The time-honored explanation for the disappearance of the dinosaurs describes perhaps the worst day in our planet’s history — a six-mile-wide space rock slammed into the ocean off the coast of the Yucatan Peninsula with such ferocity that its debris blocked out the sun and triggered a global nuclear winter. Locally, it sparked voracious wildfires and sent a tsunami roughly 200 miles inland of the Gulf of Mexico.
Renne was a graduate student when the physicist Luis Alvarez first proposed this hypothesis. “I thought it was a bunch of bull----, frankly,” he recalls. “But gradually the impact theory took hold and became very much the dogma.”
All the while, though, evidence accumulated for the role of eruptions in other major die-offs. In the 1990s, the two mass extinctions preceding the Cretaceous (the Permian and Triassic) were linked to volcanic activity, marked by layers of flood basalt just like those in India. These eruptions exuded massive quantities of carbon dioxide, which warms the planet and acidifies the oceans, as well as sulfur dioxide, which conversely filters out sunlight and cools the planet. The Permian was even deadlier than the Cretaceous, killing 95 percent of marine species. “In the face of that,” Renne says, “people started to think, ‘Gee, if flood basalts alone are capable of doing this, maybe we ought to rethink the Deccan Traps.’”
In contrast, annihilation by asteroid has no precedent, leaving some experts unconvinced. Gerta Keller, a Princeton geologist and paleontologist, has drawn attention for being especially hostile to the theory. She has likened it to a fairy tale, saying “it has all the aspects of a really nice story. It’s just not true.”
Some have tried to carve out credit for both disasters. Greg Wilson, a paleobiologist at the University of Washington and curator of vertebrate paleontology at the Burke Museum, thinks of the extinction as a one-two punch: Global warming — and, perhaps, cooling — may have destabilized the biosphere, making it more susceptible to the incoming asteroid’s devastation and less able to rebound in the aftermath. “I think the impact by itself would have had an effect on ecosystems,” Wilson says, “but the severity was amplified by ... climate change related to volcanism.”
Wilson’s view is based partly on the fact that dinosaurs seem to have dwindled toward the end of the Cretaceous Period, hinting at some level of ecological disruption. In Hell Creek, the prolific Montana fossil site where he often works, their diversity declines in the run-up to the impact. “It seems to me there’s mounting evidence that it’s more subtle than just an asteroid hitting the earth ... and everything else before that was hunky-dory,” Wilson says.
Read the full article in Discovery Magazine.