a flood? [PDF]

In North America, the onset of the Younger Dryas is marked by a "black mat" layer at more than 50 prehistoric sites. The black mat is thought to consist mostly of dried algae forming in stagnant pools as rainfall and the water table rose with the onset of the Younger Dryas. Recently, archeologist C. Vance Haynes, Jr. summarized his many years of investigating these black mat sites:

Younger Dryas "black mats" and the Rancholabrean termination in North America [PDF]

According to the scientists who are proposing the comet-impact hypothesis, a thin layer at the base of the black mat also contains charcoal, nanodiamonds, and other materials claimed to be the signature of a cosmic explosion. The scientists have suggested that the comet or its fragments may have hit the great ice sheet covering Canada and destabilized it, or perhaps launched the Lake Agassiz flood, either of which could have led to the shutdown of the Atlantic conveyor and plunged the climate into the deep freeze of the Younger Dryas. However, until an impact crater or other evidence is found, this aspect of the impact hypothesis remains only a speculation. The idea is reviewed, along with more general background about the Younger Dryas, in the blog at

Younger Dry-as dust?

The Younger Dryas comet-impact hypothesis: gem of an idea or fool's gold?

Finally, today's concerns about global warming are fueling an intense scientific quest to understand the causes of the Younger Dryas and many other similar abrupt climate shifts in earlier periods. The evidence of these ancient climate shifts poses a disturbing question: Will today's steady build-up of carbon dioxide emissions lead to a predictable, equally steady gradual rise in global temperatures? Or could the climate system suddenly "flip" to a disastrously warm state?

Among many popular books examining this question and reporting on the evidence for rapid climate change, the following are particularly useful:

The Ice Chronicles: The Quest to Understand Global Climate Change
by Paul A. Mayewski and Frank White

The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future
by Richard B. Alley

The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate
by David Archer

Thin Ice: Unlocking the Secrets of Climate in the World's Highest Mountains
by Mark Bowen


See a list of references for all journal articles and books mentioned in this article.

To join the debate yourself, see our discussion board.

Evan Hadingham is NOVA's senior science editor. For a general article about the extinction debate on this "Megabeasts' Sudden Death" website, see End of the Big Beasts.

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