Last day of the dinosaurs' reign captured in stunning detail

Rocks from deep inside the Chicxulub impact crater show what happened in the minutes to hours after one of our planet’s most catastrophic events.Tuesday, 10 September 2019

In an instant, the Chicxulub impact forever changed life on Earth—blasting out a huge crater, vaporising and flinging up tons of rock, and devastating plant and animal life. Now, researchers have constructed a jaw-dropping timeline of the chaos by studying the rocks laid down inside the crater on that fateful day.
In an instant, the Chicxulub impact forever changed life on Earth—blasting out a huge crater, vaporising and flinging up tons of rock, and devastating plant and animal life. Now, researchers have constructed a jaw-dropping timeline of the chaos by studying the rocks laid down inside the crater on that fateful day.
photo by Illustration by Science Photo Library, Alamy Stock Photo

Inch by inch, the team pulled up the skinny core of ghostly white limestone from the ocean floor, gazing at the compressed remains of ancient organisms that died tens of millions of years ago. But then a stark divide appeared as the layers abruptly darkened.

“It was nothing like the stuff above,” recalls Sean Gulick, a co-chief scientist of the expedition and a researcher at the University of Texas at Austin.

This change in the rock marks one of the most catastrophic events in Earth’s history, some 66 million years ago – when an epic asteroid slammed into the sea just offshore of Mexico’s Yucatán Peninsula. The impact triggered a nightmarish sequence of events that sent some 75 percent of plant and animal species spiralling to extinction—including all the nonavian dinosaurs.

Now, by subjecting the rocky core to a battery of tests, including geochemical study and x-ray imaging, the research team has assembled a meticulous timeline chronicling events on that fateful day—sometimes down to the minute. As they report today in the Proceedings of the National Academy of Sciences, the dark layers reveal stunning details, including the sheer amount of material that piled up mere hours after the strike, along with bits of charcoal later left by raging wildfires.

“They can put their fingers on moments in that event,” says Jennifer Anderson, an experimental geologist who studies impact cratering at Winona State University. “The level of detail kind of blows you away.”

While it’s unlikely another asteroid smashup of this magnitude will happen in our lifetimes, significant impacts are inevitable in the larger arc of our planet’s evolution, says Purdue University’s Jay Melosh, who is not part of the study team but who worked on other sections of the crater core. Studying these events helps us more strongly grasp the vulnerabilities of life on Earth, he says.

“It’s not a matter of whether there will be big impacts,” he says, “it’s just a matter of when.”

Drilling into disaster

Previous studies have been slowly piecing together what happened after the so-called Chicxulub impact using a combination of computer models and the geological fallout found at a smattering of sites around the world. One controversial locale in North Dakota may even capture an entire ecosystem catastrophically tossed by the seismic waves that rippled out from the impact zone.

But the exact details of the chaos that ensued have been an enduring mystery, one that scientists hoped to solve by closely examining the impact crater itself. Layers of sediment had buried the crater over millennia, which prevented roaring winds and water from wearing it away, but also hid it out of reach of eager scientists. To actually touch this infamous moment in our planet’s history, researchers needed to drill.

Scientists started exploring the crater’s structure in 1996 via seismic surveys led by Joanna Morgan, who co-led the latest drilling efforts with Gulick. Along with a second expedition in 2005, that work confirmed the presence of what’s known as a peak ring—a circle of buried mountains that rapidly forms within the largest of impact craters. Such a structure is an ideal place to drill, Gulick says. Not only can it reveal the fundamental processes behind the formation of mega-craters, its elevation places it relatively close to the modern ocean floor, which means easier access.

In the spring of 2016, the team at last sunk metal teeth into the Chicxulub crater, and over the course of two months, they extracted sections of core 10 feet at a time. In total, they collected a slice of Earth about a half-mile long that captures the shocked rocks that were below the impact, layers of melted rock, and the transition back to normal seafloor sediments.

“It was amazing to be on the ship and see those cores first coming up and realise we had some really exciting things to say,” Gulick marvels.

Mounds of melted rock

The new study of that core sample combines the rocky record with computer models to create an unprecedented timeline of the geologic chaos on the day sparking the dinosaurs' demise.

“To say that we’re looking at something that happened the day the impact happened 66 million years ago, that’s a kind of resolution that we almost never see in geology,” Anderson says.

One of the most striking finds is the rate at which material was re-deposited after the impact. The asteroid strike excavated miles of ocean floor, vaporising rock and water in a flash. A ripple of shockwaves inside the crater sent solid rock flowing like liquid to form a towering peak, which then collapsed outward to form the peak ring. Just tens of minutes later, a jumble of debris piled onto the peak ring in a layer some 130 feet thick. Some of this material came from a sheet of melted rock that splashed into place within minutes as the peak collapsed.

Then, as the ocean rushed back into the yawning molten gap, pockets of steam burst forth, flinging up more fragments of rock. Within an hour, the crater was likely covered in a churning vat of rocky oceanic soup, periodically sloshed by the collapse of the crater’s steep wall.

“Just like if you pour a bucket of water into a bathtub, it doesn’t sit quiet, it sloshes around,” Melosh explains. “Each slosh as it went back and forth deposited more material.”

Rocky bits slowly settled out from the stew, piling up hundreds of feet of more debris. In total, the event laid down nearly 430 feet of new material in a single day.

Sulphur surprise

The team also found a notable lack of sulphur in the crater’s rocks. About a third of the rocks surrounding Chicxulub are sulphur-rich minerals known as evaporites, but these minerals are conspicuously absent from the core sample the team drilled.

Instead, the impact seems to have vaporised the crater’s sulphur-bearing rocks, backing up past work that suggests the event released as much as 325 gigatons of sulphur. Yet the element’s near total absence hints that even this gargantuan number may be too low. This gas could have formed a haze of sulfuric acid that blotted out sunlight and triggered years of global cooling. Or, Melosh says, it might have created acid rain that abruptly acidified the oceans. Either way, the effects would have devastated life of all kinds.

What’s more, the rock core offers clues to how the collision instantly affected life on land. Hurtling to Earth at some 45,000 miles an hour, the impact likely sent out a flash of energy that ignited landscapes within a 900 miles radius.

“Mexico was on fire immediately,” Anderson says. The impact also flung geologic shrapnel high into the skies that plummeted back around the globe, igniting fires even farther from the impact zone. And in the top few inches of the core’s sediment, the scientists found bits of charcoal, likely created by those raging wildfires.

Intriguingly, the researchers also found biomarkers from the fungal breakdown of wood, which further suggests that these burned bits came from a landscape set ablaze. The team thinks a mighty tsunami rippled across the Gulf of Mexico—and perhaps around the world—and that the watery wall bounced back after crossing the Mexican highlands, dragging with it charred terrestrial remains.

Opening salvo

There are still many more questions to answer about how the impact and its aftermath rippled around the world, says Kirk Johnson, director of the Smithsonian National Museum of Natural History. But he praises the new work for providing such a stunningly preserved record of this terrifying day.

“In a way, it’s telling us things that we thought we knew, but it’s telling it with the data that underpins it for the first time,” Johnson says.

“I consider this to be an opening salvo,” adds the University of Washington’s Jody Bourgeois, who has studied the impact’s tsunami deposits in Texas and Mexico. Further study of the core samples and other evidence in the coming years will likely fill in many more details in the tumultuous tale.

“It’s heady,” Gulick says of finally publishing the first few papers from the drilling project. “The discoveries keep coming.”

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