What Happened the Day a Giant, Dinosaur-Killing Asteroid Hit the Earth

Science
دوشنبه 18 شهریور 1398 ساعت 21:41
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Smith sonian mag
One of the greatest scars on our planet is hidden beneath the Yucatán Peninsula and the Gulf of Mexico. The buried crater, over 90 miles in diameter, was created when a massive asteroid struck the planet 66 million years ago and brought a calamitous end to the reign of dinosaurs. Now, thanks to a new analysis of core samples taken from the crater’s inner ring of mountains, called a peak ring, geologists can create a detailed timeline of what happened on the day after impact.
The immense Chicxulub crater is a remnant of one of the most consequential days in the history of life on Earth. The asteroid strike triggered the Cretaceous-Paleogene, or K-Pg, mass extinction. The catastrophe not only decimated the dinosaurs, leaving only birds to carry their legacy, but also annihilated various forms of life from flying reptiles called pterosaurs to coil-shelled nautilus relatives called ammonites. Lizards, snakes, mammals and more suffered their own setbacks. The best clues to what happened now lie buried in rock layers stacked 12 miles deep.
Using a core sample collected in 2016, University of Texas at Austin geologist Sean Gulick and a team of dozens of other researchers have further pieced together the story of the Cretaceous-Paleogene extinction. “We interpret this section to represent the first day post impact, which by the definition of the geologic time scale, makes it the first day of the Cenozoic since the Cretaceous ended the moment the asteroid struck,” Gulick says. The team’s study, “The first day of the Cenozoic,” was published today in the Proceedings of the National Academy of Sciences .
The drill site was selected to investigate the series of events that followed the impact. When an asteroid the size of the Chicxulub impactor, estimated to be more than six miles wide, strikes a planet, material is ripped up from below the surface and tossed into the air, collapsing in circular mountain range within the crater. Such devastating upheaval triggers a cascading sequence of natural disasters, sending tsunamis rolling across the oceans and ejecting an immense amount of debris into the atmosphere.
The core sample is a geologic document stretching hundreds of feet long. Under a thin ring of overlying material is over 400 feet of melt rock that was laid down during the day following the impact.
A scientific mission led by IODP (International Ocean Discovery Program) studies rock cores from the Chicxulub impact crater in the Gulf of Mexico, May 7, 2016. (Ronaldo Schemidt/ AFP / Getty Images)
“This isn’t the first drill core from Chicxulub,” says University of New Mexico geologist James Witts, “but because of its position on the peak ring, which is essentially a range of mountains created in the moments after the impact event, it provides a really unique picture of the sort of dynamic geological processes that operated over short timescales.” An event of this scale has never occurred in human history, he adds, so the rock record is essential to parsing out the details.
Within minutes of the asteroid strike, Gulick and colleagues found, the underlying rock at the site collapsed and formed a crater with a peak ring. The ring was soon covered by over 70 feet of additional rock that had melted in the heat of the blast.
The sea battered against the new hole in the planet, and in the minutes and hours that followed, surges of water rushing back into the crater carried laid down more than 260 additional feet of melted stone atop the already accumulated rock. Then a tsunami hit. The wave, reflected back toward the crater after the initial impact displaced water across the globe, added another distinct layer of rock—sediments of gravel, sand and charcoal—all within the first 24 hours of the strike.
The planetary collision triggered wildfires inland, burning forests that were later doused by devastating waves. Debris from the charred woods washed out to sea, and some accumulated in the crater.
“What we have from drilling at ground zero is a fairly complete picture of how the crater formed and what the processes were within the crater on the first day of the Cenozoic,” Gulick says.
The impact affected life far from the site. The heat pulse would have raised temperatures over 900 miles away, Gulick says, and “at farther distances the ejecta could also have caused fires by frictional heating as it rained down in the atmosphere.”
Liftboat Myrtle, upon which the IODP/ICDP expedition to drill into the peak ring of the Chicxulub impact crater took place. (Image courtesy of the Jackson School of Geosciences, University of Texas at Austin)
The rocks that the asteroid struck were rich in sulfur, which was ejected and vaporized, mixing with water vapor and creating what Gulick calls a sulfate aerosol haze. Geologists had detected and studied this effect before, but the new research reinforces the role this atmospheric disruption played in the extinction that followed.
“Our results support this scenario where first you burned parts of the continents, and then you had global dimming of the sun and plummeting temperatures for years to follow,” Gulick says. These events account for the loss of 75 percent of known species at the end of the Cretaceous. Had the impact occurred elsewhere, or in a place of deeper ocean water, the extinction may have happened differently, or not at all.
Cores from Chicxulub crater reveal the planet-wide devastation that the large impactor caused, but the timing of these events will likely spur debate and discussion, Witts says. “The complication with relating individual deposits in the core to specific types of events is that clearly the crater wasn’t a static environment after formation,” Witts says, meaning that earthquakes, waves and other events have altered the rock record over the course of 66 million years. Still cores like the one taken from the peak ring show that we can get a close-up look at short-term events in the rock record, down to minutes, hours and days.
Scientists knew the first day of the Cenozoic started with a bang, and now they have a better sense of the fallout.
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