What killed the dinosaurs? Scientists have long debated whether it was an asteroid that crashed into Earth 66 million years ago or a powerful wave of volcanic eruptions at that time.
Two papers published today in Science say the real answer is— both, in a catastrophic coincidence. But the two teams of researchers disagree on a key point: whether the impact from space came first and boosted the eruptions into a climate-altering, dinosaur-killing frenzy, or whether they were two unrelated disasters with remarkably bad timing for the beasts that once stalked our planet and still stomp through our minds.
After decades of arguments between asteroid advocates and volcano boosters, in 2015 some scientists suggested both might be right, because an asteroid impact in Mexico—marked by a crater named Chicxulub—may have created seismic waves that shook the planet so violently that it sped up ongoing volcanic activity under India. That magma, in a region called the Deccan Traps, exploded in sunlight-dimming eruptions that chilled the climate, and then their release of carbon dioxide would have warmed it—a whiplash few creatures could survive. The idea was eruptions and impact together may have wiped-out the dinosaurs along with nearly 70 percent of species in a mass extinction at the end of the Cretaceous period.
The papers published today, revealing newly refined dates for both the lava flows from the eruptions and traces of the asteroid impact in other rocks, were supposed to reinforce this notion. But the sets of dates—one from scientists at the University of California, Berkeley, and the other from a group at Princeton University—come from different ways of dating rocks, and they disagree. The Berkeley team, led by Courtney Sprain (a geochronologist now at the University of Liverpool), used a method called argon–argon dating on samples from lava flows in India that occurred near the end of the Cretaceous, and compared them with dates for the asteroid impact drawn from other rocks. They were able to put the date for the asteroid impact at 66.052 million years ago, give or take 8,000 years, and timed the lava dates just after that point in time. The sequence convinced them there was indeed a boost to the eruptions right after the impact, validating the asteroid-to-eruptions idea.
But the Princeton team, led by geochronologist Blair Schoene and using another method called uranium–lead dating, concluded the opposite. The techniques are equally accurate, but the uranium-lead method can identify more details. The Princeton scientists used it to compare the age of volcanic ash from the Deccan Traps lava flows to rocks found in Colorado that bore the mineral signatures of the asteroid impact. The researchers found the Deccan Traps erupted in four huge pulses, separated by quiet periods lasting 100,000 years or more. But the key finding was that the impact date fell in one of those peaceful moments of geologic time, not right before any of the pulses. That timing, they say, makes it hard to argue the impact preceded and thus caused the eruptions. “It is highly unlikely that there is a relationship between eruption rates of the Deccan Traps and the Chicxulub impact, and that the coincidence…is one of the most remarkable coincidences in Earth history,” they wrote.
Despite their differences about the primary cause, the dates are still close enough for both teams to blame a combination of the eruptions and the asteroid for the demise of the dinosaurs. “Deccan volcanism probably made the mass extinction worse and made ecosystems more susceptible to the abrupt climate changes that came with the Chicxulub impact,” Sprain says. And Schoene agrees that “the evidence for coincidence between the impact and the big pulse of extinctions is pretty strong.”
If the impact did boost the eruptions, as the Berkeley researchers conclude, then their simultaneous effect would have been calamitous and hard to disentangle in the rock record. If the impact instead happened in between eruption pulses, as the Princeton team found, then the repeated and extreme environmental changes would have been devastating, but the main extinction event was caused by the impact. So whereas attempts to single out a dino killer may have failed, for now, they do point to a conspiracy of culprits.