Scientists at Purdue University say they have worked out the sequence of events that killed the dinosaurs after an asteroid struck Earth 66 million years ago, concluding that worldwide wildfires incinerated most animals within hours, with a global winter wiping out any survivors afterward.
The findings, described by lead author Professor Brandon Johnson, aim to settle a long-running scientific debate over whether the asteroid impact itself or its after-effects were responsible for the mass extinction.
"With the dust cloud trapping thermal radiation, it was like the surface and everything on it was being charbroiled," Johnson said. "And that's what killed the dinosaurs and all the other animals."
The Chicxulub impact
The Chicxulub asteroid, a space rock between six and nine miles wide, smashed into Mexico's Yucatan Peninsula 66 million years ago. Geology and impact simulations show the collision threw an enormous cloud of dust and rock vapor into the atmosphere.
Some of that rock vapor cooled and condensed into tiny droplets called spherules, which flew away from the impact crater before falling back to the surface. Earlier studies had suggested these spherules were hot enough to burn exposed animals but not hot enough to set plants alight, leading many scientists to argue the immediate aftermath of the impact was not what caused the mass extinction.
The Purdue team says it has found the missing piece. Based on evidence from layers of rock, the researchers now believe some of the rock vapor never condensed into spherules and instead lingered in the air as fine dust.
That lingering "dust blanket" would have made the surface 3.5 times warmer, the researchers say, making the falling spherules hot enough not only to kill animals directly in their path but also to spark spontaneous forest fires.
"Without fine dust, this still wouldn't have been a good day for dinosaurs," Johnson said. "It would have killed some off, but it probably wasn't enough to start many wildfires."
"With the fine dust, the Cretaceous animals received 17 times the dose of thermal radiation that is 100% lethal to humans," he said. "It was enough to ignite grass, pine needles, lichen and maybe even directly ignite wood."
Johnson said the clouds would have blocked daylight, so the surface would have looked dark to any animal that could not see into the infrared, with only a reddish glow visible from the fires. "But there really wouldn't be anything alive to do the seeing anyway, except for any animal that was able to burrow or swim or shelter in some way, to find a way to survive," he said. "It would have been terrible."
Smoke and suffocation
Not every animal would have died in the flames, according to the researchers, who say many would have choked to death on smoke instead.
Alexandria Johnson, a researcher on the study who specializes in clouds, said animals that survived the initial heat would likely have suffered lasting health effects from smoke particulates. "But when you look at the time scales of the cooking versus the inhalation risks, the cooking is much faster," she said.
A global winter follows
The researchers say their dust-blanket theory also supports another long-standing idea: that a global winter followed the fires. Once the spherules had finished falling and the wildfires burned out, the dust would have blocked warmth from the sun for decades, leaving surviving animals to starve.
To confirm the theory, the team now hopes to find geological records of the global wildfires.
Alfio Alessandro Chiarenza, a paleontologist at University College London who was not involved in the study, said it would be valuable to identify which animals survived the initial impact and what biological traits helped them do so. "This could finally tell us why some animals made it when others, like the dinosaurs, didn't," he said.
One of five mass extinctions
The Chicxulub impact is one of five events scientists describe as mass extinctions, in which a majority of the world's species died out. The End-Ordovician extinction, around 540 million years ago, wiped out roughly 85% of species, nearly all of them marine. The Late Devonian extinction, about 375 to 359 million years ago, killed off major fish groups and halted new coral reef formation for 100 million years.
The End-Permian extinction, known as the Great Dying, struck 252 million years ago and eliminated as much as 97% of species with a fossil record, making it the most severe of the five. The End-Triassic extinction, 201 million years ago, ended the dominance of large amphibians and mammal-like reptiles, paving the way for dinosaurs. The End-Cretaceous extinction, caused by the Chicxulub asteroid 66 million years ago, is the one most often linked to the dinosaurs' demise.
