May 262015
 
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The Younger Dryas Event (YDE) is a climatological phenomenon that happened roughly 13,000 years ago. In a span of a few years, the temperature in Western Europe and North America dropped sharply and stayed low for over a millennium. The effect was more diffuse in northern America and less pronounced in the southern hemisphere.
Nonetheless, the YDE is associated with the mass extinction of large mammals in North America. At this time, humans had already spread around the globe and started civilizations. One of these civilizations was the so called Clovis culture that also vanished during the YDE. It is entirely possible that the extinctions coinciding with the YDE are not due to climatological changes, but rather human overkill [1]. The decline in mammal population in turn led to the decline of the Clovis. But as life is complicated, the activity of humans likely conspired with the changing climate to cause the extinction and the downfall of the Clovis.
In contrast to our current civilization, the people 13,000 years ago did not have the means to affect such dramatic climatic changes (going far beyond even our current level of climate change).

The question now is: what caused the YDE in the first place? A widely held belief is that the melting of the North American ice caps disrupted the thermohaline circulation (the ocean circulation that nowadays brings warm water from the Gulf of Mexico to Western Europe), by dumping large quantities of fresh water into the north Atlantic. In 2007 Firestone et al. proposed an interesting trigger for the melting of the ice caps [2]: An impact of an asteroid, or rather the explosion of an asteroid in the atmosphere. This would have been much stronger version of the 2013 Chelyabinsk meteor. Support for this hypothesis comes from the presence of nanodiamonds in the geological layers associated with the YDE. The only other strata where these diamonds are present is the K-T boundary that marks the global extinction event that killed the dinosaurs [3].

The impact hypothesis is however hotly debated [4]. Many of the original markers used to determine that an impact took place have later been discredited, as it turned out they can also be produced by earthly phenomena, e.g. volcanism. The markers left over on the other hand could not be consistently reproduced by other research groups. One problem is that different groups include different kinds of nanodiamonds in their analysis or use different calibration scales for the dating of samples. The uncertainty in the dating is often several hundreds of years so that it is not clear if potential impact markers have been deposited at the same time or in independent events. Additionally the uncertainty in the age of the samples makes it hard to pin them to the relatively narrow time frame for the beginning of the YDE. The impact event might thus have happened significantly before or after the onset of the YDE, or it might not have happened at all. After all new climate models suggest that the melting of the North American ice sheets could have occurred without a specific trigger such as the proposed impact.

So where are we left if the impact is not necessary to explain the behavior of the climate and the evidence for an impact is disputed? There is certainly the possibility that an impact took place without changing the climate, but the main question seems to be if the impact ever occurred.

Stephan Koehler

Read more:
[1] Samdom et al., Proceedings of the Royal Society B 281, 20133254 (2014).
[2] Firestone et al., PNAS 104(41), 16016-16021 (2007).
[3] Kinzie et al., Journal of Geology 122(5), 475-505 (2014).
[4] van Hoesel et al., Quaternary Science Reviews 83, 95-114 (2014).

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