From “Eos” : “Impact Crater off the African Coast May Be Linked to Chicxulub”

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From “Eos”



Katherine Kornei

Scientists hope to drill into a newly discovered impact crater off the west coast of Africa to explore how it’s linked—if it is—to the famous Chicxulub impact 66 million years ago. Credit:

In the world of impact craters, Chicxulub is a celebrity: The 180-kilometer-diameter maw, in the Gulf of Mexico, was created by a cataclysmic asteroid impact at the end of the Cretaceous that spelled the demise of most dinosaurs. But researchers have now uncovered another crater off the coast of West Africa that might well be Chicxulub’s cousin. The newly discovered feature, albeit much smaller, is also about 66 million years old. That’s a curious coincidence, and scientists are now wondering whether the two impact structures might be linked. Perhaps Chicxulub and the newly discovered feature—dubbed Nadir crater—formed from the breakup of a parent asteroid or as part of an impact cluster, the team suggested. These results were published in Science Advances [below].

Rocks of Concern

Every day, tons of cosmic dust rain down on our planet. That microscopic debris poses no danger to life on Earth, but its larger brethren are very much cause for concern: A space rock measuring hundreds of meters in size is apt to cause regional destruction, and the arrival of something measuring kilometers in size could spell global havoc. That’s what happened 66 million years ago when a roughly 12-kilometer-wide asteroid slammed into a shallow reef in the Gulf of Mexico. That event, now known as Chicxulub after the small town that’s grown up nearby in Mexico, launched shock waves, powerful tsunamis, and blasts of superheated air that decimated life in the vicinity. And airborne particles—bits of dust, soot, and sulfate aerosols born from the sulfur-rich rocks that existed at the Chicxulub impact site—choked the atmosphere and plunged the entire planet into a sunlight-starved “impact winter” that lasted for years. When the air finally cleared, over 75% of all species had gone extinct.

The newly discovered Nadir crater appears to have formed right around the same time as that cataclysm. Uisdean Nicholson, a sedimentary geologist at Heriot-Watt University in Edinburgh, Scotland, and his colleagues discovered the candidate crater while they were poring over observations of seafloor sediments originally collected for oil and gas exploration. The team spotted the roughly 8-kilometer-wide structure in seismic reflection imaging data obtained off the coast of West Africa. “It was pure serendipity,” said Nicholson.

Signs of an Impact

The putative crater is buried under roughly 300 meters of sediments topped by 900 meters of water, and its appearance strongly suggests it was created by a hypervelocity impact, said Nicholson. For starters, it’s circular in shape, with a pronounced rim. Second, it contains a small central peak, a feature that often arises in large impact craters. And perhaps most important, there’s clear evidence of deformed sediments—caused by faulting and folding—persisting hundreds of meters below what would be the crater floor. “There’s a lot of things that suggest it’s an impact,” said Gavin Kenny, a geochemist at the Swedish Museum of Natural History in Stockholm who was not involved in the research.

Fig. 1. Map and regional seismic sections showing location of Nadir Crater.
(A) Regional bathymetry map of the Guinea Plateau and Guinea Terrace showing location of 2D seismic reflection and well data used in this study. JS, Jane Seamount; NS, Nadir Seamount; PS, Porter Seamount. The white dashed line shows the NE extent of high-amplitude discontinuous seismic facies at the top Maastrichtian interpreted as ejecta deposits and associated tsunami deposits. The north-east limit of this facies closely corresponds with the Maastrichtian shelf-slope break at the landward margin of the Guinea Terrace. Inset map shows a paleogeographic reconstruction of the Atlantic near the end of the Cretaceous, ~66 Ma ago, made using GPlates software (58). Ch, Chicxulub Crater; Nd, Nadir Crater; Bo, Boltysh Crater. (B). Regional composite 2D seismic reflection profile extending from the GU-2B-1 well in the east to the deep Atlantic basin in the west, showing the structural and stratigraphic character of the Guinea Plateau and Guinea Terrace. (C) North-South seismic profile from the salt basin in the north to the Nadir Seamount, south of the Guinea Fracture Zone. Data courtesy of the Republic of Guinea and TGS.

Fig. 2. Seismic characteristics of the Nadir Crater.
(A) Seabed depth map of crater showing seismic line locations and the mapped extent of the crater rim and damage zone. (B) W-E seismic section (pre-stack depth migration – depth domain) across the crater, highlighting the crater morphology and damage zone, and the extent of subsurface deformation. Data courtesy of the Republic of Guinea, TGS and WesternGeco. Stratigraphic key is on Fig. 1. (C) Detailed seismic stratigraphic and structural elements of the crater. KP, Cretaceous-Paleogene sequence (KP1 equivalent to Top Maastrichtian); KU, Upper Cretaceous seismic horizons. KU1 and KP1 “regionals” are schematic reconstructions of these seismic horizons before formation of the crater at the end of the Cretaceous and are used to reconstruct a conceptual model of crater formation (Fig. 5). (D) SW-NE seismic section (pre-stack time migration – time domain) across the crater, showing crater morphology and seismic facies outside the crater, including high-amplitude seismic facies sitting above a ~100-ms-thick unit of chaotic reflections, interpreted to have formed as a result of seismic shaking following the impact event. Data courtesy of the Republic of Guinea and WesternGeco Multiclient.

