Mass extinction is box office, a darling of the popular press, the subject of cover stories and television documentaries, many books, even a rock song…At the end of 1989, the Associated Press designated mass extinction as one of the “Top 10 Scientific Advances of the Decade.” Everybody has weighed in, from the economist to National Geographic.
For every problem, there is a solution that is simple, neat, and wrong.
The great tragedy of Science—the slaying of a beautiful hypothesis by an ugly fact.
I vividly remember running into my good friend, Jim Kennett (now retired from the University of California, Santa Barbara), at the 2007 meeting of the Geological Society of America. Kennett is still one of the giants and pioneers of the fields of marine geology and paleoceanography and climate change, with a career that goes back to the early 1970s when the Deep Sea Drilling Project began to revolutionize our understanding of oceans and climate. As a co-author on the paper, Jim was excited about this hot new idea that an impact had struck about 12,900 years ago and was responsible for the extinction of the Ice Age mega-mammals—one of the most interesting and controversial events in earth history. I tried to sound enthusiastic, but I’d seen many versions of the impact hypotheses for other mass extinctions crash and burn, so I didn’t want to pronounce judgment yet.
This idea is the most recent entry in the scientific bandwagon that impacts caused all mass extinctions. Firestone et al. (2007) claimed that the extinction of the Ice Age “megamammals” (large mammals over 40 kg in weight) was due to the impact of an extraterrestrial object about 12,900 years ago. Naturally, when this idea was first proposed, the media had a field day, and almost no dissenters or critics were heard at all. Some geology textbooks even inserted this untested idea into their new editions without waiting to see if it would pan out or not. And just like every other half-baked idea from the impact advocates, the “late Pleistocene impact” scenario has been shot down by a whole range of observations.
The late Pleistocene impact hypothesis was born from observations that there was a distinctive “black mat” organic layer in several localities across the southwestern U.S., immediately above the last appearance of Ice Age megamammal fossils. These include not only the huge mammoths and mastodonts, but also ground sloths, horses, camels, two genera of peccaries, giant beavers, plus predators such as short-faced bears, dire wolves, and sabertoothed cats—but not bison, deer, pronghorns, and a number of other large mammals still found in North America today. The “black mat” is also above the first known artifacts of the Clovis culture, which were thought to be the first human arrivals from Eurasia, and allegedly responsible for overhunting the megamammals to extinction. Firestone et al. (2007) also claimed to have found “nanodiamonds”, iridium, helium-3, “buckyballs,” and a number of other geochemical and mineralogical “impact indicators” in the “black mat” layer, and then painted a variety of different (and conflicting) scenarios about the impacting object (they are not consistent as to whether it is a comet or an asteroid) hitting near the Carolina Bays region. This supposedly affected the Laurentide ice sheet in the northeastern part of North America and triggered the Younger Dryas cooling event at 12,900 years ago.
The entire scenario has been completely demolished by a number of lines of evidence. As Pinter and Ishman (2008) showed, there is no evidence that there was an impact in the Carolina Bays, and most of the alleged “impact evidence” is questionable when analyzed by other labs. Firestone et al. (2007) argued that the impact was an airburst, since there is no crater, no tektites, no shocked quartz or other high-pressure minerals, which are the best indicators of a true impact. Most of the material that was allegedly impact derived (nanodiamonds, iridium, helium-3, “buckyballs”, and so on) is also consistent with the normal rain of micrometeorites, and not abundant enough to be good evidence of an impact.
The claim that the “black mat” was an impact layer has also been debunked. It is more likely an indicator of a high water table and wetter conditions associated with the abrupt Younger Dryas cooling event (Haynes, 2008). The supposed “instantaneous” extinction of megamammals at this horizon has also been debunked, since the extinctions were scattered across a wide geographic area with different genera going out locally at different times (Grayson and Meltzer, 2003; Fiedel, 2008; Scott, 2010). Mammoths, mastodons, giant deer (“Irish elk”), ground sloths, and many other megamammals did not die out at 12,900 years ago, but survived in most cases to 10,000 to 11,000 years ago. This is fatal to the idea that a single impact killed them all off. In fact, none of the well-dated extinctions occur at 12,900 years ago. Most of the extinctions are either significantly younger than that interval, or there are no good final dates for their last appearance—but very little appears to happen to the megamammals at precisely 12,900 years ago.
