Scientists gear up to drill into ‘ground zero’ of the impact that killed the dinosaurs

 Artist's reconstruction of Chicxulub crater soon after impact, 66 million years ago.  CREDIT: DETLEV VAN RAVENSWAAY/SCIENCE SOURCE

Artist’s reconstruction of Chicxulub crater soon after impact, 66 million years ago. CREDIT: DETLEV VAN RAVENSWAAY/SCIENCE SOURCE   This month, a drilling platform will rise in the Gulf of Mexico, but it won’t be aiming for oil. Scientists will try to sink a diamond-tipped bit into the heart of Chicxulub crater—the buried remnant of the asteroid impact 66 million years ago that killed off the dinosaurs, along with most other life on the planet. They hope that the retrieved rock cores will contain clues to how life came back in the wake of the cataclysm, and whether the crater itself could have been a home for novel microbial life. And by drilling into a circular ridge inside the 180-kilometer-wide crater rim, scientists hope to settle ideas about how such “peak rings,” hallmarks of the largest impact craters, take shape.

“Chicxulub is the only preserved structure with an intact peak ring that we can get to,” says University of Texas, Austin, geophysicist Sean 
Gulick, co–chief scientist for the $10 million project, sponsored by the International Ocean Discovery Program (IODP) and the International Continental Scientific Drilling Program. “All the other ones are either on another planet, or they’ve been eroded.”

At the end of March, a specially equipped vessel will sail from the Mexican port of Progreso to a point 30 kilometers offshore. There, in water 17 meters deep, the boat will sink three pylons and raise itself above the waves, creating a stable platform. By 1 April, the team plans to start drilling, quickly churning through 500 meters of limestone that were deposited on the sea floor since the impact. After that, the drillers will extract core samples, in 3-meter-long increments, as they go deeper. For 
2 months, they will work day and night in an attempt to go down another kilometer, looking for changes in rock types, cataloging microfossils, and collecting DNA samples (see figure, below). “We’ve got one shot to try and get this down to 1500 meters,” says David Smith, the IODP operations manager at the British Geological Survey in Edinburgh, U.K.



Although this is the first offshore attempt to drill into the crater, roughnecks have sunk wells into it on land—even before scientists knew a crater was there. In the 1950s, geologists for Pemex, Mexico’s national oil company, conducted gravity and magnetic surveys of the Yucatán Peninsula and were intrigued to see underground circular structures—possible oil traps. They drilled several exploratory wells but lost interest when they got volcanic rocks instead of oil-bearing sediments. “When they found the igneous rocks, they said, ‘Oh, this is a volcanic center,’” says Alan Hildebrand, a geologist at the University of Calgary in Canada.

In 1980, however, Nobel laureate Luis Alvarez and others called attention to a thin layer of iridium—possible material from an asteroid—found all over the world in rocks from the time of the dinosaur extinctions. It was the signature, they said, of a previously unsuspected cause of the extinctions: a giant impact. In 1991 Hildebrand and colleagues fingered the village of Chicxulub as the site of the cataclysm, finding quartz crystals shocked by an impact in samples from the Pemex wells—samples that had sat around for more than a decade. “Some people are a little embarrassed about that these days,” he says.

The data from the Pemex wells were spotty, and so scientists have always wanted to go back for a detailed look at the impact and its aftermath, says co–chief scientist 
Joanna Morgan of Imperial College London. “It seems like a lifetime’s ambition coming true,” says Morgan, who first proposed a scientifically cored well to the IODP in 1998. Although offshore drilling is expensive, she says that working at sea means the team will face fewer hassles with environmental permitting and won’t have to cope with the Yucatán’s poor roads. In 2005, Morgan and Gulick led a $2 million remote-sensing campaign that used small seismic explosions to help illuminate the subterranean structures and pinpoint the best spot to reach the peak ring.

As the drill approaches the crater, 
800 meters down, scientists expect to find fewer species of the shell-producing animals that make up the limestone, because life was just recovering from the impact. Some scientists think that carbon dioxide released by the impact would have acidified the oceans, contributing to the extinctions, so the drill team will look at whether seafloor animals just after the impact were species that tolerate low pH.

Just above the crater lies an impact layer, 100 meters or more thick, that would have been deposited in the weeks after the cataclysm. At its base, scientists expect to find a hodgepodge of chunks of bedrock blasted up by the impact and once-molten rock that fell back into the crater in the minutes after impact. Above that would be sediments, since hardened into rock, that were swept in as the ocean rushed into the vast new depression. The impact layer may be capped by hardened deposits of ash that persisted in the atmosphere for weeks or more before falling out.



For many of the IODP scientists, the main event will be reaching the peak ring. Peak rings abound on the moon, Mercury, and Mars. But on Earth, there are just two craters larger than Chicxulub that should also have peak rings: the 2-billion-year-old Vredefort crater in South Africa, and the 1.8-billion-year-old Sudbury crater in 
Canada—yet they are so old that the peak rings have eroded away.

The IODP team wants to test a leading model for peak ring formation, in which granite from Earth’s depths rebounds after a major impact, like water struck by stone, to form a central tower, taller than the crater rim. In minutes, the tower would collapse and collide with material slumping in from the rims to form the peak ring. Confirmation for the model could come from finding rocks “out of order”: deep rocks, probably granite, brought up in the central tower, lying atop originally shallower younger rocks. “They’re going to test whether our numerical models are making any sense or not,” says Jay Melosh, a planetary scientist at Purdue University in West Lafayette, Indiana, who helped develop the model.

The team is interested not just in the structure of the peak ring rocks but in what life they might host. Remote sensing has already suggested that the peak ring is less dense than expected for a granite—a sign that the rocks are porous and fractured in places. It is possible that these fractures, in the wake of the impact, were filled with hot fluids. “Those will be preferred spots for microbes to grow, but it depends whether the fractures have energy and nutrients,” says Charles Cockell, an astrobiologist on the IODP team at the University of Edinburgh. When the drill bit encounters mineral veins or other fracture zones in the peak ring, Cockell and his colleagues will take a subcore from the core: a biopsy on the geopsy. They will count and culture any microbes still living in the fractures, and sequence DNA to look for the genes responsible for metabolic pathways.

Those genes might show that peak ring microbes—descendants of those that lived after the impact—derive their energy not from carbon and oxygen, like most microbes, but from iron or sulfur deposited by hot fluids percolating through the fractured rock. And that would mean the impact crater, harbinger of death, was also a habitat for life.

DOI: 10.1126/science.aaf4138



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