More mapping images are available in the science paper.

Numerical simulations run by team member Veronica Bray, a planetary scientist at the University of Arizona, have suggested that the impactor was about 400 meters in diameter. The arrival of such an object moving at roughly 20 kilometers per second would have produced tsunami waves more than a kilometer in height and ground shaking equivalent to that of a magnitude 7 earthquake, Bray estimated. But the mayhem that ensued, intense as it was, was mostly limited to a regional scale, said Bray. “This wasn’t a global killer.”

Computer Simulation of the Nadir Impact Event.
Hydrocode simulation of the impact of a 400m asteroid into an 800m ocean, performed by Veronica Bray at the University of Arizona. This is the best-fit simulation from our Nadir Crater discovery paper. In future, we are aiming to drill into the crater. This will allow us to confirm whether the crater is due to an asteroid impact, and to determine its age. Current estimates of its age, based on its position in the rock layers, suggest it is of similar age to the Chicxulub – Dinosaur killer – impact. But we’ll only be sure when we get that all-important drill core!

On the basis of assemblages of microfossils unearthed close to Nadir crater, Nicholson and his colleagues estimated that this feature formed at or near the end of the Cretaceous period. But it’s too simplistic to assume that a pair of gravitationally bound asteroids—a binary asteroid—formed Chicxulub and Nadir crater in a one-two punch, the authors suggested. That’s because of the extreme distance between the two sites 66 million years ago: roughly 5,500 kilometers. (They’re even farther apart now—about 8,000 kilometers—because of spreading of the Atlantic seafloor.) Binary asteroids tend to hit much closer to one another: The one example on Earth of a so-called “impact doublet” formed by a binary asteroid is characterized by craters just a little over 10 kilometers apart. “So Chicxulub and Nadir couldn’t have formed from a direct hit of a binary asteroid,” said Nicholson.

Looking to Jupiter

A more likely scenario, Nicholson and his collaborators suggested, is something akin to what happened to comet Shoemaker-Levy 9. In 1992, the roughly 2-kilometer-diameter comet had fragmented into more than 20 pieces after passing very close to Jupiter. Two years later, those fragments slammed into the gas giant over the course of several days, creating a series of dark scars that stretched across a wide swath of the planet.

Perhaps a similar breakup of a common parent asteroid occurred on Earth 66 million years ago, Nicholson and his colleagues proposed. An asteroid—there’s good evidence that the Chicxulub impact was due to an asteroid rather than a comet—orbiting Earth could have been torn apart by our planet’s gravity. Those fragments could have then dispersed sufficiently in space such that they smashed into Earth within days of one another yet in very disparate locations, the researchers suggested.

Another possibility is that one or more asteroids collided somewhere in deep space—most likely in the asteroid belt between Mars and Jupiter—and an ensemble of cosmic shrapnel traveled en masse to Earth. The result would have been an uptick in cratering that persisted not over days, as in the case of the breakup of a common parent asteroid, but over a million or so years. Scientists are aware of only one such event—known as an impact cluster—in Earth’s history, and it occurred roughly 460 million years ago. “We think an asteroid parent body broke up somewhere in the solar system and sent material flying towards Earth,” said Kenny.

The impact cluster scenario might be more likely, Nicholson and his colleagues suggested. That’s because a third large crater—the 24-kilometer-diameter Boltysh crater in central Ukraine—also dates to around 66 million years ago. Research published last year suggested that Boltysh formed just 650,000 years after the Chicxulub impact.

There’s also the possibility that Nadir crater was simply created by an unrelated impact, Nicholson and his colleagues acknowledged. Perhaps a stroke of bad cosmic luck led to Earth being pummeled in relatively close succession.

Going Deep

It’s clearly key to more precisely constrain the age of Nadir crater, Nicholson and his collaborators maintain. Right now, the uncertainty in the structure’s age is about a million years, and that’s too large to discriminate between the breakup of a common parent asteroid and impact cluster scenarios. Drilling sediment cores from the crater would allow scientists to look for stratigraphic signatures like the iridium layer from Chicxulub that could yield a much more precise date. Nicholson and his colleagues recently submitted a drilling proposal to the International Ocean Discovery Program to do just that.

Science paper:
Science Advances

See the full article here .


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