One of the underlying rules of science could be phrased as "simpler is better." Scientists don't look for two causes when a single one will do, and they are trained to find correlations between events. One of the results of these tendencies is that scientists are unifiers: they tend to try to piece together disparate facts into a single coherent picture. Last week's announcement of the discovery of small diamonds that suggest that a comet struck North America roughly 13,000 years ago, however, complicates the interpretation of an event that was already very challenging.
The event is called the Younger Dryas period. As you can see from the graph here, all indications are that the earth was emerging from a glacial period about 15,000 years ago, with temperatures approaching modern levels in many areas. Suddenly, however, at 13,000 years, that trend reversed. Glacial conditions returned for another thousand years before a sudden climb occurred that brought temperatures close to our current levels. Those basic outlines are pretty clear, but getting into cause and effect from there gets extremely messy.
The bottom line in the graph shows what has been a leading contender for a cause: a sudden influx of fresh water from the North American glaciers. At the time, much of what's now central Canada was essentially a giant lake (called Lake Agassiz), the contents of which were prevented from flowing into Hudson Bay by the remaining ice sheets. Right around this time, the ice sheets failed, releasing a megaflood into the North Atlantic that played havoc with ocean currents. The change in the ocean currents, which would trap carbon dioxide in the deep basins, was thought to be enough to alter global climate.
But a new paper, published last week in Science, makes a mess out of that nice, single-cause explanation. It builds on earlier evidence of widespread carbon deposits in the sediments of this era, which suggested widespread fires that, to the authors at least, suggested an extraterrestrial impact. The research team has now found that these deposits are associated with nanodiamonds—small diamonds that form under high temperatures and pressures. Those conditions don't occur on the planet's surface, but they would occur if a carbon-rich body hit the atmosphere.
The widespread nature of the deposits—the authors found them in sites in Manitoba, Arizona, and Oklahoma—suggests that North America took a hit from several bodies at roughly the same time, as might happen if a comet or carbonaceous chondrite asteroid had broken up before striking earth.
Obviously, this could also have a significant effect on the climate, as the debris and resulting fires would place lots of aerosols into the atmosphere. It would also have a profound effect on the wildlife of North America, which was suffering from the extinction of a lot of the megafuana, such as the mammoth. Confusing the matter still further, human hunters in the form of the Clovis culture had only recently arrived on the North American scene, which undoubtedly made matters uncomfortable for those same animals.
So, did an impact or the draining of glacial lakes set off the Younger Dryas? Was it climate change, the impact, or the first North American humans that triggered the major extinctions of the period? A plausible case can probably be made for any of them, and partisans for each of the explanations will undoubtedly do so. But, of course, there's always the chance that no single factor was dominant in contributing to the events of the Younger Dryas.