Extraterrestrial Non-Impact...

Extraterrestrial Impact Hypothesis: A Critique

Today we will briefly discuss an alternative theory for megafaunal extinctions in the late-Quaternary, which I have not previously mentioned. This is known as the extraterrestrial impact hypothesis. This impact apparently interrupted a period of warming with the onset of 1300 years of cooling climate known as the Younger Dryas (Daulton et al., 2016). 

Did the impact of a comet near the end of the Pleistocene cause Younger Dryas cooling and subsequent extinctions?

 

What does this hypothesis entail?

Firstly, this hypothesis contains several key parts:

• A comet (comets) hit the Earth 12,900 years ago.
• This impact forced Younger Dryas cooling.
• Sites from this time period possess a layer that contains: “magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with extraterrestrial helium” (Firestone et al., 2007 p. 16016). This indicates an extraterrestrial impact.
• This layer can be found at +50 sites across the US.
• Several comets may have hit the Laurentide Ice Sheet over North America, triggering destabilization and causing Younger Dryas cooling.
• This contributed to late-Pleistocene extinctions.
• Some megafaunal remains are radioactive near this time, in comparison to those from other time periods (Firestone et al., 2007).

Figure.1: Example of carbon spherules, found in 13 of the sampled sites.

(Firestone et al., 2007)

Evidence for no such impact

• An impact capable of damage to this extent would need to be >4km wide, no such evidence has been found of a crater this size (Firestone et al., 2007)
• There is evidence that some of the characteristics identified in the carbon rich layer or ‘impact layer’ may have been misinterpreted, and are not indicative of a collision. Lonsdalite and an increase in nanodiamond presence were used as evidence for impact, this is not found under re-examination (Daulton et al., 2016; Surovell et al., 2009).
• Impact-caused fires in the Arlington Canyon are often used as evidence for a comet. These fires are found to be natural and not induced by a collision (Scott et al., 2016).
• It is likely that even with an impact, effects would not have been large enough to have catastrophic effects on the megafauna. 

This hypothesis has now been largely put to rest. It’s more exciting to consider the idea of extraterrestrial impact as the cause of megafauna extinction, than it is to consider the much more robust science behind other explanations.

So as much as this is a cool picture…

(Source)

 …It has been pretty much discredited.

Cya next time!