Of all the places in the world an asteroid could have walloped ancient Earth, the Yucatán Peninsula was possibly the worst.
That’s the premise of a new study examining what happened 66 million years ago, after a 7.5-mile-wide asteroid crashed into the ocean near what’s now the port town of Chicxulub, Mexico. The impact brought the age of dinosaurs to an abrupt end, wiping out the vast majority of the iconic beasts along with about three-quarters of all life on Earth.
According to the paper, this mass extinction happened because the space rock slammed into an oily tinderbox, blasting enough soot into the atmosphere to cause extreme global cooling.
The impact chilled the planet by a global average of 14 to 18 degrees Fahrenheit, with a drop of 18 to 29 degrees over land, the study finds.
Only 13 percent of Earth’s surface is made up of rocks that could have burned off that much soot, the team argues this week in Scientific Reports. That means if the asteroid had landed almost anywhere else, the nonavian dinosaurs may not have died out after all.
“This is a fascinating paper that … argues that even given the large size of the impactor, the mass extinction itself was of low probability,” says Paul Chodas, manager of the Center for Near Earth Object Studies at NASA’s Jet Propulsion Laboratory.
“We have often remarked on how unlucky this massive impact was for the dinosaurs, and how lucky it was for us, as the top of the mammal family, but now we have a measure of just how unlucky the dinosaurs were and how lucky we were!”
Lead author Kunio Kaiho estimates that the Chicxulub impact burned enough oil-rich sedimentary rocks to inject around 1.7 billion U.S. tons of fine-particle black carbon into the atmosphere, or enough soot to fill a covered baseball stadium.
Though rain would have quickly washed most of the low-drifting soot from the sky, about 385 million tons would have remained circulating high in the atmosphere, blotting out life-giving sunlight. (Read about the theories for dinosaur extinction proposed before the Chicxulub impact.)
Kaiho made his estimations based on a reconstructed map of areas that could have been rich in hydrocarbon-bearing sedimentary rocks at the end of the Cretaceous. Generally coastal, these areas more or less align with modern oil- and gas-bearing regions.
Kaiho previously looked at post-impact soot in rock layers around the world. He found that soot from samples taken in Haiti, relatively close to the Chicxulub impact crater, resembles soot in samples from Spain, thousands of miles distant.
“The [similarities] indicate a single source of the soot, which suggests that it was sourced from the target rocks of the Chicxulub asteroid impact,” Kaiho says. “The amount of hydrocarbon in sedimentary rocks at the impact site could have decided cooling levels across land and ocean.”
The prevailing theory for why soot appears in many places in the fossil record from this time—though not everywhere—is from widespread wildfires caused by superheated rock falling back to earth post-impact.
Kaiho says his recent work refutes that idea, and that ground fires alone could not have generated enough high-flying soot to create a global cooldown. He adds that asteroid-generated soot would not have been evenly distributed, which fits with data that suggests the Northern Hemisphere experienced more severe cooling, while the planet’s southern half recovered sooner.
Sulfur, Not Soot?
But there’s one problem with Kaiho’s new work: Recent drilling into the actual rocks of the Chixculub impact crater didn’t turn up very many hydrocarbons.
Any immediate cooling was more likely due to vaporized sulfur, not soot, says Sean Gulick, a University of Texas at Austin geologist who has been part of expeditions to drill rock samples from the underwater portions of the Chicxulub crater.
In a separate study published last week, expedition co-lead Joanna Morgan found that the impact likely released around 325 gigatons of sulfur, more than enough to temporarily chill the planet, and that’s probably a conservative estimate.
Gulick notes that the soot from Haiti—400 miles away from Chicxulub—could still have been deposited by ground fires, and that upcoming analysis of the Chicxulub cores will help tell that story more clearly.
He does, however, accept Kaiho’s basic premise: The asteroid hit in a hugely unlucky spot. Other big asteroids have hit Earth in the past, leaving their marks in the Chesapeake Bay and in western Bavaria in Germany, for instance. But they caused no mass extinctions as far as the fossil record shows—probably because those impact sites just didn’t have the right mix of volatile rocks.
“There are relatively rare areas of the planet that you can drop a 12-kilometer asteroid on and get the same level of atmospheric change,” Gulick says.
And whether the culprit was sulfur or soot, Kaiho’s work can still be useful in testing climate models that simulate change in ancient Earth.
“We can run these single factors through the models—what does it look like if there’s a huge release of sulfur, or soot, or carbon dioxide—and test those questions about the chemistry of our atmosphere,” Gulick says.
“The ability to do that is of huge importance to testing the effects of modern-day climate change.”