Particularly striking is the persistence of mammoths and ground sloths well into the Holocene (as young as only 6000 years ago), and of course, the bison, deer, grizzly bear, cougars, peccaries, and pronghorns that are still with us, while elk and moose came to North America at this time (rather than being wiped out). In fact, studies of DNA trapped in soils from the Canadian Arctic shows that many of these “extinct” Ice Age mammals persisted well into the Holocene, even though there are no bones preserved in beds that young. The impact hypothesis does nothing to explain the selectivity of this extinction. In addition, the South American, Australian, and Eurasian-African megafaunal extinctions are not synchronous with the alleged “impact,” so it does nothing to explain their demise.
The claim that the “impact” had a severe effect on human cultures has been completely shot down as well (Buchanan et al., 2008), since there is no evidence whatsoever that human cultures changed dramatically at this time, or that there was a major population decline. Clovis culture was gradually transformed into Folsom, Dalton, and Eastern U.S. Paleoindian cultures, and they apparently spread widely at this time, rather than declining. And just before my 2011 Catastrophes! book came out, Jacquelyn Gill of the University of Wisconsin presented a paper at the Ecological Society of America meeting analyzing the details of lake sediments from the northeast, which preserve a high-fidelity record of that time. She found no evidence of the impact debris that was supposed to be common—and her data were gathered even closer to the alleged impact site than the evidence garnered from the western U.S. Nor was there any great shift in vegetation, pollen, spores, or any other biotic signal that would be consistent with the impact hypothesis.
Finally, if the authors of the Pleistocene impact scenario had paid any attention to the past decade of research on impacts and extinctions, they would have realized that the “impacts cause extinction” notion is passé. As I discussed in Chapter 11 of my new book Catastrophes!, none of the great extinctions of past (except possibly the end-Cretaceous event) are associated with impacts. It feels like the Firestone et al. (2007) impact scenario is a bad rehash of the debates from the 1980s. Apparently, the authors are still stuck on a bandwagon that has long since ground to a halt—except in the popular media. As Barnosky et al. (2004) showed, the causes of the late Pleistocene megafaunal extinctions are complicated, and probably involve a combination of both human overhunting and climatic change. One thing that doesn’t seem to be relevant is an impact.
Like many other trendy ideas in science, it made a big splash when it first came out in 2007, and some textbooks even jumped the gun and featured it in their new editions. But eventually the scientific review process works through all the hot ideas that have made it past the first level of peer reviews. After 2-3 years, the majority of these faddish proposals die a quiet death as they are debunked, one claim after another. Yet the public and press only remember the splashy coverage when the idea was first proposed, and don’t realize that it has been quietly discredited in the scientific community.
- Barnosky, A.D., P.L. Koch, R.S. Feranec, S.L. Wing, and A.B. Shabel. 2004. Assessing the causes of late Pleistocene extinctions on the continents. Science 306: 70–75.
- Buchanan, B., M. Collard, and K. Edinborough. 2008. Paleoindian demography and the extraterrestrial impact hypothesis. Proceedings of the National Academy of Sciences 105:11651–11654.
- Fiedel, S. 2009. Sudden deaths: the chronology of terminal Pleistocene megafaunal extinction, in Haynes, G. (Ed.), American Megafaunal Extinctions at the End of the Pleistocene. New York: Springer, pp. 21–38.
- Firestone, R.B., and 25 others. 2007. Evidence for an extraterrestrial impact 12,000 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences 104, 16016-16021.
- Grayson, D.K., and Meltzer, D.J. 2003. A requiem for North American overkill. Journal of Archeological Science 30:585–593.
- Haynes, G. 2009. Estimates of Clovis-era megafaunal populations and their extinction risk, in Haynes, G. (Ed.), American Megafaunal Extinctions at the End of the Pleistocene. New York: Springer, pp. 39–54.
- Pinter, N., and Ishman, S.E. (2008) Impacts, mega-tsunami, and other extraordinary claims. GSA Today, 18(1):37–38.
- Scott, E. 2010. Extinctions, scenarios, and assumptions: changes in latest Pleistocene herbivore abundance and distribution in western North America. Quaternary International 217:225–239.