May 6, 2016

by Eric Hand

Scientists have reached ground zero for one of the world’s most famous cataclysms. Burrowing into the impact structure responsible for the demise of the dinosaurs, a team of researchers has achieved one of its main goals, with rocks brought up from 670 meters beneath the sea floor off the coast of the Yucatán Peninsula in Mexico. These core samples contain bits of the original granite bedrock that was the unlucky target of cosmic wrath 66 million years ago, when a large asteroid struck Earth, blasted open the 180-kilometer-wide Chicxulub crater, and led to the extinction of most life on the planet.

“We’re feeling pretty good,” said co–chief scientist Sean Gulick in an interview from the deck of a drilling platform 30 kilometers offshore in the Gulf of Mexico. “I’m not getting much sleep out here, so we’re little delirious.”

Although scientists have drilled into the buried crater before on land, this is the first offshore effort, and also the first to go after the crater’s “peak ring”—a circular ridge inside the crater rim that’s characteristic of the solar system’s largest impact craters. Astronomers see peak rings on the moon, Mars, and Mercury, but they have never been able to sample one on Earth until now. The team has already been charting the return of life after the worldwide die-off in cores from higher up in the hole. By examining peak ring rocks closely, they hope to test models of crater formation and determine whether the crater itself was one of the first habitats for microbial life after the impact.

The peak ring formed in a matter of minutes. Just after the impact, deep granite bedrock, flowing like a liquid, rebounded into a central tower as tall as 10 kilometers before collapsing into the circular ridge. Next, the peak ring was covered by a layer of jumbled-up rocks, called a breccia, that contains chunks of blasted-up rock and impact melt. Then, in the hours that followed, ocean tsunamis dumped huge amounts of sandy sediment in the giant hole in Earth. Further deposition would come slowly, as life returned to the seas, and layers of limestone were built up in the ensuing millions of years.

Another bone of contention for the team concerns the boundary between the Cretaceous—the last age of the dinosaurs—and the Paleogene, the period that began 66 million years ago. Traditionally, Morgan says, the K-Pg boundary, as it is known, has been defined by appearance of fossils of small shelled creatures called foraminifera. By that definition, the team crossed the K-Pg boundary last week, at a depth of 620 meters, when drillers left fossil-containing limestone layers and entered sandy tsunami deposits. But Gulick points out that the tsunami deposits and impact breccia found between 620 and 670 meters all came after the impact itself, so they could technically be considered part of the Paleogene. He suggests that scientists instead call this thick section between the Cretaceous and Paleogene an “event layer.”

The team of scientists living on board the drilling platform is now investigating the fractures and veins of minerals that precipitated out of hot solutions in the wake of the impact. Some of the minerals they’ve found suggest that, initially at least, the fluid-filled cracks were way too hot for life. But they are hoping to find signs of ancient and modern DNA. As hellish as the impact was, the team suspects that the buried peak ring itself may have been an early place for life to return, because of the nutrients in the hot fluid-filled fractures.

As of 1 May, the team has reached a depth of 700 meters. It is funded to drill through the first week of June, and hopes to go as deep as 1500 meters. As the researchers move deeper into the harder granites of the peak ring, they will core more slowly, obtaining a 3-meter core about every 2 hours. They will look for evidence that the peak ring rocks are flipped “out of order,” with deeper rocks lying on top of rocks that originally sat more shallowly and contain minerals with higher levels of shocking. This would confirm one of the main models for peak ring formation.

The drilling effort began at the beginning of April and is sponsored by the International Ocean Discovery Program (IODP). To avoid choppy ocean waters, the scientists are using a special vessel called a lift boat that has jacked itself up off the sea floor on three pylons. Morgan, who just arrived on the drilling platform over the weekend, says the mission is the culmination of years of effort that began with her first proposal to IODP in 1998. “I had this dream we would drill this impact crater many years ago,” she says. “To see this immense structure and all the people here, it’s been really amazing.”