The Younger Dryas impact hypothesis

The cause(s) that led to the extinction of most of the larger mammals that roamed the Pleistocene world are still today unknown. Many different hypotheses were proposed, from human overkill to climate change, but more unusual was an idea largely publicized in 2007.

Geophysical studies presented in spring 2007 suggested that perhaps an extraterrestrial bolide vaporized in the Earth's atmosphere caused the extinction of the North American Megafauna some 13.000 years ago.
In 2009 this hypothesis seemed to be further confirmed by subsequent findings. This and other interdisciplinary research presented various sedimentological features found in peat layers at nine excavated sites of the North American continent and one site in Belgium and thought to be associated with impacts of meteorites on earth.

Fig.1. Profiles of sites in North America with proposed sedimentological evidences for an impact event some 12.900 years ago. The diagrams show the concentration of magnetic particles, microspheres, particles of coal and glass and some elements rare in the earth's crust but common in meteorites (such as iridium, chromium and nickel). The highest values are apparently found in a distinct single thin layer, after FIRESTONE et al. 2007.

1) An increased concentration of iridium, a rare element in earth's crust but common in meteorites.

2) Metallic particles and grains, also carbon microspheres concentrated in a thin layer, interpreted as remains of the impacted meteorite and the recrystallized molten rocks of the bolide and crust.

Fig.2. and 3. The metallic micrometeorites with a diameter between 100 to 150 µm found in one of the profiles (Blackwater Draw) showed a high content of titanium and nickel typical for extraterrestrial material (FIRESTONE et al. 2007), also shown the carbon microspheres interpretated as the remains of the molten and recrystallized bolide.

3) A particular and rare modification of carbon - Lonsdaleite - in shape of microscopic nanodiamonds with a hexagonal crystalline structure formed only under very high pressure as experienced during an impact.

4) Another exotic modification of carbon, the Buckminsterfullerene or buckyballs, a modification of carbon that supposedly can be created only under great heat conditions.

5) Dark layers of peat or sediments rich in organic matter were interpreted as the remains of burned vegetation by megafires ignited by the heat of the impact.

6) Recovered Pleistocene bones of mammoth and bison showed features that were interpreted as direct effects of the explosion - small, 2 to 3 mm in diameter, holes in the bones with a burned halo and penetrated magnetic particles with a high content of iron and nickel of unusual isotopic composition.

To explain the lack of the most compelling evidence - the impact crater- it was suggested that the bolide exploded above or on the Laurentide ice shield, leaving behind no visible trace.
7) An alternative suggestion positioned a debris field in the Carolina Bay area along the south-eastern coast of the United States. The Carolina Bays include thousands of circular to elliptical depressions across the coastal plain of still unknown origin (some authors suggested even spawning fish).

The most intriguing conclusion of the Younger Dryas impact hypothesis: The heat released and the shock waves of the impact caused the extinction of the North American Megafauna and the annihilation of the Clovis-culture possibly in two ways, by directly killing animals and igniting large scale firestorms and in a second moment by the partial melting of the ice shield of North America and Greenland. The large amounts of fresh water released in the Atlantic Ocean caused an arrestment or a slowdown of the warm Gulf Stream, starting a 1.500 years long climatic reversal recognized especially in Europe as Younger Dryas stadial (12.900-11.600 cal BP), an important phase of cooling recognized in glacier advances and vegetation shift.

Fig.4. The isotopic values as proxy of climate recovered in ice cores from Greenland show a distinct phase of climatic reversal between 12.900 and 11.600 years ago. The sudden beginning of this period named Younger Dryas stadial is still poorly understand, the influence of changing patterns of the marine currents in the Atlantic at the end of the Pleistocene are the most suggested and likely triggers of such an abrupt climate change (Greenland Ice Core Chronology 2005 released 10. March 2006).

The hypothesis experienced a positive and large attention in the popular media but got mixed reception from earth scientists during it´s official presentation in May 2007 at the congress of the American Geophysical Union.
The meteorite idea was however not completely new and already published in 2001 and in 2006 even in a own book - however with some pseudoscientific implications like glacial landscapes with drumlins or the North American Great Lakes as the direct results of the impact.

Also focusing only on the research published in 2007 soon problems arouse. For example the methods used to identify some of the most compelling impact evidence, like the nanodiamonds, were questioned because the results of the analyses were explainable also by other, more earthly, materials.
More important - the impact hypothesis could only explain a local decline and extinction event for the American continent in a very short interval, maybe in few decades or centuries. However dated fossils seem not to support a unique and sudden extinction as proposed by this and many other catastrophist hypotheses. In a survey on 4.532 archaeological sites in Europe and Siberia and 1.177 dated remains of mammoth and mastodons in Europe, Siberia and North America the dates scatter between 45.000 to 12.000 years.
Estimating the development of the population of single species there seems to be various phases of increase and decline in numbers of individuals. The woolly mammoth for example reaches a population maximum some 16.000 to 15.500 years ago, this phase is followed by a slow decline 14.500 to 13.500 years ago, however isolated populations survived on islands and in northern regions of Asia even until historic times.

