Researchers Rethink Dinosaur Die Off Scenario

David Braun
National Geographic News
February 26, 2002
Dinosaurs may not have been killed off by asteroid impact dust blocking
out sunlight, a geologist says. Instead, the mass extinction associated
with an asteroid impact 65 million years ago might have been caused by
soot from global wildfires or sulfuric acid clouds that were a
consequence of the collision.

Whether ash, soot, or acid clouds
from the impact, what difference does it make how the dinosaurs and
other life forms died in the mass extinction event?

"It probably doesn't seem important what mechanism was triggered; either way it still seems that the impact caused the extinction," says Kevin Pope from Geo Eco Arc Research in Aquasco, Maryland. "But the difference is important because it may have implications for the predictions of the consequences of future asteroid impacts, as well as explain why impact extinction events are so rare."

To understand the difference, consider some of the mechanisms triggered when a large object from space hurtles into the Earth.

A large asteroid enters the atmosphere at extremely high speed, glowing red hot as the friction of the air turns it into a fiery cannon ball. Its impact with the ground results in a massive explosion, vaporizing the space object and launching perhaps over a trillion tons of gas, ash and rock dust into the atmosphere.

If the asteroid is big enough— Pope says about three kilometers (two miles) in diameter— the energy released by the impact would hurl enough debris into space to envelop the Earth in a rain of fire.

The ejected debris would re-enter the atmosphere like billions of meteorites, raining burning balls of fire back to Earth in a giant display of planetary fireworks. The brilliant glow from these billions of fireballs would ignite forest fires across the globe, generating vast, thick clouds of smoke and soot.

The asteroid that is associated with the mass extinction of the dinosaurs is believed to have been the one that created the Chicxulub crater in Yucatan, Mexico. It was certainly bigger than three kilometers across (more like ten to 15 kilometers or six to ten miles) and it would have caused global fires, Pope says.

"Another important factor is that the Yucatan, where the giant asteroid hit, was especially rich in sulfur-bearing rocks (calcium sulfate). The impact vaporized the sulfate rock and deposited billions of tons of sulfur dioxide gas in the atmosphere.

"Studies of volcanic eruptions have shown that this gas would convert to sulfuric acid clouds in the atmosphere, and that such clouds could remain in the atmosphere for years. These clouds may have initially been thick enough to shut down photosynthesis for a year, and perhaps they blocked the sun long enough (several years) to cause major global cooling. This mechanism helps explain why the impact was especially devastating," Pope says.

The original theory, proposed by Luis Alvarez and his colleagues in 1980, is that asteroid dust from the Yucatan impact formed dense clouds that surrounded the Earth, obscuring the sun. The prolonged period of darkness that shrouded the planet caused the plants to die, breaking the food chain and starving the animals. Many of them, including the dinosaurs, died out.

But now Pope is challenging this theory. Arguing in the February issue of Geology that the assumptions behind the asteroid dust theory are wrong, he says that the damage estimates from future asteroid impacts are also amiss.

Model Used to Show Dust Dispersal

Pope used a model to show how the large dust particles found in the K-T layer [the geological term for the layer of Earth that dates to the time of the asteroid impact associated with the mass extinction] could disperse. From the results of his test he extrapolated how the finer dust particles, the ones that were supposed to have surrounded the Earth and altered its climate, would have dispersed.

He believes that the Yucatan impact could not have produced enough dust particles of a size that it would take to shut down photosynthesis for any significant length of time and therefore the original extinction theory is not valid.

Instead, Pope believes it may have been sulfur gases produced from impacted rocks and soot from global fires that shut down photosynthesis and caused global cooling.

The original studies of the clay layer found at the K-T boundary assumed much or all of this layer was derived from fine impact dust, he says. "More recent studies of this layer have shown this not to be the case. Furthermore, earlier estimates were based on extrapolations of data from surface atomic bomb blasts, which had about 100 million times less energy than the Chicxulub impact. Extrapolation over eight orders of magnitude is risky business. "

Pope, who was involved with the identification of the Chicxulub crater as the dinosaur killer in 1989-1990 when he worked at the NASA Ames Research Center, says that the current widely held theory suggests that the ash particles caused by the impact were so fine that they would have remained suspended in the air for a long time, making the Earth dark for an extended period.

But his model indicated that not enough ash could have been generated to do that. "In any event, the ash would not have dispersed in that way," Pope says. "Most of the ash would have fallen rather quickly near the impact area, causing substantial regional damage but having less effect with increased distance from the site," he says.

"The implication is that asteroids of a smaller size— with a diameter of under three kilometers— would not necessarily have the dire consequences for the planet that is currently believed," Pope says. "They would cause heavy regional damage, but the ash fall-out would not be as great as previously believed."

Pope says some scientists have challenged his theory. "They say there may be some other extinction mechanisms that smaller impacts trigger besides dust. That may be true, but no one has done the detailed studies to back up such arguments."

Recent National Geographic News stories on dinosaurs:
Researchers Melt Polar Dinosaur Mysteries
Scientist's Finds Spur New Thinking on Dino Evolution
Dino-Era Vomit Fossil Found in England
Study Paints New Picture of Dinosaur's Nose
Skeleton of New Dinosaur "Titan" Found in Madagascar
"Tidal Giant" Roamed Coastal Swamps of Ancient Africa
"Feathered" Fossil Bolsters Changing Image of Dinosaurs
Oddly Angled Teeth Make Masiakasaurus Stick Out

Additional dinosaur resources from National Geographic:
Paul Sereno: NG explorer-in-residence and dinosaur hunter
Wanted: Albertosaurus
Dinosaur Eggs
Destinations: Dinosaur National Monument

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