It’s one of the greatest whodunits of all time: What killed the dinosaurs, along with three-quarters of all other species on Earth?
We never tire of the story of this grisly extinction. Perhaps the demise of the dinosaurs gives us a sense of geologic schadenfreude. Or perhaps it’s just the opposite: If it happened to them, could we meet a similar fate?
Scientists thought they had cracked the case in the 1990s, after discovering a vast crater in Mexico. It seemed to confirm the idea that a 6-mile-wide rock smashed into the Earth and plunged the planet into a nuclear winter. Based on fossil records, many species disappeared immediately after the impact.
But some researchers have pointed the finger at another culprit: a spurt of intense volcanism in India that released gases into the atmosphere, potentially changing the climate and stressing ecosystems long before the fatal collision.
The volcano hypothesis has been gaining traction, and many investigators now think both events may have contributed to the cataclysm approximately 66 million years ago.
“There’s such a tight coincidence now between the volcanism and impact, it’s very hard to tell them apart,” said Paul Wignall, a professor of paleoenvironments at the University of Leeds in England. “Which is why the debate is still going.”
Two new studies, published this week in Science, provide fresh clues to this unsolved mystery. They attempt to pin down the timing of the eruptions to uncover whether they played a significant role in the mass extinction.
Broadly speaking, both studies find that volcanism was going strong when the die-off happened, and they offer various accounts about how it might have impacted life.
But neither finds a smoking gun.
In fact, the studies disagree on crucial details — such as whether there was a big burst of lava right before the asteroid impact, or shortly after it.
“One of them is wrong,” said Wignall, who was not involved in either effort. “They can’t both be right.”
Above all, the studies illustrate just how hard it is to solve a geologic cold case.
Scientists are trying to reconstruct events that unfolded over thousands of years, millions of years ago, said Will Clyde, a geologist at the University of New Hampshire who was not involved in the studies. “In that context, it’s kind of amazing we are even asking these questions.”
The eruptions in question did not come out of a single volcano, like Mount St. Helens in Washington. They were more like the Columbia River Basalts of the Pacific Northwest, where magma gushed through cracks in the Earth’s crust, unleashing flood after flood of molten rock.
These kinds of eruptions occurred during four of the five mass extinctions in the last 600 million years. That made them prime suspects, even though scientists don’t fully understand how volcanoes wipe out life. (The event that killed off the dinosaurs, on the other hand, is the only one linked to an asteroid impact.)
Eruptions probably do damage mainly by changing the climate. They release sulfur compounds that can cool the planet for years, or even centuries. They also release massive amounts of carbon dioxide, which causes global warming that lasts for many thousands of years. And, as we are learning today, high concentrations of CO2 can also lead to other problems, such as ocean acidification.
But were the events in India potent enough to help bring down the dinosaurs and their contemporaries?
Part of the answer may lie in the resulting volcanic rocks. Known as the Deccan Traps, they cover an area the size of Texas in western India. In some places, they’ve been carved into rugged mountains whose striped faces reveal the layers of lava from which they are built.
For the new studies, both teams of researchers collected samples from these layers. They wanted to figure out when the eruptions occurred, and whether the order of events suggested that the volcano had any part in causing the mass extinction.
The two groups used different techniques, just as one detective might study analyze video from a security camera to place a suspect at a crime scene, while another might scour his or her phone records.
Schoene’s team, which includes his Princeton colleague Gerta Keller — one of the staunchest proponents of the volcano hypothesis — relied on crystals called zircons. They form in magma before it erupts, and they incorporate atoms of uranium as they grow. Because uranium is radioactive, some of it decays into lead. And because scientists know how long that takes, the amount of lead in a zircon gives an estimate of its age.
The eruptions that created the Deccan Traps didn’t produce many zircons. But other volcanoes in the region did, and the crystals wound up in layers of sediment and ash that collected between lava flows. This allowed Schoene’s group to put bounds on when individual basalt layers formed.
The geologists estimated that the Deccan Traps began erupting about 300,000 years before the extinction, and continued for about 600,000 years afterward, perhaps slowing the recovery of life. The researchers also concluded that the layers of rock were produced in three huge pulses of volcanism, one of which occurred just before the fateful asteroid impact.
“Does that mean that that pulse really helps drive the extinction event? We don’t know, but it certainly is suspicious,” Schoene said.
Based on other geologic records of what the planet was like back then, it’s clear that the climate started changing at approximately at the same time that the Deccan Traps began erupting. But it’s unclear whether the pulse that immediately preceded the extinction had any effect. That will require more research, he said.
Schoene’s results do seem to rule out one recently proposed idea: that the asteroid impact invigorated volcanism at the Deccan Traps and beyond. But the other study, led by Courtney Sprain, a geoscientist at the University of Liverpool in England, does not.
Sprain’s team employed a different method to date the lava flows. The technique is similar to the uranium-lead approach that Schoene used, except that it’s based on potassium and its radioactive offspring, argon. Also, the researchers studied a mineral in the basalt called plagioclase, not zircon.
Argon dating gives less precise ages, but Sprain’s results generally agree with Schoene’s. However, there are some important differences.
Instead of several big pulses of volcanism, Sprain’s and her colleagues saw evidence for fairly constant eruption rates.
They also found that 75% of the lava erupted after the extinction.
That may be consistent with the idea that the asteroid impact stimulated more volcanic activity, which Sprain and her coauthors have previously suggested. But the result complicates the question of whether the Deccan Traps played much of a role in killing off life.
Sprain said it’s possible that that volcanism began releasing climate-altering gases long before the bulk of the lava actually erupted.
“We still do see a very clear coincidence in time between the onset of Deccan volcanism and the onset of climate change,” she said. “That’s kind of leading us to believe that it still was involved, but that our assumptions of how gas is released in these systems needs to be modified.”
Sprain’s group and Schoene’s plan to collaborate to try to reconcile their results. “We agree more than we disagree with our ages, and that’s really exciting,” Sprain said.
Right now, either team could have the story right, which is why the studies appeared back-to-back in Thursday’s issue of the journal.
“This is how science works,” Wignall said. Both papers present new measurement and careful analyses, he said. “There’s no reason to reject them.”
However, some researchers are skeptical that either investigation offers a compelling indictment of the Deccan Traps as a major cause of the mass extinction.
“It’s very ambiguous,” Clyde said. He welcomed the new data, but said, “the relationship between these eruptions and the climate change, to me, is not clear.”
Wignall agreed that the volcanoes were probably an accessory to the crime rather than the perpetrator.
“If there hadn’t have been a meteorite impact, there wouldn’t have been a mass extinction,” he said.
Historically, this has been one of the most acrimonious debates in earth science. But those days may be ending, Schoene said.
Researchers are starting to study the whole, complicated picture at once — the asteroid impact, the volcanism and the potential links between them.
“I think more people want to understand it in that way, rather than shout at each other about whether it was one or the other.”
In the end, they may find one obvious culprit, Schoene said, “but we’re not going to figure that out unless we study both.”