Sixty-six million years ago, life on our planet was going on just as it had on any other day – enormous reptiles dominated the landscape. Herbivorous behemoths up to 40 metres long co-mingled with bi-pedal carnivorous titans, seas teemed with fanged leviathans and the skies were navigated by leathery-winged creatures larger than any birds in history.
But in moments, after 180 million years of prosperity, this extraordinary abundance of life was all but obliterated.
The impact set alight vast wildfires stretching thousands of miles, triggered towering tsunamis and blasted so much sulphur into the atmosphere it blocked the sun, causing the catastrophic global cooling that ultimately doomed the dinosaurs.
This cataclysmic scenario is what scientists have largely hypothesised ended the reign of the fearsome creatures.
Now, a new study led by the University of Texas at Austin has confirmed the theory by finding hard evidence in the hundreds of feet of rocks that filled the impact crater within the first 24 hours after impact.
Core samples taken at the crater also contain charcoal and jumbles of rock brought in by the tsunami’s backflow. However, sulphur is conspicuously absent. They are all part of a rock record that offers the most detailed look yet into the aftermath of the catastrophe which wiped out 75 per cent of life on Earth.
Sean Gulick, a research professor at the University of Texas Institute for Geophysics (UTIG), who worked with an international team of more than two dozen scientists, said the research “tells us about impact processes from an eyewitness location”.
“It’s an expanded record of events that we were able to recover from within ground zero,” he said, describing it as a short-lived inferno at the regional level, followed by a long period of global cooling.
The asteroid “fried them, then [it] froze them. Not all the dinosaurs died that day, but many dinosaurs did.”
The research, published in the Proceedings of the National Academy of Sciences, builds on earlier work by the Jackson School of Geosciences which described how the crater formed and how life quickly recovered at the impact site.
Most of the material that filled the crater within hours of impact was produced at the impact site or was swept in by seawater pouring back into the crater from the surrounding Gulf of Mexico.
In just one day about 425 feet of material was deposited – a rate that’s among the highest ever encountered in the geologic record.
This breakneck rate of accumulation meant the rocks recorded what was happening in the environment within and around the crater in the minutes and hours after impact and provide clues about the longer-lasting effects of the asteroid strike.
The research details how the blast from the impact ignited trees and plants stretching thousands of miles away and triggered a massive tsunami that reached as far inland as Illinois (over 500 miles).
Inside the crater, as well as charcoal, the research team found a chemical biomarker associated with soil fungi within or just above layers of sand that shows signs of being deposited by resurging waters. This suggests the charred landscape was deluged by the tsunami, then pulled into the crater as the floodwaters retreated.
Jay Melosh, a Purdue University professor and expert on impact cratering, said finding evidence for wildfire helps scientists know their understanding of the asteroid impact is on the right track.
“It was a momentous day in the history of life, and this is a very clear documentation of what happened at ground zero,” said Professor Melosh, who was not involved with this study.
One of the most important findings from the research is the lack of sulphur in the core samples. The area surrounding the impact crater is full of sulphur-rich rocks. But there was no sulphur in the core.
The discovery supports the theory the asteroid impact vaporised the sulphur-bearing minerals present at the impact site and released it into the atmosphere, where it wreaked havoc on the Earth’s climate, reflecting sunlight away from the planet and causing global cooling.
Researchers estimate at least 325 billion metric tons of sulphur would have been released by the impact. That’s about four orders of magnitude greater than that spewed out during the 1883 eruption of Krakatoa – an event which cooled the Earth’s climate by an average of 1C for five years.
Although the asteroid impact created mass destruction at the regional level, it was this global climate change that caused the mass extinction, killing off the dinosaurs along with most other life on the planet at the time.
“The real killer has got to be atmospheric,” Professor Gulick said. “The only way you get a global mass extinction like this is an atmospheric effect.”
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