Which came first: the impact or the eruptions? That question is at the heart of two new studies in the Feb. 22 Science seeking to answer one of the most hotly debated questions in Earth’s geologic history: Whether an asteroid impact or massive volcanism that altered the global climate was mostly to blame for the demise of all nonbird dinosaurs 66 million years ago.
The dinosaur die-off is the only known mass extinction that coincides with two cataclysmic events: an asteroid impact linked to the massive Chicxulub crater in Mexico, and a gigantic volcanic eruption, evidenced by kilometers-thick layers of hardened lava at India’s Deccan Traps. The extinction marks the boundary between the Cretaceous and Paleogene periods, or the KPg boundary.
Using two different geochemical dating techniques, two separate teams dated the lava flows. The goal was to try to determine whether the bulk of the lava predates or postdates the KPg boundary. Both estimated that the eruptions lasted in total about 1 million years. But one team, using uranium-lead dating, found that some of the biggest pulses of lava erupted tens of thousands of years before the KPg boundary. The other team, using argon-argon dating, determined that three-fourths of the lava erupted afterward.
“We were excited that in both of our studies, the start and end dates for the eruption were very similar,” says Courtney Sprain, a geoscientist now at the University of Liverpool in England who led the argon-argon study while she was at the University of California, Berkeley. “It’s in the middle that it’s different.”
That’s because the tricky part is figuring out where the KPg boundary falls amid all that lava, Sprain says. Elsewhere in the world, the KPg boundary is easily identifiable in sediments by an unusually large bump in iridium, a by-product of the asteroid impact 66 million years ago. But it’s difficult to preserve any such marker in a high energy, superhot environment like the eruptions that formed the Deccan Traps. “The only way to identify the KPg is to date it,” Sprain says.
A few years ago, the uranium-lead dating team, led by geologist Blair Schoene of Princeton University, reported very precise start and end dates for the Deccan Traps eruptions, using the radioactive decay of uranium to lead inside zircon crystals as a clock (SN: 1/10/15, p. 12). In the new study, Schoene’s team wanted to determine the timing of different pulses of lava that made up the million-year-long eruption.
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Analyzing zircons embedded in ash flows that occurred in between pulses of molten lava, the team distinguished four main pulses, each lasting about 100,000 years, with the most voluminous events occurring before the KPg boundary. Those events, the researchers suggest, would have sent vast amounts of greenhouse gases such as carbon dioxide and methane into the atmosphere tens of thousands of years before the Chicxulub impact. That would have dramatically altered Earth’s climate, the researchers say, leading to the extinction event.
However, one source of uncertainty in this method is that the zircons may have crystallized before the lava erupted, while the magma was still underground, Sprain says. If so, that would have started the radioactive decay clock ticking sometime before the eruption, throwing off the dates a bit.
So Sprain’s team used a different technique, called argon-argon dating, that can be used to directly date the lava itself. Basalt lava contains a potassium-bearing mineral called plagioclase. The argon-argon technique measures the decay of two different forms of that potassium into two forms of argon to determine a date. Most of the Deccan Traps lava erupted after the extinction event, the argon-argon dating suggested. In fact, Sprain says, the asteroid impact may have sent seismic shock waves around the world that actually kicked the eruption into high gear.
But the argon-argon method has its own source of uncertainty: There isn’t a lot of potassium in the basalt, and as a result her study’s dating precision was on the order of 0.1 percent to 0.2 percent, compared with about 0.05 percent for the uranium-lead system, Sprain says. That lower precision also meant that the team wasn’t able to identify distinct pulses of lava.
Getting dueling dates from two world-class labs just shows how challenging it will be to pinpoint the ultimate culprit in the dinosaurs’ demise, says geologist Seth Burgess of the U.S. Geological Survey in Menlo Park, Calif., who wrote a commentary in the same issue of Science. The uncertainty inherent in both dating methods “leaves a little bit of room for subtle differences in interpretation,” he says. “And that’s what we have here.”
Geochemical dating has improved dramatically in recent years, in terms of both accuracy and precision, he adds. “We’ve got a more powerful microscope than we’ve ever had, that allows us to look in more detail at the timing of the KPg than ever before,” he adds. “But this is showing us that we need an even more powerful microscope.”