Throughout Earth鈥檚 history, our planet has been bombarded with a countless number of meteorites, ranging in size from rock fragments that barely leave an imprint to huge kilometre-wide boulders that wiped out vast swathes of life. But do all large impacts lead to mass extinctions? Perhaps not, as geologists have found that the mineral make-up of the rocks that a meteorite hits matters just as much as its size.
Large impacts on Earth鈥檚 surface produce ejecta, in which huge amounts of dust are thrown into the atmosphere, blocking out the sun. This, in turn, can see global temperatures temporarily plummet and plants falter, a scenario known as an 鈥�impact winter鈥�. These can last up to a year and are thought to trigger mass extinctions.
But that isn鈥檛 the full picture, says Matt Pankhurst at the Instituto Volcanol贸gico de Canarias in Spain. He and his colleagues looked at 44 of the largest known recorded impacts on Earth and, after discounting 11 of them due to a lack of precise data on their ages, compared the timings of the remaining 33 with 24 mass extinctions events that left their mark in the geological records over the past 600 million years.
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They chose this period because it encompasses the entirety of multicellular life and extends back to include the earliest known link between mass extinctions and impacts 鈥� the Acraman impact, which struck Australia about 580 million years ago, around the same time as microorganisms known as the acritarchs experienced a mass extinction.
鈥淎ge dating improvements mean we can now look at very precise timing of events in the meteorite impact and fossil records,鈥� says Pankhurst. 鈥淭his means we can test the strength of any correlation with much higher confidence.鈥�
The researchers found that 15 of these 33 impacts, including the famous Chicxulub event thought to have wiped out the dinosaurs, created ejecta blankets rich in a mineral called potassium-feldspar (Kfs). All were associated with mass extinctions.
In contrast, the other 18 meteorites hit rocks deficient in Kfs, and correspond to times of ecological stability, irrespective of the size of the impact. This suggests that only ejecta blankets rich in Kfs lead to mass extinctions.
Kfs acts as a catalyst for ice forming in the atmosphere. Its presence increases the formation of ice in clouds and decreases clouds鈥� ability to reflect sunlight. This allows more of the sun鈥檚 heat to reach the surface and so causes warming effects on Earth.
Clouds that contain a higher proportion of ice crystals can also suppress atmospheric stabilising feedbacks. This leaves the climate more vulnerable to warming from other factors, such as changes in the concentration of greenhouse gases.
Once produced, the Kfs ejecta blankets can influence the planet鈥檚 climate for tens of thousands of years, as the surfaced minerals can be repeatedly swept up into the atmosphere.聽 This is a much better match to the timescales of extinction events than an impact winter scenario, says Pankhurst, as the climate is affected over a much longer period, making it harder for species to recover, if at all.
鈥淥ur explanation helps resolve other puzzles in the rock record, such as why the relationship between volcanism and extinction events is so hit-and-miss, and why meteorite impact size does not correlate with extinction intensity,鈥� he says.
Confirming this idea will be difficult, says at the University of Oslo, Norway. 鈥淟arge meteorite impacts are a small dataset due to their rarity, and there are inherent uncertainties in accurately dating mass extinction events in the rock record.鈥�
Further tests on the cloud-forming abilities of Kfs could help provide more evidence, and may even prove useful in combatting climate change by reducing Earth鈥檚 reflectivity, he says. 鈥滻t could be a powerful geoengineering tool with less drawbacks to other cloud condensation methods.鈥�
earthArXiv
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