Study: Dinosaurs were charbroiled after Chicxulub impact

Some 66 million years ago, an asteroid impact wiped out three-quarters of all plant and animal species on Earth, most notably taking down the dinosaurs. But exactly how that impact caused those extinctions has been the subject of fierce debate. According to a new paper published in the Journal of Geophysical Research: Biogeosciences, the impact would have produced an immense cloud of dust that superheated the upper atmosphere, charbroiling living organisms unable to take shelter and igniting widespread wildfires. As previously reported, the most widely accepted explanation for what triggered that catastrophic mass extinction is known as the “Alvarez hypothesis,” after the late physicist Luis Alvarez and his geologist son, Walter. In 1980, they proposed that the extinction event may have been caused by a massive asteroid or comet hitting the Earth. They based this conclusion on their analysis of sedimentary layers at the Cretaceous-Paleogene boundary(the K-Pg boundary, formerly known as the K-T boundary) found all over the world, which included unusually high concentrations of iridium—a metal more commonly found in asteroids than on Earth. (That same year, Dutch geophysicist Jan Smit independently arrived at a similar conclusion.) Since then, scientists have identified a likely impact site: a large crater in Chicxulub, Mexico, in the Yucatan Peninsula, first discovered by geophysicists in the late 1970s. The impactor that created it was sufficiently large (between 11 and 81 kilometers, or 7 to 50 miles) to melt, shock, and eject granite from deep inside the Earth, probably causing a megatsunami and ejecting vaporized rock and sulfates into the atmosphere. It has been estimated that the impact would have released energy over a billion times higher than the atomic bombs dropped on Hiroshima and Nagasaki in 1945. This, in turn, had a devastating effect on the global climate, leading to mass extinction. In 2022, scientists suggested that one reason so many species perished while others survived may have been because the impact occurred in the spring (at least in the Northern Hemisphere), thereby interrupting the annual reproductive cycles of many species. This latest paper combines previous simulations of the impact with geologic observations. After the initial impact, tiny droplets of rock called spherules formed when some of the rocky vapor cooled and condensed. These spherules would have generated an intense pulse of heat as they fell through Earth’s atmosphere to the surface—equivalent to an oven broiler. But scientists have long assumed that the pulse would not have been hot enough to cause wildfires or roast the Cretaceous creatures to death. Credit: Purdue University/Kelsey Lefever Credit: Purdue University/Kelsey Lefever “That much kinetic energy has to go somewhere, and eventually it is converted to heat,” said Johnson. “With the dust cloud trapping thermal radiation, it was like the surface and everything on it was being charbroiled. And that’s what killed the dinosaurs and all the other animals. It’s not the blast wave from the impact or the fireball—those don’t go very far. It’s the global ejection of this vapor plume material that makes this a global extinction event. Without the dust cloud, it would have been bad. It would have killed a lot of creatures. But it wouldn’t have been a planetary catastrophe.”