UNIVERSITY PARK, Pa. — A giant, speeding asteroid crashed into a shallow sea 66 million years ago and plunged Earth into a mass extinction that claimed 75 percent of all species, including the dinosaurs, scientists have long believed.
Researchers have found new evidence that supports the theory and paints the clearest picture yet of the Cretaceous-Paleogene extinction event. Core samples taken from ground zero, the Chicxulub crater in the modern-day Gulf of Mexico, reveal what happened in the days after the massive asteroid impact, according to a recent study published in the Proceedings of the National Academy of Sciences.
“As geologists, the things we study usually take place over thousands or millions of years,” said Timothy Bralower, professor of geosciences at Penn State and a co-author on the new study. “Here we are seeing incredible changes that took place over months, including the extinction of about 75 percent of lifeforms on our planet.”
The impact liquefied rock and lifted up earth, forming a new underwater mountain chain within seconds. Molten rock spewed into the atmosphere and rained back down on the planet, the scientists said.
A massive tsunami traveled hundreds of miles from the crater, reaching to modern day Florida and Texas. When the waters rushed back into the crater, they carried charred evidence of the destruction. Bits of burned vegetation sunk to the bottom of the sea, becoming trapped in sediment layers.
Scientists with the International Ocean Discovery Program uncovered those layers in 2016 while digging core samples from the peak ring of the crater. An analysis of those samples is the basis of the new study.
“The core samples show how fast changes took place in the crater — from this massive collision to the uplifting of a mountain chain in 90 seconds to the deposition of tens of meters of rock on top of that within a matter of months,” Bralower said. “These are remarkable findings.”
Charcoal reveals clues about devastation
Bralower served on the expedition that drilled the core samples, helping the crew identify the age of the rocks brought to the surface.
Later, studying pieces of the core to learn how long it took for plankton to recover from the impact, Bralower made an unexpected discovery — pieces of charcoal.
“Imagine this rock hitting the Earth at a velocity that would have created a massive amount of thermal radiation,” he said. “A fire ball traveled from the center of the crater to the edges of the Gulf of Mexico, frying all the vegetation or animals it encountered.”
Water from the tsunami swept up the charcoal and washed it back toward the crater, where it floated down to the sea floor and became buried along with other sediments, Bralower said.
“I never expected it,” he said. “Charcoal has been very elusive at the K-Pg boundary layer. To actually find it in the crater was a big surprise.”
Shelby Lyons, a graduate student at Penn State, and her adviser, Katherine Freeman, Evan Pugh University Professor of Geosciences, analyzed charcoal samples in the study. They are continuing the research to help better identify the sources of the burned material, Bralower said.
Material from the cores can help scientists piece together the regional destruction caused by the impact, but what was missing from the samples tells another important story about the global consequences.
While rocks in the region are rich in sulfur, the chemical was absent in the cores. The findings indicate that the impact vaporized rocks containing the sulfur, allowing the chemical to enter the atmosphere, where it reflected sunlight away from Earth and led to global cooling.
“The real killer has got to be atmospheric,” Sean Gulick, a research professor at the University of Texas Institute for Geophysics and lead author on the study, said in a press release. “The only way you get a global mass extinction like this is an atmospheric effect.”
The National Science Foundation partly funded this research.