The Earth still bears the scar of the gigantic asteroid impact that wiped out the dinosaurs, along with three quarters of all life – but a new study suggests that killer rock wasn’t alone. Scientists have discovered a new crater in the seabed of the North Atlantic Ocean that seems to correspond to around the same time, suggesting the extinction event could have been triggered by multiple impacts.

Nadir crater, as it’s been named, measures around 8.5 km (5.3 miles) wide and lies about 400 km (250 miles) off the coast of Guinea, West Africa. Geologist Uisdean Nicholson was studying seismic reflection data of the Guinea Plateau, a large expanse of flat seabed, when he spotted a strange structure buried some 300 to 400 m (985 to 1,310 ft) underground.

“I’ve interpreted lots of seismic data in my time, but had never seen anything like this,” said Nicholson. “It has particular features that point to a meteor impact crater. It has a raised rim and a very prominent central uplift, which is consistent for large impact craters. It also has what looks like ejecta outside the crater, with very chaotic sedimentary deposits extending for tens of kilometers outside of the crater. The characteristics are just not consistent with other crater-forming processes like salt withdrawal or the collapse of a volcano.”

Studies of the sediment stirred up during impact suggest that Nadir crater was formed about 66 million years ago, at the end of the Cretaceous period. That’s famously marked by the extinction event that took out the dinosaurs, believed to have been caused by a huge asteroid impact that’s still visible as the Chicxulub crater in Mexico.

The researchers used computer simulations to investigate what kind of impactor it would take under which conditions to produce the crater seen, as well as how bad the fallout might have been. They determined that the asteroid responsible was likely about 400 m (1,300 ft) wide, and it appears to have struck a body of water between 490 and 790 m (1,600 and 2,600 ft) deep.

“This would have generated a tsunami over 3,000 ft (915 m) high, as well as an earthquake of more than magnitude 6.5,” said Veronica Bray, co-author of the study. “Although it is a lot smaller than the global cataclysm of the Chicxulub impact, Nadir will have contributed significantly to the local devastation. And if we have found one ‘sibling’ to Chicxulub, it opens the question: Are there others?”

The finding could change our understanding of this key extinction event. Instead of just one big rock, it could indicate that Earth encountered a swarm of asteroids around that time, or that the parent body broke into several pieces before hitting.

Of course, it’s hard to pin down the exact timing of the two impacts – they could have occurred thousands, even millions of years apart. The team plans to investigate further by drilling into the seabed at the site, to confirm that it is an impact crater and pin down its age more precisely.

The research was published in the journal Science Advances.

Source: University of Arizona