Were the dinosaurs seen off by an asteroid, or a flurry of volcanic eruptions? Two new studies on the timing of volcanic events help us piece together the story of Earth’s most famous mass extinction, but they leave it unclear exactly what triggered the demise of so many species.
Around three-quarters of the species on Earth are thought to have perished in the Cretaceous-Palaeogene extinction event 66 million years ago, most famously including all dinosaurs except the ancestors of modern birds.
In the geological records, the event coincides with a layer of rock with high levels of iridium – evidence of an asteroid impact. Most geologists think this impact created a huge crater at Chicxulub, Mexico. The impact could have caused a global soot cloud that blocked out the sun.
However, the extinction also coincided with a burst of intense volcanic activity that formed a huge rock formation known as the Deccan Traps in western India.
Similar volcanic events have been implicated in other mass extinctions in Earth’s history. Eruptions can warm the climate by releasing massive quantities of greenhouse gases or cause cooling by putting sun-blocking aerosols into the high atmosphere.
To get a more precise idea about the timing of the Deccan eruptions, Courtney Sprain of the University of California, Berkeley, and colleagues used argon-argon dating to estimate the age of the lava flows. Another team, led by Blair Schoene of Princeton University, New Jersey, used a different method, uranium-lead dating.
Both studies agree that the Deccan eruptions took place over a period lasting around a million years, beginning around 400,000 years before the extinction event. But on precise details, their conclusions differ.
Schoene and his colleagues suggest the Deccan eruptions occurred in four bursts. The second was the most rapid and it began tens of thousands of years before the asteroid impact.
Sprain and her team, meanwhile, concluded that three-quarters of the lava volume at Deccan erupted after the mass extinction, and that the eruption rate increased after the impact.
The discrepancy may be down to the strengths and weaknesses of the two methods. Schoene’s offers greater precision, but instead of dating lava flows directly, it dates the formation of zircon crystals in the lava, which may occur in magma chambers before the eruption.
We still don’t know what the relative contributions of the Deccan Traps and the asteroid impact were to the extinction, says Schoene. But having a more precise timeline helps us get closer. “Before, we had no record with enough detail from the volcanic rock to compare to the climate records,” he says.
Although her results suggest most of the lava erupted after the extinction, Sprain still thinks volcanism played an important role. Fossils and temperature records show that changes in the climate and signs of ecological stress coincided with the start of the volcanic activity, some 400,000 years before the impact and the extinction.
“The Deccan Traps likely weakened late Cretaceous ecosystems to make them susceptible to the rapid environmental changes that came with the Chicxulub impact,” says Sprain.
It may be that the earlier eruptions released more gas, causing changes in the climate, while more of the lava came later, she says.
Although the mystery remains unsolved, Sprain is confident that we are getting closer to an answer. “I do think in the future we will get a much clearer picture of what’s going on.”
Journal reference: Science, DOI: 10.1126/science.aau2422
Journal reference: Science, DOI: 10.1126/science.aav1446
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