Why an asteroid impact is unlikely to have caused the Late Quaternary mass extinction.
The Pleistocene world was dominated by large mammals, flightless birds and reptiles. These included mammoths, mastodons, giant ground sloths, camels, sabre-tooth cats, giant beavers, and giant deer with antlers spanning 3 m (10 ft.). In Australia, there lived the hippopotamus-sized Diprotodon optatum that weighed in at 2.8 tonnes, and was the largest marsupial of all time. These animals are collectively known as the megafauna, a term applied to animals with an adult weight of 45 kg (100 lb.) or more. Between 50,000 and 10,000 years ago, many of these great beasts vanished in one of the largest extinction events since the demise of the dinosaurs. Australia and the Americas were hardest hit, but no habitable continent remained unscathed. Overall, about 180 large mammal species and over 100 entire genera perished (Barnosky, et al., 2004; Lyons, et al., 2004; Koch & Barnosky, 2006; Barnosky, 2008). Usually referred to as the Late Quaternary extinction event, it was recognised by early geologists towards the end of the eighteenth century.
The cause of this mass extinction has long been debated and there remains a lack of consensus to this day. Climate change and human activity were put forward as possible causes as far back as the early nineteenth century, and both are hotly championed to this day. One of the more controversial theories, first proposed in 2007, is that around 12,900 years ago, Earth suffered multiple airbursts and surface impacts from fragments of a comet or asteroid that had previously broken up in space. In North America, the bombardment caused devastating shock-waves and continent-wide forest fires that brought about the extinction of the megafauna. While the overall effects were far less severe than those of the impact now believed to have killed off the dinosaurs, they were still sufficient to trigger a global ‘impact winter’. This in turn precipitated Younger Dryas climatic downturn.
Evidence for the supposed impact was claimed in the form of a 12,900-year-old carbon-rich layer or ‘black mat’. The layer has been identified at around fifty Clovis sites in North America. It is said to contain material consistent with an impact, including magnetic mineral grains, soot, carbon spherules and so-called nanodiamonds. The latter are minute diamonds formed when carbon particles are subjected to intense heat and pressure by an explosion (Firestone, et al., 2007; Haynes, 2008; Kennett, et al., 2009). More recently, evidence for impacts has also been claimed from Younger Dryas boundary sites in Mexico, Belgium, the Netherlands, Germany and Syria (Israde-Alcántara, et al., 2012; Bunch, et al., 2012). Anomalous levels of platinum, said to be due to the impact of a large iron meteorite, have also been reported from Greenland ice core samples dating to the Younger Dryas boundary (Petaev, et al., 2013).
In theory this is all sounds highly feasible, but in practice the timing is a little suspicious. Other factors were in play at the time, and it is not necessary to invoke an extraterrestrial impact to explain the onset of the Younger Dryas. Named for the arctic-alpine flowering plant Dryas octopetala that flourished in the northern tundra at that time, the Younger Dryas marks the final stage of the Pleistocene. It lasted from 12,900 until 11,600 years ago and both its onset and termination were fairly abrupt (Taylor, et al., 1997; Severinghaus, et al., 1998). During the preceding Bølling-Allerød warm period, the North American ice sheets retreated. The resulting meltwater formed a vast glacial lake known as Lake Agassiz, larger than all the modern Great Lakes put together. Beginning 13,000 years ago, Lake Agassiz released a series of freshwater discharges into the Arctic Ocean, through what is now the drainage basin of the Mackenzie River (Murton, et al., 2010). The conventional view is that the great volume of freshwater disrupted the Gulf Stream, halting the flow of warm seawater from the tropics to higher latitudes. The result was to plunge the Northern Hemisphere back into glacial conditions. Effects in the Southern Hemisphere are less certain, though evidence of cooling has been found there also (Moreno, et al., 2001).
That an extraterrestrial impact should occur at more or less the same time as the freshwater discharge strikes me as being a rather implausible coincidence. It should also be noted that other studies have failed to find evidence for the nanodiamonds (Daulton, et al., 2010), and that there is no evidence for the continent-wide conflagration supposedly triggered by the impact. Evidence for burning is better attributed to climate change-generated increases in natural wildfires (Marlon, et al., 2009). Similarly, the magnetic grains can be accounted for by a constant influx of micrometeorites from space (Surovell, et al., 2009). The ‘black mats’ do not occur throughout the whole of North America, but rather are located predominantly in the west. They may be algal mats or ancient soils associated with regional increases in moisture (Gill, et al., 2012). The source of platinum anomaly may be extraterrestrial, but this remains unproven and more evidence is needed. All in all, I am sceptical about the impact theory, although it certainly cannot be ruled out.
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