When the cosmos shoots pool, it plays for keeps. It sunk a 6-mile-wide rock in our pocket of the solar system 66 million years ago. The smack of the asteroid against Earth released energy on the order of billions of atomic bombs. Dinosaurs were the cataclysm’s most famous victims, joined by sea creatures, plants and microorganisms. All told, Earth’s biodiversity shrank by 75 percent in what is known as the Cretaceous-Paleogene, or K-Pg, extinction.
A large asteroid strike happens only once every 100 million years. And a controversial new report suggests the K-Pg impact was an exceptionally unlikely shot. In a paper published Thursday in the journal Scientific Reports, researchers calculated the asteroid had little more than a 1-in-10 chance of triggering a mass extinction.
Soot was the impact’s most lethal symptom, argued paleontologist Kunio Kaiho of Tohoku University and Naga Oshima, an atmospheric chemist at Japan’s Meteorological Research Institute. The asteroid hit Earth near the Yucatán Peninsula in Mexico. There, researchers say, vast reservoirs of crude oil and hydrocarbons were tucked beneath a shallow sea, waiting to be set ablaze.
Kaiho and Oshima’s previous work, published in 2016, modeled what would happen if an asteroid turned lots of organic matter into soot — millions and millions of tons of it, injected into the stratosphere. In the scenario, Earth’s temperature plunged beneath the soot cloud that blocked the sun’s radiation. Plants, trapped in this carbon choke hold, wilted and died. Starving animals soon followed.
Sixty-six million years ago, only 13 percent of Earth’s surface contained enough organic material to generate this doomsday soot, the authors concluded in the new study. Had the asteroid hit the other 87 percent of Earth, Kaiho said, “I think dinosaurs could be alive today.”
Timothy Bralower, a Penn State paleoceanographer who was not involved with the work, said he doubted that a soot cloud alone could explain why the asteroid was so lethal.
The extinction asteroid theory, widely accepted as the most plausible explanation for the dinosaurs’ disappearance, is the result of four decades of research. In the late 1970s, scientists Luis and Walter Alvarez, a father-son duo at the University of California, Berkeley, began to investigate rocks on the border between the Cretaceous and Paleogene geologic periods. The Alvarez team discovered the element iridium, at levels found only in asteroids, in Italian clay that dated to the ancient divide. Cretaceous soot, too, was mixed in with the red clay.
Iridium appeared in 66 million-year-old clay around the world. In 1990, scientists announced they’d found the entry wound. It was a giant pockmark in the Yucatán Peninsula, centered near the Mexican town Chicxulub.
Kaiho and Oshima based their soot cloud calculations on geologic layers in Haiti, near the peninsula. In the late Cretaceous, these rocks were rich in hydrocarbons. That, they said, was the ammunition the asteroid needed.
Scientists have found a diary of horrors burned into geologic layers at the time of impact. Hypothesized “kill mechanisms” include toxic heavy metals brought by the asteroid, acidic oceans and global firestorms. Hot asteroid bits would have rained down on forests and started wildfires across the planet. This might explain the soot found in the clay, Bralower said.
Bralower pointed to a paper in Geophysical Research Letters that contends that the asteroid released killer amounts of gas. Gases blown high into the atmosphere would have the opposite of a greenhouse effect, surface temperatures plummeting.
“If you cool the planet by 26 degrees Celsius in five years you’re going to cause a lot of extinction,” Bralower said. To release these climate-altering gases, the asteroid needed to hit a shallow sea above sedimentary rock. In other words, it would have had to strike a place just like Chicxulub.