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.