A second asteroid may have struck Earth when the dinosaurs died out

A possible impact crater under the sea off West Africa might have been made by a smaller piece that broke off the asteroid that wiped out most dinosaurs

Earth



17 August 2022

By Michael Le Page

Dinosaurs may have been hit by two or more impacts in a short period of time

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What appears to be a 9 kilometre-wide crater has been discovered buried beneath the sea floor near the coast of West Africa. It was made around the time of the larger Chicxulub impact that wiped out most dinosaurs, leading to speculation that it was caused by a chunk that broke off the Chicxulub asteroid.

“It definitely fits the bill for an impact crater,” says Uisdean Nicholson at Heriot-Watt University in Edinburgh, UK.

Nicholson spotted the feature in seismic reflection data supplied by the oil and gas industry. The likely crater, named the Nadir crater after a nearby seamount, is on the continental shelf a few hundred kilometres off the coast of Guinea, buried beneath around 300 metres of sediment in an area where the water is 900 metres deep.

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The structure has all the features characteristic of an impact crater of this size, says Nicholson, including a raised rim and signs of ejected material outside the crater itself. Modelling by team member Veronica Bray at the University of Arizona in Tucson suggests it was caused by the impact of an asteroid around 400 metres in diameter.

The Nadir crater appears to have formed around 66 million years ago, the same time as the 180 kilometre-wide Chicxulub crater in what is now Mexico. That has led the team to speculate that it was made by a chunk that broke off the Chicxulub asteroid, which is estimated to have been 13 kilometres in diameter.

If this had happened just before impact, the two craters would be very close. Instead, Nicholson suggests that gravity could have broken the asteroid apart during an earlier orbit that passed closer to Earth, leading to two impacts within a few days of each other.

This is what happened with the Shoemaker-Levy comet, says Nicholson, which was ripped into fragments by Jupiter’s gravity in 1992 and then struck the planet in 1994. At least 21 fragments hit Jupiter over a six-day period.

It is possible that the Chicxulub asteroid broke up into several fragments too, says Nicholson. Other impact craters may remain to be discovered or could have been destroyed by tectonic processes. Craters don’t form when asteroids hit water several kilometres deep, as most of our oceans are.

“This is an exciting discovery,” says Gareth Collins at Imperial College London, who has studied the Chicxulub impact. “It certainly has lots of features consistent with an impact origin.”

However, Collins isn’t convinced that the event is linked to the Chicxulub impact, pointing out that there is a lot of uncertainty in the dating. “I think that the two events are more likely to be unrelated,” he says.

Nicholson thinks this is a possibility. His team has put in a proposal to drill through the Nadir crater and retrieve cores that will confirm if it is an impact structure and allow the event to be much more precisely dated.

The Nadir impact alone wouldn’t have caused a major extinction, says Nicholson. The event would have mostly affected the surrounding region, not least by creating tsunami waves that would have been 500 metres high near the impact site, according to the study. “It would have been a very significant regional event,” he says.

However, the Nadir asteroid could have triggered some global warming by releasing carbon from the black shale rocks it struck, and also by destabilising methane hydrates on the sea floor.

The Chixculub impact was around 1000 times more powerful and blanketed the entire planet in dust within a few hours. It wiped out all dinosaurs apart from birds and brought the long Cretaceous Period to a decisive end. Palaeontologists have discovered a site in North Dakota that they think contains fossils of animals killed by the impact.

Journal reference: Science Advances, DOI: 10.1126/sciadv.abn3096

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