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 sank 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 realize 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, vaporizing 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.
Sulfur surprise
The team also found a notable lack of sulfur in the crater’s rocks. About a third of the rocks surrounding Chicxulub are sulfur-rich minerals known as evaporites, but these minerals are conspicuously absent from the core sample the team drilled.
Instead, the impact seems to have vaporized the crater’s sulfur-bearing rocks, backing up past work that suggests the event released as much as 325 gigatons of sulfur. 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.”
(Sources: National Geographic)
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