The causes that led to the extinction of most of the larger mammals - also referred as Megafauna- that roamed the Pleistocene world are still highly controversial and fiercely discussed. 35 mammals and 19 bird genera became extinct during the transition from the Pleistocene to the Holocene 10.000 years ago, in the last decades many different hypotheses were proposed to explain this event, from human overkill to climate change, more unusual was an idea largely publicized by mass media in 2007.
Geophysical studies presented in spring 2007 suggested that perhaps an extraterrestrial meteorite vaporized in the Earth's atmosphere caused the extinction of the North American Megafauna and associated human cultures some 13.000 years ago. This idea was not all new, already in 1990 archaeologist William Topping found in Michigan spherules in glacial sediments - supposedly of extraterrestrial origin, even if he and nuclear physicist Richard Firestone suggested a nearby supernova as origin of these features. Based on this discovery Firestone later developed as alternative explanation the meteorite hypothesis, according to him the hypothesis could not only explain the extinction of the large mammals, but also the observed fluctuation of the Younger Dryas at the end of the Pleistocene and the beginning of the Holocene. The Younger Dryas is a 1.500 years (12.900-11.600 cal.B.P.) long cooling event, recognized especially in Europe by the advance of glaciers and a vegetation shift. The Younger Dryas is a "freak" event - unique of the termination of the last glacial - similar events are not recorded in the ice cores of Antarctica for the previous glacial phases.
Fig.1. 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 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 March 2006).
In 2009 the "Younger Dryas Impact Hypothesis" seemed to be further supported by subsequent discoveries - interdisciplinary papers by various authors presented various sedimentological features found in peat layers from more than 50 sites of the North American continent and one site in Belgium associated supposedly to an impact of a meteorite on earth.
1 ) A distinct layer of carbon-rich clays and silts and "black mats" (a diatomaceous earth deposited in a lacustrine environment) found on more than 50 sites across North America, dated samples in 9 sites provided an age of approximately 13.000 yrs B.P. In one case this layer was overlying artefacts of the Clovis-culture and fossil remains of a mammoth. The authors suggested that this layer was deposited very fast as results of burned vegetation and that these sediments covered and preserved the underlying fossils.
2 ) The presence of singular forms of soot on two of the investigated sites, similar to particles found in sediments of the Cretaceous-Palaeogene transition (associated to notoriously known Chicxulub-Impact).
3) An increased content of minerals like allanite or monzanite, rich in elements like Uranium and Thorium, associated to an increase in radioactivity of the layers and fossils underlying the carbon-rich layer, minerals presumably of extraterrestrial origin.
4 ) An increased concentration of nickel, and especially iridium in the carbon-rich layer, a rare element in the earth's crust, but common in meteorites. A peak could indicate a single great impact that provided enough of this metal to become deposited in the sediments.
5 ) Metallic and carbon microspherules concentrated in a thin layer, (figure from FIRESTONE et al. 2007) interpreted as remains of the impacted meteorite and the recrystallized molten rocks of the meteorite and crust.
6 ) A particular and rare modification of carbon with a hexagonal crystalline structure - Lonsdaleite - and nanodiamonds, both minerals can supposedly form only under very high pressure as experienced during an impact.
7 ) Another exotic modification of carbon, the Buckminsterfullerene or "buckyballs", a modification of carbon that supposedly can be created only with great heat as experienced during an impact.
8 ) Some 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 magnetic particles with a high content of iron and nickel of unusual isotopic composition penetrated in the bones.
9 ) To explain the lack of the most compelling evidence - the impact crater- it was suggested that the meteorite exploded above or on the Laurentide ice sheet, leaving behind no visible trace.
10 ) One suggestion positioned a debris field of fragments of the meteorite 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 Laurentide ice sheet, at the time still covering large parts of the North American continent. The large amounts of fresh water released in the Atlantic Ocean caused an arrestment or a slowdown of the warm Gulf Stream, plunging the slowly warming earth back in a 1.500 years long cooling event.
The hypothesis experienced a positive and large attention in the popular media but got mixed reception from earth scientists during its official presentation in May 2007 at the congress of the American Geophysical Union.
The at the conference suggested particular scenario was not completely new and already published in 2001 and in 2006 even in a own book by Firestone - however with some disputable implications like glacial landscapes of North America and the 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 published by UGAN & BYERS in 2007 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 results in ages scatter between 45.000 to 12.000 years. The authors estimated the development of the population of single species and concluded that there were phases of increase and decline in numbers of individuals over a long time interval. 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 meteorite:
1 ) Sediments rich of organic matter are not necessary products of a wildfire; common depositional environments like swamps can also produce thick layers with encoaled plant remains. HAYNES 2007 examining the "black mats" suggests also that the layer was caused by a change of depositional environment as result of a climate change from wet to dry conditions.
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 none unequivocally evidence that the layers are connected to any wildfires or to an impact.
2 ) Soot was reported only on two sites and as evidence such rare findings is not to compelling; also soot is not related exclusively to an firestorm, but simply to wildfires, which are not related exclusively to an impact.
3 ) Peaks in radioactivity or presence of radioactive minerals are not known from other impacts or supposed impact deposits, there is also no physical explanation why radioactivity should be increase after an impact, however diagenetic processes can concentrate particular minerals in single layer - the presumed evidence therefore doesn't support the impact claim.
4 ) 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, as the fragments could become distributed inhomogeneous by sedimentological processes. Apart the possible methodological error, there are also contradicting results - on some studied sites the concentration of iridium in the supposed Dryas interval was surprisingly low - this could indicate that the apparent concentration anomalies are imputable to diagenetic alteration of the sediments and the iridium concentration is more likely of terrestrial origin. However the greatest problem is that subsequent research failed to reproduce the single iridium peak as proposed by the supporters of the Younger Dryas Impact.
5 ) Some of the extraterrestrial carbon spherules resulted to be fungal spores or coprolites of arthropods. Subsequent research could not reproduce a peak or concentration of spherules 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.
6 ) 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 studied sediments.
7 ) 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.
8 ) 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 also 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.
10 ) The explanation of the Carolina Bays as debris field is not supported by any presence 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 this formation, ranging from 6.500 to 700 years ago. The authors suggested 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 however mostly rejected as evidence even by most impact supporters.
The Younger Dryas Impact Hypothesis was adopted mainly by non earth-sciences related researchers and especially the mass media, who dedicated to the scenario various TV-shows - even if it was stated that some of the results were preliminary. The hypothesis is still under scrutiny by the geological community - as for example during the actual edition of the INQUA meeting in Bern. It seems however that most of the proposed evidence for the Younger Dryas Impact Hypothesis so far could not be reproduced by other teams and were evidence is available there are terrestrial, non impact related, interpretations possible.
Apart the sensationalist presentation of the scenario in 2007 by the media it is important to remember that every new hypothesis has to be discussed and the evidence tested by other scientists - this phase is maybe not so spectacular for the general public, but not less important.
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)
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HAYNES, C.V. (2007): Younger Dryas ''black mats'' and the Rancholabrean termination in North America. Proceedings of the National Academy of Sciences of the United States of America 105(18): 6520-6525
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(31): 12623-12638
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KERR, R.A. (2008): PLANETARY IMPACTS: Did the Mammoth Slayer Leave a Diamond Calling Card? Science 323: 26
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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
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