Apart of these general critics in 2011 a paper By PINTER et al. focused explicit on the single evidences as presented in 2007 and subsequent years and concluded that most of the claims can not be reproduced and the few reproduced evidences are not unequivocally related to an impact of an extraterrestrial bolide:

1) The iridium concentration was not measured in the bulk sediment but on single fragments or spherules thought to be of impact origin - this could falsify the apparent peak in the stratigraphic column. On some studied sites the concentration of iridium in the supposed Dryas interval was also surprisingly low. Despite the methodological error, these contradicting results are imputable to diagenetic alteration of the sediments and the iridium concentration is more likely of terrestrial origin.
More notable subsequent research failed to reproduce the single iridium peak.

2) Some of the carbon spherules resulted to be fungal spores or coprolites of arthropods. Subsequent research could not reproduce a peak or concentration of micrometeorites in a single layer, but the particles resulted to be distributed homogenous in the stratigraphic column, as more likely explained by the common background sedimentation from the interplanetary space occurring during geological times.

3) The supposed nanodiamonds resulted by further and more detailed investigations to be amorphous to polycrystalline carbon aggregates as produced during common wildfires, the presence of the particular carbon modification Lonsdaleite could not unequivocally proven in the sediments.

4) The presence of buckyballs was questioned because of methodological problems already in the original research and later investigations could not reproduce the results. Despite the dubious presence of the fullerenes, it is known that small amounts of this carbon modification can be produced by common wildfires, so even if buckyballs will be found, these are not unequivocally evidence of an impact.

5) Sediments rich of organic matter are not necessary produced by wildfires; common depositional environments like swamps can also produce thick layers with encoaled plant remains. Some proposed impact-related dark layers, supposedly rich of organic matter, resulted even to be coloured not by organic remains but by minerals. There is today no unequivocally evidence that the layers are connected to any wildfires after an impact.

6) Some of the bones with the supposed fragments of the bolide resulted to be older by nearly 20.000 years than the previously specified impact date. The fragments in the bones were not reanalyzed after the first claims and doubts arouse of the proposed origin.
It seems unlikely that such minuscule and fragile particles could penetrate earth's atmosphere and still impact on such a hard material as are the bones. In alternative it is well possible that the discovered particles are more likely diagenetic iron concentrations.

7) The explanation of the Carolina Bays as debris field is not supported by any discovery of extraterrestrial material in the area; also relative dating efforts showed that these depressions were formed probably during a long time interval. So if these features still remain mysterious an impact origin seems the most unlikely cause of origin.
Even radiocarbon ages achieved by the impact supporters showed significant fluctuations in the ages of formation, ranging from 6.500 to 700 years ago. This lead to the excuse that "the impacting object was ejected by a recent near-Earth supernova in which case carbon [was] enriched" modifying the radiocarbon age of the sediments.
Realizing the improbability and problems of such claims the Carolina Bays were rejected as evidence by most impact proponents.

The Younger Dryas Impact Hypothesis was adopted mainly by non earth-sciences related researchers and especially the mass media, who dedicated to the scenario even various TV-shows. Even if it was stated that some of the results were preliminary, it is still surprising how catastrophic theories are accepted uncritically by popular media.
However three years later it seems that most of the proposed evidence for the Younger Dryas Impact Hypothesis could not be reproduced by other teams and were evidence is available there are terrestrial, non impact related, interpretations possible.

Bibliography:

BECKER (2007): Abstract: The End Pleistocene Extinction Event - What Caused It? Eos Trans. AGU, Abstract PP41A-03
BECKER (2007): Ice Age Impact. mp3 (4MB). (Interview by the Canadian Broadcast)
FIRESTONE, R.B.; WEST, A.; KENNETT, J.P.; BECKER, L.; BUNCH, T.E. 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 of the United States of America 104(41): 16016-16021
KENNETT, D.J.; KENNETT, J.P.; WEST, A.; WEST, G.J.; BUNCH, T.E. et al . (2009): Shock-synthesized hexagonal diamonds in Younger Dryas boundary sediments. Proceedings of the National Academy of Sciences of the United States of America 106: 12623-12638
KERR, R.A. (2007): Mammoth-Killer Impact Gets Mixed Reception From Earth Scientists. Science 316: 1264-1265
KERR, R.A. (2008): PLANETARY IMPACTS: Did the Mammoth Slayer Leave a Diamond Calling Card? Science Vol.323 : 26
LEVY, S. (2006): Clashing with Titans. BioScience Vol. 56(4) : 292-298
PINTER, N.; SCOTT, A.C.; DAULTON, T.L.; PODOLL, A.; KOEBERL, C.; ANDERSON, R.S.; ISHMAN, S.E. (2011): The Younger Dryas impact hypothesis: A requiem. Earth Science Reviews. Article in Press
UGAN, A. & BYERS, D. (2007): Geographic and temporal trends in proboscidean and human radiocarbon histories during the late Pleistocene. Quaternary Science Reviews.26: 3017-3440

Online Resources:

MORRISON, D. (2010): Did a Cosmic Impact Kill the Mammoths? (Accessed on 23.04.2011)