Dark days doomed dinosaurs

Though the catastrophe that destroyed the dinosaurs’ world may have begun with blazing fire, it probably ended with icy darkness, according to a Purdue University research group.

By analyzing fossil records, a team of scientists including Purdue’s Matthew Huber has found evidence that the Earth underwent a sudden cooling 65 million years ago that may have taken millennia to abate completely. The fossil rock samples, taken from a well-known archaeological site in Tunisia, show that tiny, cold-loving ocean organisms called dinoflagellates and benthic formanifera appeared suddenly in an ancient sea that had previously been very warm. While some scientists have long theorized that a meteorite’s fiery collision with Earth was in some way responsible for the mass extinction of many dinosaur species, the discovery provides the first physical evidence of the global cooling that likely followed the impact.

“This is the first time anyone has found a fossil record indicating the Earth cooled significantly at that time,” said Huber, an assistant professor of earth and atmospheric sciences in Purdue’s School of Science. “It is likely that the object that struck the Earth hurled huge quantities of sulfate aerosols high into the atmosphere, which darkened and cooled the planet’s surface for several years afterward.

“This discovery, which certainly has relevance to theories about dinosaur extinction, is also significant because it confirms our computer models of the Earth’s climate – they predict that the climate would respond in this way under the circumstances. That’s encouraging for those of us who are trying to understand what our climate is doing now.”

The research, which Huber conducted with first author Simone Galeotti of the University of Urbino, Italy, and Henk Brinkhuis of the University of Utrecht, the Netherlands, appears in the current (June 2004) issue of the scientific journal Geology.

Though dispute continues over what caused the dinosaurs’ extinction, many scientists are convinced that a meteorite several miles wide struck the Earth at Chicxulub (pronounced “CHIX-a-lube”) off Mexico’s Yucatan Peninsula, causing a global catastrophe that wiped out nearly all large land animals. The details of this catastrophe are still poorly understood, though the heat from the explosion likely caused a worldwide atmospheric firestorm that within hours killed many large land animals – most famously, the dinosaurs (for recent evidence supporting this theory, see related Web site).

The evidence Huber’s team has uncovered provides a complimentary story: After the initial firestorm abated, the particles hurled into the atmosphere from the impact cooled the Earth’s surface by filtering out much of the sunlight.

“Whatever dinosaurs survived the initial cataclysm, whether by burrowing underground or hiding in the water, would have emerged to find their world rapidly growing cold and dark,” Huber said. “Without warmth or sunlight, nourishment got scarce in a hurry.”

The team found evidence of the cooling in rocks found at El Kef in Tunisia, a site that shows the boundary in time between the Cretaceous and Tertiary periods 65 million years ago. This so-called “K-T boundary” is well-known as the time of the mass extinction that wiped out most dinosaurs. In the El Kef rocks, which during the Cretaceous were submerged beneath a warm-water ocean, Huber’s colleagues found fossilized dinoflagellates that ordinarily appeared only in colder regions.

“The fossils indicate that something suddenly made the water cold enough to support these tiny critters,” Huber said. “We theorize that the meteor strike produced huge quantities of sulfate particles, such as are often blown high into the atmosphere during a volcanic eruption, and these particles shielded the Earth’s surface from sunlight. The decrease in solar energy ultimately caused a long cold spell, called an ’impact winter,’ that persisted for years.”

A reasonable theory, Huber said, is that the oceans cooled because they lost most of their heat to the chilly atmosphere above, which was no longer being heated by the sun. Had this cooling effect continued long enough, the surface of the oceans might have frozen solid, turning Earth into a giant snowball.

“The oceans evidently retained enough residual heat to remain liquid while the aerosols slowly left the atmosphere,” he said. “Our climate models indicate that a snowball Earth would develop after an eight-year-long impact winter, but as the oceans did not freeze completely at the K-T boundary, the winter probably lasted five years or less.”

Huber said that while life on the planet’s surface was probably back on the road to recovery 30 years or so following the impact, the fossil records show the cold-loving dinoflagellates were present at El Kef for as long as 2,000 years afterward.

“It took much longer for the oceans to get back to normal,” Huber said. “Prolonged feedback effects may have kept the ocean depths cold for many centuries.”

The research results are good news for scientists, Huber said, because they bolster existing theories about the behavior of Earth’s climate.

“This evidence is encouraging because it suggests that our computer models are correct in predicting the climate’s response to a major perturbation,” he said. “Our computer simulations indicate that if you turned off the sun today, this sort of winter would engulf the planet. Finding data about an impact winter 65 million years ago is encouraging, because it means that historical evidence lines up with our theory and models of climate.”

These models need to be as accurate as possible, he added, if we are to comprehend the effects of aerosol particles on global warming.

“The results point to the critical role of the potential cooling effects of aerosols, which is very important for predicting the effects of humans on climate,” he said. “Although human influences on aerosols are much more subtle than those thought to have resulted from the K-T boundary event, coal-fired power plants and biomass burning are also important aerosol sources. A better representation of aerosols’ effects is crucial for understanding future climate changes as well as those in the deep past.”

Huber is affiliated with the Purdue Climate Change Research Center, which promotes and organizes research and education on global climate change and studies its impacts on agriculture, natural ecosystems and society.