New insights about the asteroid thought to have killed off the dinosaurs suggest it may have just been the final blow, and that the reptiles were already suffering from a finicky climate prompted by volcanic eruptions long before the meteorite struck.
"The [asteroid] impact was the coup de grace," Paul Renne, a geologist at the University of California, Berkeley, said in a statement.
The research, detailed in the February 8 issue of the journal Science, adds to the ongoing scientific debate over what exactly killed off the dinosaurs.
That debate, which once revolved around the question of whether the culprit was an asteroid or volcano-induced climate changes, has evolved to consider the possibility that perhaps multiple environmental factors were involved.
Renne and his team recently determined the most precise date yet for the asteroid strike, which occurred in the Yucatán Peninsula in what is now Mexico.
Using a high-precision dating technique on tektites—pebble-sized rocks formed during meteorite impacts—from Haiti that were created during the event, the team concluded that the impact occurred 66,038,000 years ago—slightly later than previously thought.
When error limits are taken into account, the new date is the same as the date of the extinction, the team says, making the events simultaneous.
Renne said the new findings should lay to rest any remaining doubts about whether an asteroid was a factor in the dinosaurs' demise.
"We have shown that these events are synchronous to within a gnat's eyebrow," he said, "and therefore the impact clearly played a major role in extinctions."
That is not to say, however, that the asteroid—which carved out the so-called Chicxulub crater—was the sole cause of the dinosaurs' extinction.
Evidence now suggests massive volcanic eruptions in India that predated the asteroid strike also played a part, triggering climate changes that were already killing off some dinosaur groups.
For example, "nobody has ever found a non-avian dinosaur fossil exactly at the impact layer," Renne said in an email. "Hence, strictly speaking, the non-avian dinosaurs"—those dinosaurs unrelated to birds—"may have already gone extinct by the time of the impact."
Death From the Skies
The idea that volcanism was responsible for the dinosaurs' demise actually predates the impact theory, and it fits well with what is known about Earth's other mass extinction events.
"Many of the other mass extinctions have been found to co-occur with large-scale volcanic eruptions," said Heiko Pälike, a paleoceanographer at the University of Bremen in Germany.
But in the 1980s, father-son team Luis and Walter Alvarez, a physicist and planetary scientist, respectively, presented a bold new theory.
After discovering that a layer of clay that's found throughout the world and that coincided with the end of the Cretaceous period is enriched in iridium—an element rare on Earth but common in space rocks—they proposed that a meteorite wiped out the dinosaurs.
"As the impact theory took hold, especially with the more physical scientists ... the volcanists lost ground," Renne explained.
The impact theory gained further momentum in the 1990s, when scientists discovered a 180-kilometer wide impact crater in the Yucatán Peninsula that dated to the boundary between the Cretaceous and Tertiary periods—the so-called KT boundary—when the dinosaurs disappeared.
The crater's size indicated that whatever created it was roughly 10 kilometers in diameter.
An asteroid of that size striking the Earth would have had devastating consequences, including destructive pressure waves, global wildfires, tsunamis, and a "rain" of molten rock reentering the atmosphere.
Additionally, "much additional particulate matter would have stayed afloat in the atmosphere for weeks, months, perhaps years, blocking incoming solar radiation and thus killing plant life and causing catastrophic drops in temperatures," explained Hans-Dieter Sues, a paleontologist at the Smithsonian National Museum of Natural History in Washington, D.C.
Hybrid Theory for Dinosaur Extinction
The once-abandoned volcanism theory has seen a revival of sorts in recent years, however, as a result of fresh insights about a period of sustained ancient volcanic activity in India and the discovery that dinosaur diversity may have already been declining before the asteroid strike.
The debate now is "whether the Chicxulub impact was the 'smoking gun,' as many researchers claim," Sues said, "or one of several causative factors, kind of like 'Murder on the Orient Express.'"
Renne belongs to the camp that thinks a series of volcanic eruptions in India that produced ancient lava flows known as the Deccan Traps caused dramatic climate variations, including long cold snaps, that may have already been culling the dinosaurs before the asteroid struck.
"It seems clear that volcanism alone, if on a sufficiently massive and rapid scale, can trigger extinctions," Renne said. "Thus my view that the impact was probably the final straw, but not the sole cause."
The new hybrid theory still has some major questions it must answer, however, like precisely how much the Indian volcanic eruptions affected the dinosaurs.
"Some people say if you look at the eruption of Mount Pinatubo [in 1991], it cooled the Earth for a short period of time due to the aerosol and the dust that was ejected," Pälike said.
But "others say in the long run volcanoes probably pump more carbon dioxide into the atmosphere and actually warm the planet, at least temporarily."
It's also unclear how the Deccan Traps eruptions were spread out in time. "We know that they started a few million years before the end of the Cretaceous and lasted for several million years after, extending even beyond the [asteroid impact]," Pälike said.
"However, some people have suggested that there were clusters of eruptions that happened within a span of a few tens of thousands of years."
Knowing the timing of the eruptions is important, Pälike added, because if they were happening close to the end of the Cretaceous, it's more likely they played a role in the dinosaurs' extinction than if most of the eruptions happened two million years before.
Pälike thinks that more precise dating of the volcanic ash layers in India could help answer some of the remaining questions: "That's the next step of the puzzle."
Pinning down the cause of the dinosaurs' extinction isn't just of academic interest, said Jonathan Bloch, associate curator of vertebrate paleontology at the Florida Museum of Natural History at the University of Florida.
"It's important for us to understand how ecosystems respond to big perturbations," Bloch said, "whether it's gradual climate change or a catastrophic event. These are all things we have to think about as humans on the planet today."