The oldest known impact structure (?)

That large, rocky bodies in the Solar System were heavily bombarded by asteroidal debris at the end of the Hadean Eon (between 4.1 to 3.8 billion years ago) is apparent from the ancient cratering records that they still preserve and their matching with dating of impact-melt rocks on the Moon. Being a geologically dynamic planet, the Earth preserves no tangible, indisputable evidence for this Late Heavy Bombardment (LHB), and until quite recently could only be inferred to have been battered in this way. That it actually did happen emerged from a study of tungsten isotopes in early Archaean gneisses from Labrador, Canada (see: Tungsten and Archaean heavy bombardment, August 2002; and Did mantle chemistry change after the late heavy bombardment? September 2009). Because large impacts deliver such vast amounts of energy in little more than a second (see: Graveyard for asteroids and comets, Chapter 10 in Stepping Stones) they have powerful consequences for the Earth System, as witness the Chicxulub impact off the Yucatán Peninsula of Mexico that resulted in a mass extinction at the end of the Cretaceous Period. That seemingly unique coincidence of a large impact with devastation of Earth’s ecosystems seems likely to have resulted from the geology beneath the impact; dominated by thick evaporite beds of calcium sulfate whose extreme heating would have released vast amounts of SO2 to the atmosphere. Its fall-out as acid rain would have dramatically affected marine organisms with carbonate shells. Impacts on land would tend to expend most of their energy throughout the lithosphere, resulting in partial melting of the crust or the upper mantle in the case of the largest such events.

The further back in time, the greater the difficulty in recognising visible signs of impacts because of erosion or later deformation of the lithosphere. With a single, possible exception, every known terrestrial crater or structure that may plausibly be explained by impact is younger than 2.5 billion years; i.e. they are post-Archaean. Yet rocky bodies in the Solar System reveal that after the LHB the frequency and magnitude of impacts steadily decreased from high levels during the Archaean; there must have been impacts on Earth during that Eon and some may have been extremely large. In the least deformed Archaean sedimentary sequences there is indirect evidence that they did occur, in the form of spherules that represent droplets of silicate melts (see: Evidence builds for major impacts in Early Archaean; August 2002, and Impacts in the early Archaean; April 2014), some of which contain unearthly proportions of different chromium isotopes (see: Chromium isotopes and Archaean impacts; March 2003). As regards the search for very ancient impacts, rocks of Archaean age form a very small proportion of the Earth’s continental surface, the bulk having been buried by younger rocks. Of those that we can examine most have been subject to immense deformation, often repeatedly during later times.

The Archaean geology of part of the Akia Terrane (Manitsoq area) in West Greenland. The suggested impact structure is centred on the Finnefjeld Gneiss (V symbols) surrounded by highly deformed ultramafic to mafic igneous rocks. (Credit: Jochen Kolb, Karlsruhe Institute of Technology, Germany)

There is, however, one possibly surviving impact structure from Archaean times, and oddly it became suspected in one of the most structurally complex areas on Earth; the Akia Terrane of West Greenland. Aeromagnetic surveys hint at two concentric, circular anomalies centred on a 3.0 billion years-old zone of grey gneisses (see figure) defining a cryptic structure. It is is surrounded by hugely deformed bodies of ultramafic and mafic rocks (black) and nickel mineralisation (red). In 2012 the whole complex was suggested to be a relic of a major impact of that age, the ultramafic-mafic bodied being ascribed to high degrees of impact-induced melting of the underlying mantle. The original proposers backed up their suggestion with several associated geological observations, the most crucial being supposed evidence for shock-deformation of mineral grains and anomalous concentrations of platinum-group metals (PGM).

A multinational team of geoscientists have subjected the area to detailed field surveys, radiometric dating, oxygen-isotope analysis and electron microscopy of mineral grains to test this hypothesis (Yakymchuck, C. and 8 others 2020. Stirred not shaken; critical evaluation of a proposed Archean meteorite impact in West Greenland. Earth and Planetary Science Letters, v. 557, article 116730 (advance online publication); DOI: 10.1016/j.epsl.2020.116730). Tectonic fabrics in the mafic and ultramafic rocks are clearly older than the 3.0 Ga gneisses at the centre of the structure. Electron microscopy of ~5500 zircon grains show not a single example of parallel twinning associated with intense shock. Oxygen isotopes in 30 zircon grains fail to confirm the original proposers’ claims that the whole area has undergone hydrothermal metamorphism as a result of an impact. All that remains of the original suggestion are the nickel deposits that do contain high PGM concentrations; not an uncommon feature of Ni mineralisation associated with mafic-ultramafic intrusions, indeed much of the world’s supply of platinoid metals is mined from such bodies. Even if there had been an impact in the area, three phases of later ductile deformation that account for the bizarre shapes of these igneous bodies would render it impossible to detect convincingly.

The new study convincingly refutes the original impact proposal. The title of Yakymchuck et al.’s paper aptly uses Ian Fleming’s recipe for James Bond’s tipple of choice; multiple deformation of the deep crust does indeed stir it by ductile processes, while an impact is definitely just a big shake. For the southern part of the complex (Toqqusap Nunaa), tectonic stirring was amply demonstrated in 1957 by Asger Berthelsen of the Greenland Geological Survey (Berthelsen, A. 1957. The structural evolution of an ultra- and polymetamorphic gneiss-complex, West Greenland. Geologische Rundschau, v. 46, p. 173-185; DOI: 10.1007/BF01802892). Coming across his paper in the early 60s I was astonished by the complexity that Berthelsen had discovered, which convinced me to emulate his work on the Lewisian Gneiss Complex of the Inner Hebrides, Scotland. I was unable to match his efforts. The Akia Terrane has probably the most complicated geology anywhere on our planet; the original proposers of an impact there should have known better …

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)

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