What killed off the dinosaurs? The answer has seemed relatively simple since the discovery a few decades ago of a large impact crater in the Gulf of Mexico. It pointed to a massive asteroid strike 66 million years ago that unleashed towering tsunamis and blotted out the sun with ash, causing a plunge in global temperatures.
But the asteroid wasn’t the only catastrophe to wallop the planet around this time. Across what is India today, countless volcanic seams opened in the ground, releasing a flood of lava resembling last year’s eruptions in Hawaii—except across an area the size of Texas. Over the course of 1 million years, the greenhouse gases from these eruptions could have raised global temperatures and poisoned the oceans, leaving life in a perilous state before the asteroid impact.
The timing of these eruptions, called the Deccan Traps, has remained uncertain, however. And scientists such as Princeton University’s Gerta Keller have acrimoniously debated how much of a role they played in wiping out 60% of all the animal and plant species on Earth, including most of the dinosaurs.
That debate won’t end today. But two studies published in Science have provided the most precise dates for the eruptions so far—and the best evidence yet that the Deccan Traps may have played some role in the dinosaurs’ demise.
There’s long been evidence that Earth’s climate was changing before the asteroid hit. Some 400,000 years before the impact, the planet gradually warmed by some 5°C, only to plunge in temperature right before the mass extinction. Some thought the Deccan Traps could be responsible for this warming, suggesting 80% of the lava had erupted before the impact.
But the new studies counter that old view. In one, Courtney Sprain, a geochronologist at the University of Liverpool in the United Kingdom, and colleagues took three trips to India’s Western Ghats, home of some of the thickest lava deposits from the Deccan Traps. They sampled various basaltic rocks formed by the cooled lava. The technique they used, called argon-argon dating, dates the basalt’s formation, giving a direct sense of the eruptions’ timing.
The researchers’ dates suggest the eruptions began 400,000 years before the impact, and kicked into high gear afterward, releasing 75% of their total volume in the 600,000 years after the asteroid strike. If the Deccan Traps had kicked off global warming, their carbon dioxide (CO2) emissions had to come before the lava flows really got going—which, Sprain adds, is plausible, given how much CO2 scientists see leaking from modern volcanoes, even when they’re not erupting.
The dates, and the increase in lava volume after the impact, also line up with a previous suggestion by Sprain’s team, including her former adviser, Paul Renne, a geochronologist at the University of California, Berkeley, that the two events are directly related: The impact might have struck the planet so hard that it sent the Deccan Traps into eruptive high gear.
The second study used a different method to date the eruptions. A team including Keller and led by Blair Schoene, a geochronologist at Princeton, looked at zircon crystals trapped between layers of basalt. These zircons can be precisely dated using the decay of uranium to lead, providing time stamps for the layers bracketing the eruptions. The zircons are also rare: It was a full-time job, lasting several years, to sift them out from the rocks at the 140 sites they sampled.
The dates recovered from the crystals suggest that the Deccan Traps erupted in four intense pulses rather than continuously, as Sprain suggests. One pulse occurred right before the asteroid strike. That suggests the impact did not trigger the eruptions, he says. Instead, it’s possible this big volcanic pulse before the asteroid impact did play a role in the extinction, Schoene says. “It’s very tempting to say.” But, he adds, there’s never been a clear idea of how exactly these eruptions could directly cause such extinctions.
Though the two studies differ, they largely agree on the overall timing of the Deccan eruptions, Schoene says. “If you plot the data sets over each other, there’s almost perfect agreement.”
This match represents a victory, says Noah McLean, a geochemist at the University of Kansas in Lawrence, who was not involved in either study. For decades, dates produced with these geochronological techniques couldn’t line up. But improved techniques and calibration, McLean says, “helped us go from million-year uncertainties to tight chronologies.”
Solving the mystery of how the dinosaurs died isn’t just an academic problem. Understanding how the eruptions’ injection of CO2 into the atmosphere changed the planet is vital not only for our curiosity about the dinosaurs’ end, but also as an analog for today, Sprain says. “This is the most recent mass extinction we have,” Sprain says. Teasing apart the roles of the impact and the Deccan Traps, she says, can potentially help us understand where we’re heading.
Source: Science Mag