Did an impact affect hunter gatherers at the start of the Younger Dryas?

Whether or not the return to a glacial climate between 12.8 and 11.7 thousand years (ka) ago, known as the Younger Dryas (YD), was triggered by some kind of extraterrestrial impact has been a hot and sometimes fractious issue since 2007 (see: Whizz-bang view of Younger Dryas; Earth-logs, July 2007). Before then the most favoured causal mechanism was a shutdown of the Gulf Stream’s Arctic warming influence as a result of some kind of catastrophic flooding of fresh water into the North Atlantic. That would have lowered the density of surface waters, thereby preventing them from sinking to drive the deep circulation that draws surface water from the tropics into high northern latitudes (see: The Younger Dryas flood; May 2010). In 2008 the melt-water flood supporters were sufficiently piqued by the suggestion of a hitherto unsuspected impact event to mount a powerful rejoinder (see: Impact cause for Younger Dryas draws flak; May 2008), casting doubt on the validity of the data that had been presented. It seemed like a repeat of the initial furore over claims for a ‘mountain falling out of the sky’ wiping out the dinosaurs and much else. Yet, like the claims by Alvarez pere et fils for the K-T impact, accumulated weight of evidence published by its protagonists eventually has given the idea of an impact trigger for the YD a measure of respectability. This began with evidence of an impact crater beneath the Greenland icecap (see: Subglacial impact structure in Greenland: trigger for Younger Dryas?; November 2018), then signs of a 12.8 ka fire storm in Chile followed by geochemical evidence from South Carolina, USA for a coinciding impact (see: More on the Younger Dryas causal mechanism; November 2019).

Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland, showing a possible impact crater (Credit: Kjaer et al. 2018; Fig. 1D)

The YD played havoc with humans who had begun to repopulate northern Europe from their Ice Age refuges in the south and those who had first ventured into the Americas  across the Beringia land bridge between Siberia and Alaska. The climate decline was extremely rapid, spanning a mere decade or so, and many would have been trapped to perish in what again became frigid steppe land. There is now evidence that late-Palaeolithic to Mesolithic hunter gatherers living far south of the reglaciated zone also suffered devastation at the start of the YD (Moore, A.M.T. and 13 others 2020. Evidence of Cosmic Impact at Abu Hureyra, Syria at the Younger Dryas Onset (~12.8 ka): High-temperature melting at > 2,200 °C. Nature Science Reports, v. 10, p. 1-22; doi: 10.1038/s41598-020-60867-w). Abu Hureyra is a tell – a mound settlement – originally on the banks of the Euphrates in northern Syria. It now lies beneath Lake Assad, but was excavated in the early 1970s to reveal a charcoal-littered habitation surface with signs of a settlement and some cultivation. Charcoal from archived samples yielded a precise radiocarbon age of 12825 ± 55 ka, coinciding with the start of the YD. The sediment from the habitation floor also contained signs compatible with ejecta from a high-energy impact: tiny diamonds and glass spherules. Analyses of the glass by the authors suggests that it formed at a temperature up to 2200°C, far greater than that of magma associated with a volcanic eruption or in hearths used by the inhabitants. However, others have analysed the glass and suggest more mundane temperatures that could be explained more simply by accidental burning of thatched huts. That possibility might explain the lack of other impact indicators, such as shocked mineral grains and anomalous geochemistry, particularly the platinum-group metals that were the original ‘smoking gun’ for the K-T boundary event and other major impacts. Incidentally, these crucial indicators have been reported from other YD sites investigated by several members of the team behind this paper. My view is that what seems to be a remarkable coincidence will not settle the matter, but will probably draw the same kind of ‘flak’ as did others on this topic. It is hardly likely that new samples will be collected from the now submerged Abu Hureyra site.

See also: Cometary Debris may have destroyed Paleolithic settlement 12,800 years ago (Science News. 2 July 2020)

A major Precambrian impact in Scotland

The northwest of Scotland has been a magnet to geologists for more than a century. It is easily accessed, has magnificent scenery and some of the world’s most complex geology. The oldest and structurally most tortuous rocks in Europe – the Lewisian Gneiss Complex – which span crustal depths from its top to bottom, dominate much of the coast. These are unconformably overlain by a sequence of mainly terrestrial sediments of Meso- to Neoproterozoic age – the Torridonian Supergroup – laid down by river systems at the edge of the former continent of  Laurentia. They form a series of relic hills resting on a rugged landscape carved into the much older Lewisian. In turn they are capped by a sequence of Cambrian to Lower Ordovician shallow-marine sediments. A more continuous range of hills no more than 20 km eastward of the coast hosts the famous Moine Thrust Belt in which the entire stratigraphy of the region was mangled between 450 and 430 million years ago when the elongated microcontinent of Avalonia collided with and accreted to Laurentia.  Exposures are the best in Britain and, because of the superb geology, probably every geologist who graduated in that country visited the area, along with many international geotourists. The more complex parts of this relatively small area have been mapped and repeatedly examined at scales larger than 1:10,000; its geology is probably the best described on Earth. Yet, it continues to throw up dramatic conclusions. However, the structurally and sedimentologically simple Torridonian was thought to have been done and dusted decades ago, with a few oddities that remained unresolved until recently.

NW Scotland geol
Grossly simplified geological map of NW Scotland (credit: British Geological Survey)

Continue reading “A major Precambrian impact in Scotland”