Tiny patches of white dot a stretch of brittle rock Brazos River in Texas. To a casual observer, the grains may appear like inconspicuous chunks of sand, but within their strange shapes lie clues to the most cataclysmic day in our planet’s history.
About 66 million years ago, a six-mile-wide asteroid crashed into the ocean off the coast of Mexico’s Yucatán Peninsula, leaving behind a 110-mile-wide crater called Chicxulub. In an instant, the trajectory of life on Earth was changed forever. The impact triggered wildfires and tsunamis thousands of miles away. Then fluctuations in global climate — a dramatic cooling period followed by a long warming period — ushered in the extinction of about 75 percent of all species, including non-avian dinosaurs.
Now a study published in the journal geology uses the tiny patches of white in Texas known as lapilli to reveal intriguing new details about what happened in the minutes after that fateful impact: The asteroid struck with so much force it instantly created a thick bed of carbonate rock below it vaporized and a superheated gas sent out a cloud that blew upward along with a curtain of rock fragments blasted from the surface.
The lapilli formed somewhere in this geological jumble of steam and dust, and then rained down on what is now Mexico, Belize. Texas and even New Jersey. “They essentially form in an instant,” says Gregor HenkesGeochemist at Stony Brook University in New York and author of the new study.
A chemical analysis found the lapilli formed as temperatures soared to about 311 degrees Fahrenheit, indicating a zone of devastation more than a thousand miles from the center of the crater within minutes, the team writes in the study .
The lapilli may also contain clues as to how much carbon dioxide remained in the atmosphere after impact, eventually leading to a period of global warming that one estimate has lasted for so long 100,000 years. The ancient waves through Earth’s ecosystems from this climatic shift remain relevant to this day.
Humans are essentially “conducting our own experiments” when we pump greenhouse gases into the sky, notes BrandonJohnson, a planetary scientist at Purdue University who was not part of the new study. “If we can understand what it did 66 million years ago, maybe we can better understand what it can do today.”
The lapilli analyzed in the new study were collected in the 1990s from a small bluff in central Texas along the Brazos River. The tiny chunks of rock have since raised many mysteries, including how they formed.
Lapilli are known in deposits of a few species volcanic eruptions, where they grow in the ash plumes when clumps of glassy fragments are held together by water. “It’s actually similar to how a hailstone might grow,” says Johnson.
Lapilli can also be found nearby some impact craters, but whether they form in the same way is less clear. Analysis of carbon and oxygen isotopes in the new study supports a previously proposed mechanism: the condensed gases from the vaporized carbonate rocks could act like lapilli glue, holding the tiny clumps together. Johnson and a colleague proposed a similar mechanism behind the formation of Lapilli on the moonwhere water is scarce.
In addition, bonds between the heavy isotopes of carbon and oxygen become rarer at higher temperatures. With this fact and a method known as clumped isotope analysisthe scientists were able to measure the temperature of the gas cloud that dissipated millions of years ago, explains the first author of the study David Burt, a Ph.D. candidate at Stony Brook University.
A challenge in this analysis is to confirm that the lapilli were not later altered, such as by heating from deep burials that would have obscured their recorded temperatures. The researchers also analyzed the carbonate shells of tiny sea creatures forums which were kept nearby at about the same time. The heavy bindings of the forams matched the expected sea surface temperatures from that period, suggesting that lapilli temperatures were also preserved.
The results suggest the tiny boulders formed at a sizzling 311 degrees Fahrenheit.
“It will be devastating for the biosphere,” says the geochemist Steven Goderis from the Vrije Universiteit Brussel, which specializes in impact craters but was not part of the study team.
Figuring out exactly how far this gaseous inferno spread from the impact site was a challenge. Researchers have long debated this exact angle and direction of the asteroid upon impact with the surface, which would help determine the areas most intensely blasted by the ejected material. Goderis notes that examining additional lapilli in Mexico could help researchers better understand variations in the spread of temperatures. But curiously, Goderis says, lapilli aren’t found in all areas around the impact crater, and scientists aren’t sure why.
Another unknown factor is when and where the lapilli formed along the impact path, the geologist notes David Kring from the Lunar and Planetary Institute, which has done extensive work at the Chicxulub impact site. “I hope that these types of studies will eventually lead us to this point,” says Kring, who was not part of the study team.
The fireball that changed the world
Previous models have suggested that the impact caused even higher atmospheric temperatures. So, in a way, the high temperatures of the Lapilli Formation are not surprising.
“What’s new is that they actually pinned a temperature to a specific type of object,” says Kring.
Some estimates suggest that the glowing gases released by the impact formed an expanding fireball that radiated so much heat that it ignited wildfires up to 1,500 km Away. Temperatures are believed to have crescendoed as the bits blasted from Debris fell back to earth. As the material “screeched through the atmosphere,” says Kring, it overheated the air and burned vast tracts of land.
Around the impact site, temperatures would have been high enough to spontaneously ignite plants. Debris also orbited the planet, concentrating on the opposite side of the world, where it likely started similar fires.
Burtt and Henkes both see the new study as a starting point. An important point for future studies concerns the pulse of carbon dioxide released when the asteroid vaporized a large amount of carbonate rock.
Lapilli formation in the vapor plume would eat up some of this carbon dioxide, potentially affecting global climate shifts in the years following the asteroid’s impact. the Mixture of emissions from the impact— including sulfur, carbon dioxide, and water vapor — sent the world on a wild swing from cooling to warming, collapsing food webs and sending countless species into a spiral of extinction. Studying how much of these climate-impacting gases were released is important to fully understand what killed so many species — something scientists are still finding out, Johnson explains.
And that aspect of history isn’t all ancient history. “There’s an anthropogenic extinction event happening right now,” Henkes says, referring to the dramatic decline in biodiversity that humans have caused through the release of greenhouse gases, land-use change, the introduction of invasive species, and more.
The current changes aren’t as sudden as an asteroid impact, he says, but the effects will reverberate through the biosphere for millennia.