Was it the asteroid that smashed into Mexico or massive volcanic eruptions that ripped open the Earth’s crust? The debate about what killed off the dinosaurs 66 million years ago has raged for years.
Paul Renne from the Berkeley Geochronology Centre and the University of California and colleagues decided to settle the question once and for all. Turns out it was both. In a one-two punch, the Mexican asteroid impact triggered a monstrous flood of volcanic flows around the world. Their work was published in Science in October.
India’s volcanoes have long been a prime suspect in the mystery of the dinosaur extinction. While no one doubts the asteroid played its part, many scientists suspected deadly gases oozing from massive volcanic lava flows were the real executioners.
This “flood volcanism” unleashed a million cubic kilometres of lava from the west coast of India, smothering an area the size of Spain, forming a distinct geological region called the Deccan Traps. “Some of these flows reached from Mumbai all the way to the other coast – about 800 kilometres,” Renne says. These flows can be seen today as distinct layers in the rock in the Deccan Traps. Along with the lava floods came deadly sulfur dioxide, which may have prevented ecosystems from bouncing back after the initial effects of the asteroid impact.
“We reasoned it would be a remarkable coincidence if these things happened simultaneously and they weren’t related to one another.”
Another camp, while acknowledging the cataclysm of the Deccan Traps, was not convinced the timing of the major flows could account for the extinction. They placed their bets on the asteroid.
For good reason. When it slammed into Mexico 66 million years ago, it released more energy than a billion nuclear bombs. The limestone-rich impact site vapourised into carbon dioxide, releasing plumes into the atmosphere and so much dust that it blocked out the Sun. Plants and animals began dying off.
Both these catastrophes would have made the dinosaurs very sick. But delivered the knock-out blow? Answering that question required fine measurements of the timing of the Indian eruptions.
So last year Renne and his colleagues paid a visit to the Deccan Traps. To measure the precise dates of the lava, Renne used a staple geological technique that relies on the fact potassium-40 – which is in a known amount in lava – decays into the gas argon-40 at a constant rate. By measuring the amount of argon, the date of the rock can be measured.
Renne and his team analysed eight samples from different rock layers that were free of contaminants. By dating them over and over, they were able to refine their estimates and narrow the lava age range. “We knew we needed exceptional results to be able to crack this nut, so we did analysis after analysis,” Renne says. The found that the maximum period between the time the asteroid struck and the Deccan traps erupted was 50,000-years – in geological terms, no time at all.
And that led them to believe that the asteroid triggered the eruptions. “We reasoned it would be a remarkable coincidence if these things happened [virtually] simultaneously and they weren’t related to one another,” Renne says. The researchers imagine the massive asteroid impact must have shaken the planet, like a champagne bottle. The weak patches in the crust, like the Deccan Traps, became escape valves for bubbling magma.
Andrew Glikson, an earth and palaeoclimate scientist at the Australian National Univeristy, thinks the volcanism triggered by the asteroid strike wasn’t confined to India. Another area worth examining is the Hawaiian chain, he says. “It was reactivated 66 million years ago, so there was some effect there, for sure.” Renne’s meticulous analyses might determine if the Hawaii boost happened before or after the asteroid, lending even more weight to the theory that the asteroid triggered volcanos that annihilated the dinosaurs – and perhaps put a decades-long debate to rest.
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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