[meteorite-list] Dark Days Doomed Dinosaurs, Say Purdue Scientists

[Ron Baalke]

Purdue University

Writer:
Chad Boutin, (765) 494-2081, cboutin at purdue.edu

Source:
Matthew Huber, (765) 494-0652, huberm at purdue.edu

Purdue News Service:
(765) 494-2096; purduenews at purdue.edu

June 23, 2004

Dark days doomed dinosaurs, say Purdue scientists

WEST LAFAYETTE, Ind. -- 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 ¡V 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.

Related Web site:
Related research on firestorm following Chicxulub meteor impact,
     http://www.colorado.edu/news/releases/2004/168.html

ABSTRACT
Records of post-Cretaceous-Tertiary boundary millennial-scale 
cooling from the western Tethys: A smoking gun for the impact-
winter hypothesis?
Simone Galeotti, Henk Brinkhuis and Matthew Huber

The record of both dinoflagellate cysts and benthic foraminifera 
across the Cretaceous-Tertiary boundary at El Kef, Tunisia, reveals 
a brief expansion of the Boreal bioprovince into the western Tethys, 
suggesting that an approximately 2,000-year cooling occurred 
during the earliest Danian. We show that this prolonged cooling 
phase is consistent with the oceanographic response to an impact 
winter.