The rocks below a famous crater

Scientists drilling into the heart of the Chicxulub impact crater in the Gulf of Mexico have discovered 130 metres of sediments laid down within hours after the site was struck by the asteroid widely believed to have killed off the dinosaurs.

In part, it’s exciting because of the link to the dinosaurs. But it also gives geologists a chance to watch how events unfolded on a time scale of minutes to hours, says Sean Gulick, a geophysicist at the University of Texas, Austin, as opposed to thousands or millions of years, “which is what normal geology would look like”.

The Chicxulub crater was formed 66 million years ago when a 10-kilometre-wide asteroid or comet ploughed into the ocean near what is now Mexico’s Yucatan Peninsula.

In 2016, Gulick co-led a team from the International Ocean Discovery Program (IODP) that drilled into the 200-kilometre wide crater in an effort to better understand its history.

The site they chose was a portion of the now-buried crater’s peak ring, formed when the impact caused rock from deep beneath the surface to splash upward, forming a plateau near the crater’s centre. 

However, because the ocean at that time was hundreds of metres deep, the peak ring never rose above sea level. 

Not that the impact zone was immediately submerged. Initially, the blast drove the water away, leaving a zone of molten rock known as impact melt – now solidified into lava. 

But soon, the water came rushing back. At first, Gulick says, it hit the impact melt and exploded into steam, creating about 10 metres of shattered rock, just above the now-solidified impact melt.

That was followed by 80 to 90 metres of gravel-like sediments, with the larger gravel at the bottom and the smaller at the top. The only way that could have happened, he says, is if the waters rushed back so quickly that they were still full of rocks from the blast – rocks that then settled to the bottom: big ones first, smaller ones later. 

There are also signs, he says, that the water sloshed around within the crater, like bathwater in a tub. Then came a 10-centimetre layer of gravel-sized material that appears to have been created by the disturbance of the sea floor by a fast-moving wave: i.e., a tsunami.

Gulick thinks it was created when the outrushing waters from the impact reflected off the nearest landmass – which at the time would have been mountains in central Mexico, 800 kilometres away – then came back to deposit sediments on top of the 130 metres of rocks already deposited in the aftermath of the impact.

Support from this, he says, comes from the fact that these deposits contain perylene, a chemical made only in soils. That, he says, “would require land, somewhere, to have been touched by water that then came rushing back”.

None of this means the Chicxulub impact killed the dinosaurs. Others have argued that climate-changing volcanism in India may instead have been the culprit.

But Gulick’s samples also contain charcoal in the layers directly above the tsunami deposits, suggesting that the impact may have set off massive wildfires. “We knew impacts can make wildfires,” he says. “But this is direct evidence that this happened at ground zero.”

In addition, the rocks returning to the crater after the impact were low in sulfur, even though geologists knew that about one-third of the ones in the impact area initially contained sulfur-rich minerals like gypsum or anhydrite. 

The sulfur from these rocks must therefore have been vaporised by the impact, Gulick says. 

And when it mixed with vaporised ocean water, it would have filled the upper atmosphere with hundreds of gigatons of sulfate aerosols, creating a bright haze that would have dropped global temperatures by more than 25 degrees Celsius, “putting most of the world below freeing for most of the year” – and possibly lasting for “a decade or two”.