A sudden collapse of global climate around 12.8 ka and equally brusque warming 11.5 ka ago is called the Younger Dryas. It brought the last ice age to an end. Because significant warming preceded this dramatic event palaeoclimatologists have pondered its cause since it came to their attention in the early 20th century as a stark signal in the pollen content of lake cores – Dyas octopetala, a tundra wild flower, then shed more pollen than before or afterwards; hence the name. A century on, two theories dominate: North Atlantic surface water was freshened by a glacial outburst flood that shut down the Gulf Stream [June 2006]; a large impact event shed sufficient dust to lower global temperatures [July 2007]. An oceanographic event remains the explanation of choice for many, whereas the evidence for an extraterrestrial cause – also suggested to have triggered megafaunal extinctions in North America – has its supporters and detractors. The first general reaction to the idea of an impact cause was the implausibility of the evidence [November 2010], yet the discovery by radar of a major impact crater beneath the Greenland ice cap [November 2018] resurrected the ‘outlandish’ notion. A recent paper in Nature: Scientific Reports further sharpens the focus.
Since 2007, a team of Chilean and US scientists has been working on a rich haul of late Pleistocene fossil mammals from Patagonian Chile that turned up literally in someone’s suburban back garden in the town of Osorno. The stratigraphy has been systematically dated using the radiocarbon method. A dark layer composed of peat with abundant charcoal gave an age of about 12.8 ka, thereby marking both the local base of the Younger Dryas episode and a cap to the rich mammalian fossil assemblage. Similar beds have been found at more than 50 sites elsewhere in the world at this stratigraphic level, including a site in Arizona carrying Clovis artifacts. Steadily, such ‘black mats’ have been yielding magnetised spherules, elevated concentrations of platinum-group metals, gold, native iron, fullerenes and microscopic diamonds, plus convincing signs of wild fires at some sites; the very evidence that most researchers had panned when first reported. The Chilean example contains much the same pointers to an extraterrestrial cause, attributed to air-burst impacts (Pino, M. and 14 others 2019. Sedimentary record from Patagonia, southern Chile supports cosmic-impact triggering of biomass burning, climate change, and megafaunal extinctions at 12.8 ka. Scientific Reports, v. 9, article 4413; DOI: 10.1038/s41598-018-38089-y)
A larger team of researchers, to which several of the authors of the Chilean paper are affiliated, claim the evidence supports some kind of impact event 12.8 ka ago, possibly several produced by the break-up of a comet. Yet the criticisms persist. For instance, had there been wildfires on the scales suggested, then there ought to be a significant peak in the proportion of charcoal in lake bed sediments from any one region at 12.8 ka. In fact such data from North America show no such standalone peak among many from the age range of the Younger Dryas. The fossil record from the last few millennia of the Pleistocene does not support a sudden extinction, but a decline. The Clovis-point culture, thought by many to have wrought havoc on the North American megafauna, may have come to an end around 12.8 ka, but was quickly succeeded by an equally efficient technology – the Folsom point. As regards the critical evidence for impacts, shocked mineral grains, none are reported, and some of the reported evidence of microspherules and nanodiamonds is not strongly supported by independent analysis – and nor are they unique to impact events. How about the dating? The evidence from ice cores strongly suggests that the Younger Dryas began with an 8° C temperature decline over less than a decade, and the end was equally as sudden. Is radiocarbon dating capable of that time resolution and accuracy? Certainly not
Related articles: Gramling, C. 2018. Why won’t this debate about an ancient cold snap die? (Science News); Easterbrook, D.L. 2012.The Intriguing Problem Of The Younger Dryas—What Does It Mean And What Caused It? (Watts Up With That); Wolbach, W.S. and 26 others 2018. Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 1. Ice cores and Glaciers. Journal of Geology, v. 126, p. 165-184; DOI: 10.1086/695703; Wolbach, W.S. and 30 others 2018. Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago. 2. Lake, Marine, and Terrestrial Sediments. Journal of Geology, v. 126, p. 185-205; DOI: 10.1086/695704.