Extra-Terrestrial Impact Causing Megafaunal Decline: Fact or Fiction?

Could the impact of a Comet cause the extinction of megafauna?

Over resent blogs we have seen an array of factors which could have caused the extinction of megafauna in the late Pleistocene. In this blog I will discuss the impact hypothesis, looking specifically at the evidence for and against the occurrence of an extra-terrestrial event.

The Younger Dryas (YD) impact hypothesis is a recent theory that suggests that a comet or meteoric body hit/and or exploded over North America 12,900 years ago, causing the YD climate episode, extinction of Pleistocene megafauna, demise of the Clovis archaeological culture, and a range of other effects (Printer et al 2011).  The authors that support the impact hypothesis suggest that a comet exploded over the great lakes, destabilizing the Laurentide ice sheet, releasing huge volumes of melt water which subsequently caused the YD re-glaciation and caused intense wildfires that led to the extinction of megafauna. The impact is also claimed to have caused major cultural changes and population decline among the Paleoindians (Buchanan et al 2008). The impact hypothesis gained widespread publicity in 2007, and extensive research has been focused on testing the sources of evidence that support this hypothesis.

The evidence (see Firestone et al 2007) that supports the impact theory includes:

1.  Particle tracks in archaeological records
2.  Magnetic nodules in Pleistocene bones- representing meteorite fragments (cosmic bullets), derived from the YD impactor and directly linking the proposed impact event with the megafaunal demise (Pinter et al 2011)
3. Impact origin of the Carolina bays
4. Elevated concentration of radioactivity. Firestone et al (2007) stated that some megafaunal bones in the YD are highly radioactive.     
5.      Carbon spheres and elongates
6. Magnetic grains and charcoal/soot (by products of catastrophic wildfire). This is mentioned by Buchanan et al (2008) who stated that the impact was accompanied by a high-temperature shock wave, changes in pressure that would have resulted in hurricane force winds, and extensive groundcover burning from the impact and superheated ejecta.
7. Nanodiamonds present across North America.
8.      Extinction of many mammalian and avian taxa occurred abruptly and perhaps catastrophically at the onset of the YD, which is believed to have been caused by a ET event.
   
   
        However, whilst there is evidence that supports a ET event, many are skeptical about this. Consequently, such evidence has been largely rejected by the scientific community. The reasons why ET evidence has not been supported is that:
   
   
   1.       Peak concentrations of magnetic grains at the start of the YD have yet to be found.
   2.       Impact markers have yet to be found (Pinter et al 2011). Similarly, there has been no evidence for charcoal peaks at the time of the YD.
   3.       None of the evidence supporting the impact hypothesis have been independently reproduced or have met the minimum threshold for scientific credibility (Pinter et al 2011)
   4.       The existence of carbon elongates have been confirmed but are ubiquitous in Pleistocene to modern sediments, and did not originate in catastrophic wildfires.
   5.       Buchanan et al (2008) found no such evidence of a population decline among the   Paleoindians around 12,000 yr BP.
   
   
   
    In conclusion, there is no doubt that the impact of a comet would have been devastating for animals and plants (Firestone et al 2007). However, the impact hypothesis has been controversial. Personally I am skeptical about an extra -terrestrial event triggering the YD, as there has been a lack of reproducible evidence supporting it (see Haynes et al 2010). Whist the majority of the scientific community discredits the impact theory, whether a ET event caused the extinction of megafauna is unknown.
   
   
   American Geophysical Union (AGU) Press Conference looking at the key issues surrounding the impact hypothesis: