New dating of a colossal series of volcanic outpourings bolsters the idea that the Chicxulub asteroid impact had help in wiping out the dinosaurs 66 million years ago.
Using crystals embedded in lava layers, geologists have deduced the most precise timing yet for the massive Deccan eruptions that poured out hundreds of thousands of cubic kilometers of molten rock in West India. The most intense volcanic activity began about 250,000 years before the Chicxulub impact and continued for another roughly 500,000 years after it, the researchers report online December 11 in Science. The finding supports the hypothesis that climate disruptions caused by the eruptions played a major role in the dinosaurs’ extinction, says study coauthor Gerta Keller, a paleontologist at Princeton University.
“For the last 30 years it has been basically ruled as the given truth that Chicxulub caused the mass extinction,” she says. “[We’re] now finding that maybe Chicxulub was not the main cause.”
But not all experts are convinced. Paleontologist David Fastovsky of the University of Rhode Island in Kingston says fossil and geological records suggest a single sudden event probably triggered the extinction, something the drawn-out climate change caused by volcanism doesn’t fit. “Those of us who are ‘impactors’ have to keep an open mind to other possibilities,” he says. “But the question that needs to be answered is, if the asteroid is good enough, why do you need this?”
The Cretaceous Period ended with a purge of more than 60 percent of animal and plant species on Earth, including all nonbirdlike dinosaurs. Around this time, the Deccan eruptions in India belched gas and oozed more than 1.3 million cubic kilometers of lava and rock. The remnants of these eruptions, known as the Deccan Traps, encompass an area the size of Spain and are 3 kilometers high in places.Two of history’s other four mass extinctions coincide with extensive volcanic outpourings known as flood basalts that were similar to the Deccan Traps eruptions. The gases spewed from these eruptions probably caused worldwide climate devastation, including short-term cooling and a progressive increase in greenhouse gases and ocean acidification. Gases from the Deccan eruptions have been suspected as a possible cause of the Late Cretaceous extinction, but scientists didn’t know exactly when the volcanism happened relative to the extinction event.
Previous dating of the Deccan Traps used the decay of radioactive potassium atoms in the rocks. This technique produced uncertainties of more than 1.4 million years — longer than the volcanic event itself. To create a more precise timeline, Keller and colleagues instead looked at crystals called zircons, which form during volcanic eruptions.
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As zircons grow inside pools of magma, they absorb uranium atoms. Over time, the embedded uranium slowly decays into lead. By comparing the relative number of uranium and lead atoms in the crystals, scientists can estimate how long ago the zircon formed. “The crystals are like little time capsules,” says lead author Blair Schoene, a geologist at Princeton.
Flood basalts such as the Deccan eruptions don’t produce many zircons, however, and two expeditions to India left the team empty-handed. In December 2013, the researchers finally uncovered a few crystals in rock layers reddened by oxidation. The oxidation, a sign of weathering, suggests the layer was at the surface for an extended period of time.
The crystals in these weathered rocks may have been blasted from unrelated explosive eruptions, Schoene says, but would still provide a time marker for the surrounding rock. The researchers collected zircon crystals from both the oldest rock layers at the bottom of the traps and the youngest rock at the top to get a complete eruptive timescale.
The researchers’ new dating resulted in uncertainties of 85,000 years or less. These precise measurements yielded an estimate that roughly 80 to 90 percent of the Deccan outpouring took place over 750,000 years, and that the eruptions began before the extinction event.
“Millions of cubic kilometers poured out in a geologic instant,” says coauthor Samuel Bowring, a geologist at MIT. The relatively brief time frame probably meant the climate had less time to recoup between eruptions, Bowring says, amplifying the global effect. The team next plans to date individual eruptions to help reconstruct the volcanism’s exact climate impacts.
The new dating demonstrates the importance of considering the Deccan eruptions alongside the Chicxulub impact, says geoscientist Paul Renne at the University of California, Berkeley. “The outstanding question is if there hadn’t been an asteroid impact, would there have been a mass extinction anyway?”
Renne and his colleagues suggest the asteroid impact and volcanism may not have been entirely unrelated. At the Geological Society of America’s annual meeting in October in Vancouver, they proposedthat seismic waves from Chicxulub’s impact in Mexico may have circled Earth to India and triggered a massive eruption of buried magma. Renne says that future dating of the Deccan eruptions should indicate whether their hypothesis has credence.