An asteroid that crashed into the earth 65 million years ago may not have been the cause of the dinosaurs‘ extinction, a group of researchers are arguing. Instead, that impact may have been just a prelude to the main event, when a wave of volcanic eruptions spewed out massive clouds of sulfur dioxide, clouding the air and bringing showers of acid rain. The researchers are basing their theory on studies of an area in India called the Deccan Traps, which was convulsed with volcanic activity around 65 million years ago. At least four waves of massive eruptions spread successive sheets of thick basalt across the land for more than 500 miles, and they piled into a plateau more than 11,000 feet high over thousands of years [San Francisco Chronicle].
The new research on the Deccan Traps volcanoes, announced at the ongoing meeting of the American Geophysical Union, are the first major challenge to the asteroid theory that has dominated dinosaur extinction studies for three decades. That theory posits that a six-mile-wide asteroid slammed into Mexico’s Yucatan peninsula, creating the Chicxulub crater and cooling the climate so drastically that the majority of life forms went extinct in what’s known as the Cretaceous-Tertiary (or K-T) extinction. But geologist Gerta Keller and her colleagues argue that the impact occurred well before the massive die-offs began. By examining sediment layers, the team found that the crater impact appears to have occurred about 300,000 years before the K-T boundary, with virtually no effects to biota. “There is essentially no extinction associated with the impact,” Keller said [LiveScience].
Meanwhile, geophysicist Vincent Courtillot determined more exact dates for the Deccan Trap eruptions by studying the magnetic signatures of the Indian volcanic deposits that lined up with the Earth’s magnetic field as they cooled. Because the orientation of the magnetic field has changed over time, lava that cooled at different times will have different signatures. The more than 2-mile thick pile of Deccan Traps deposits has several major pulses that occurred over the course of several decades each, almost certainly less than a hundred years [Wired Science].
The researchers say they detected individual pulses of eruptions at 67.5 and 65 million years ago, with two more quickly following. After the first flow, “the species disappear; we have essentially very few left,” Keller said. The two subsequent flows prevented any recovery, and “by the fourth flow, the extinction is complete,” Keller said [LiveScience]. The researchers also argue that an asteroid impact wouldn’t kick up enough dust and sulfur dioxide to alter the climate around the planet, but says that these supervolcanoes may have spewed 10 billion to 150 billion tons of sulfur dioxide into the air with each pulse of eruptions.
However, the proponents of the asteroid impact theory aren’t going to quietly accept the junking of their thesis. “There was volcanism at the time. There’s always volcanism, but that impact is so significant that you can’t ignore it,” said Rick Firestone of Lawrence Berkeley National Laboratory who studies the link between impacts and extinctions. “The only question is, were there other things that happened as result of it” [Wired Science].
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Image: Wikimedia Commons