Science

Discovery of an 8 km wide underwater crater that must have formed when the dinosaurs disappeared.

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This 8.5 km diameter crater was probably formed by an asteroid that hit Earth 66 million years ago. But this is not the killer asteroid that caused the mass extinction in the Cretaceous-Paleogene period! However, both could have come from the same asteroid stream or were formed as a result of a parent body rupturing, the researchers who made the discovery say.

66 million years ago, an asteroid with a diameter of several tens of kilometers hit the Earth, causing the extinction of dinosaurs – one of the largest extinctions in the history of the Earth. The event left an indelible mark – the Chicxulub crater, located near the Yucatan Peninsula in Mexico. Experts estimate that large asteroids (about 50 meters in diameter) collide with the Earth approximately every 900 years; collisions with even larger asteroids (more than 1 km), fortunately, happen less often: they occur once every million years.

To date, only a small fraction of high-velocity impacts have been preserved or discovered. Thus, collisions of large asteroids with the Earth are still poorly understood, despite the danger they pose. There are about 200 craters on Earth, of which only 15 to 20 are submarine impact craters—an astonishing number considering that most of our planet is covered in water. But soon a new underwater crater may be added to this list.

Features compatible with impactor

The researchers discovered this new crater by analyzing seismic data from the Guinea continental shelf. “I have never seen anything like it. Instead of the flat sedimentary strata that I expected offshore, I found an 8.5 km depression under the seabed with very unusual characteristics,” says Dr Wisdeen Nicholson, a geologist at Heriot-Watt University in Edinburgh.

Regional bathymetric map of the Guinean Plateau showing the location of the recently discovered Nadir crater. © W. Nicholson et al.

The crater, named Nadir after a nearby seamount, is buried under Paleogene deposits 300 to 400 meters thick off the coast of West Africa, 400 km off the coast of Guinea. This trough has characteristics consistent with a large asteroid impact crater: an elevated rim above a stepped bottom (or terraces), pronounced central uplift, and extensive subsurface deformation, the researchers report in the journal Science Advances. They also observed ejecta outside the crater with very chaotic sediments extending for tens of kilometers around.

The team has no doubt that this crater was formed by an asteroid. These characteristics are indeed inconsistent with other cratering processes (eg removal or dissolution of salt below the surface, leakage of gases or liquids, or collapse of calderas). “These processes are either incompatible with local geology and stratigraphy, […]or lead to crater morphology and scale ratios that differ significantly from those observed,” the study authors explain.

seismic section of the crater

Seismic section of the crater highlighting the morphology of the crater and the damaged area, as well as the degree of deformation of the subsurface layer. © W. Nicholson et al.

Numerical modeling of the formation of this crater shows that the asteroid had a diameter of at least 400 meters and ran aground at a depth of 500 to 800 meters. “Depending on the nature of the affected area and the proximity of settlements, an impact of this size should lead to an average of more than 300,000 deaths,” the team notes.

Cascade of catastrophic events

According to simulations, the impact would likely have triggered a giant tsunami over a kilometer high, as well as an earthquake with a magnitude of about 6.5. According to Dr. Veronica Bray, a planetary scientist at the University of Arizona, the energy released would be about 1,000 times greater than the energy from the January 2022 eruption and tsunami in Tonga.

The climatic consequences of such an event depend, in turn, on the amount of volatiles/aerosols released into the atmosphere, the researchers note. They believe the impact may have released significant amounts of greenhouse gases from shallow, organic-rich black shale deposits.

asteroid impact sequence

Impact sequence based on seismic observations and numerical models. (A) The impactor hits the water surface at about 20 km/s, causing a tsunami. (B) After a few seconds, a temporary crater forms while the impactor and a large body of water evaporate. Shock waves cause damage below and around the impact site, and seismic waves propagate through the plateau. (C) Strong uplift occurs, resulting in an elevated crater peak. (D) Radial collapse of a damaged section of the subsurface leads to the formation of terraces on the surface. © W. Nicholson et al.

Seismic evidence also suggests that the crater formed at or near the Cretaceous–Paleogene boundary about 66 million years ago; therefore, this asteroid must have hit at about the same time as the Chicxulub asteroid that wiped out the dinosaurs. However, due to uncertainty in seismic data resolution, the dating has yet to be confirmed.

But if the hypothesis is correct, it means that the two asteroids are fragments of the same parent body, or that there was a massive asteroid shower at the time. “While much smaller than the Chicxulub extinction impactor, its very existence forces us to explore the possibility of a Late Cretaceous impact cluster,” says Dr. Sean Gulick, an impact scientist at the University of Texas at Austin. research co-author. The team now plans to drill in situ to confirm the origin of this impact and more accurately determine its age.

W. Nicholson et al., Science Advances.

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