Comets, Climate Change, and Extinctions—1

by Emil Morhardt

At the end of the last ice age as the Earth was warming to its present condition there was an unexplained 1000-year pause and partial reversal in the warming (called the Younger-Dryas stadial). The result was a millennium of very cold weather in the Northern Hemisphere. The cause was widely attributed to the abrupt stoppage of the Gulf Stream; warm water was no longer transported from the equator north past the US east coast and Europe toward Greenland. The physical cause of the stoppage was presumably the melting of the Laurentide Ice Sheet covering Canada; enough freshwater flowed out over the North Atlantic near Greenland, that it formed a thick layer on top of the ocean that was not dense enough to sink through the underlying salt water. It is sinking saltwater off Greenland that drives the major global ocean currents—the Meridional Overturning Circulation (MOC)—of which the Gulf Stream is the last leg. Scientists are somewhat worried that under the current warming conditions, enough meltwater could flow off the Greenland Ice sheet to wreak the same sort of havoc…a much colder North America and Europe in the midst of a generally warming globe. In 2007, Firestone et al. presented an unexpected theory that the trigger for the freshwater outflow 12,900 years ago was an extraterrestrial (ET) impact event—a comet or meteorite—that also directly led to the extinction of the North American megafauna (mammoths, mastodons, ground sloths, native horses and camels) rather than overhunting by PaleoAmericans. The evidence consisted of a black mat of carbonaceous silts or dark organic clays found in sediment cores at just that time, accompanied by various substances that seemed likely to be of extraterrestrial origin (carbon spherules, nanodiamonds, etc.). The whole idea is interesting to contemplate because it not only also explains the abrupt disappearance of the Clovis people (as evidenced by the cessation of production of Clovis artifacts), it also gives one pause—what would such an ET impact event do today?

The Firestone et al. theory was not embraced widely, but its authors have persisted, publishing supporting evidence regularly, and its detractors have been busy digging up evidence to the contrary. This controversy seems to me to epitomize the scientific effort—any new interesting theory will be strongly disputed and the theorists will have to work long and hard to overcome the doubt. Such has been happening ever since the 2007 paper, and from time-to-time I’ll use this forum to recapitulate what is a truly fascinating tale. Next time I’ll describe in more detail that original paper, and proceed chronologically from there.

Firestone, R.B., West, A., Kennett, J., Becker, L., Bunch, T., Revay, Z., Schultz, P., Belgya, T., Kennett, D., Erlandson, J., 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 104, 16016-16021.

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