Title: Impact wars.   
Subject(s): ASTEROIDS; MACLEOD, Norman
Source: Science News, 3/5/94, Vol. 145 Issue 10, p156, 2p, 1c
Author(s): Monastersky, Richard
Abstract: Comments on the belief of some former skeptics on the idea
that an asteroid or comet hit the Earth 65 million years ago. Views of
paleontologist Norman MacLeod; When the impact debate began; The
Cretaceous (K) and Tertiary (T) boundary; Evidence that the impact
spawned a huge tsunami in the Gulf of Mexico; More.
AN: 9403087778
ISSN: 0036-8423
Note: Tucson-Pima Public Library subscribes to this magazine.
Database: MasterFILE Elite

IMPACT WARS

Debate rages over killer comets and dinosaur deaths

The setting was scholarly but the university auditorium felt more like
a revival tent as Norman MacLeod addressed a scientific conference in
Houston last month.

"Although I'm a paleontologist, I'm here to tell you I BELIEVE,"
MacLeod proclaimed, drawing out the last syllable in true evangelical
tradition. "I believe in the impact," announced the researcher from
London's Natural History Museum.

While designed to draw a laugh, MacLeod's theatrical conversion also
provided a fitting symbol for the Houston meeting, convened to examine
the calamity that befell Earth at the end of the Cretaceous period,
when the last dinosaurs and two-thirds of living species disappeared.
After 14 years of often rancorous debate, nonbelievers such as MacLeod
admitted before their peers that they have come to accept the idea
that a huge bolide - an asteroid or comet blindsided Earth 65 million
years ago.

"The number of skeptics has really decreased, and that is due to the
fact that a lot more evidence has come forward in recent years," says
Princeton University paleontologist Gerta Keller, another researcher
who had long criticized the bolide theory

Despite the retreat of Keller, MacLeod, and the rest of the
anti-impact squad, the Houston meeting was anything but peaceful.
Instead of arguing whether a crash had indeed occurred, participants
in the dispute have simply shifted the intellectual battleground,
ensuring that the conflict will continue unabated for many years. In
terms of resolution, the meeting ended without success. But it offered
a revealing glimpse of how ideology and personality are shaping the
course of a revolution in the earth sciences.

   The impact debate began in 1980, when investigators from the
University of California, Berkeley, reported the discovery of an
unusual feature in ancient ocean rocks preserved in Italy, Denmark,
and New Zealand. While studying a thin layer of clay deposited at the
end of the Cretaceous period, Nobel laureate Luis W. Alvarez, Walter
Alvarez, Frank Asaro, and Helen V. Michel measured a substantial
enrichment in the metal iridium, an element exceedingly rare in
Earth's crust.

The Berkeley researchers knew that some meteorites carry relatively
high concentrations of iridium. They also recognized that the boundary
between Earth's Cretaceous (K) and Tertiary (T) periods marks a mass
extinction. Putting the two facts together, the Berkeley team proposed
that a mountain-size meteorite slammed into Earth at that time,
pulverizing enough rock to create a globe-circling dust cloud that
blocked out sunlight. Such a catastrophe, they claimed, caused the
well-known extinctions at the K-T boundary

Many researchers rejected the impact hypothesis from the outset,
preferring to stick with previous explanations that blamed the
extinctions on volcanic eruptions, changes in sea level, or climate
shifts. Paleontology provided many of the impact critics, who believed
that species disappeared gradually over millions of years instead of
virtually overnight.

Through the 1980s, impact supporters strengthened their case by
finding numerous other clues preserved in the K-T boundary clay layer
around the world. In 1991, their cause received a dramatic boost when
investigators identified what they called "the smoking gun" - an
apparent crater buried beneath the northern tip of Mexico's Yucatan
Peninsula. They proposed that the 180-kilometer-wide circular
structure could be the long-sought scar carved by the K-T blow (SN:
1/25/92, p.56).

Subsequent work has backed that suggestion, convincing many on both
sides of the impact issue that the so-called Chicxulub crater does
indeed mark ground zero for a colossal crash 65 million years ago (SN:
8/15/92, p.100).

Although agreement about the impact might suggest some degree of
reconciliation between opposing camps, the Houston conference showed
that the community remains fundamentally split, both philosophically
and physically In the meeting hall, researchers unconsciously divided
themselves, with the skeptical minority assembling on the speaker's
left and the believers on the right.

As agreement over the impact has grown in recent years, much of the
research and debate regarding the K-T mystery has shifted to questions
of severity: How devastating was the collision and how did it affect
life?

Stolid supporters of the impact hypothesis blame the disaster for most
of the extinctions at the end of the Cretaceous. The skeptics, while
now accepting the crash, question its role. When they view the
extinction record, they see a variety of different traumas plaguing
the planet at nearly the same time, with the extraterrestrial wallop
playing either a major or minor part.

Which theory eventually prevails will depend largely on the timing and
pattern of the die-offs. If animal and plant species disappeared over
hundreds of thousands of years, starting before the impact, that would
implicate down-to-earth problems and diminish the bolide's role. Some
researchers who favor this interpretation point an accusing finger at
a series of huge lava flows in India that date to the time of the
impact. Such eruptions could have polluted the atmosphere and altered
Earth's climate, they say.

   While the dinosaurs' demise has long excited the public, one of the
most spectacular extinctions at the K-T boundary swept away a much
smaller, but no less important organism: planktonic foraminifera.
These tiny animals form a critical link in the marine food chain, and
they suffered tremendous losses at the end of the Cretaceous (SN:
2/l/92, p.72). Researchers on both sides of the debate have argued
that this vulnerable group could offer critical information about the
cause of the mass extinction.

Six years ago, at the last conference devoted to the K-T question, two
paleontologists reported diametrically different findings concerning
the speed of foram extinctions, prompting calls for a test to resolve
the discrepancy With that aim, the two main figures in the debate,
Princeton's Keller and Jan Smit of the Free University in Amsterdam,
traveled to Tunisia in 1992. Working with Tunisian geologists, they
collected samples of marine sediments at various levels above and
below the K-T boundary, then sent the rocks to the test supervisor,
Robert Ginsburg of the University of Miami.

Ginsburg distributed the sediments to four "blind" testers, who
recorded the types of forams they found in each sample. To keep the
results free from prejudice about extinction rates, the testers did
not know the original positions of the samples relative to the
boundary

This procedure might seem definitive. But like opposing political
parties reacting to a public opinion poll, Keller and Smit still found
room to put different spins on the test results when they were
announced at the Houston meeting.

The four paleontologists did not verify Keller's original finding that
12 large, highly ornamented species of forams died out before the
boundary. Each analyst did find a few species disappearing early, but
the particular species differed from one investigator to the next.

Smit and others assert that the discrepancy is the result of sampling
and analysis problems. Any single investigator might find some species
dying out early, because one researcher can miss a few species when
looking at a limited number of microscope slides. But the combined
results from the four testers indicate that all the species in
question survived until the boundary, they contend.

"Taken at face value, the results of the blind test suggest that there
were no extinctions prior to the impact horizon," says paleontologist   
Steven D'Hondt of the University of Rhode Island in Narragansett.

   Keller disagrees: "From this test, you don't show anything about
   the time before the K-T boundary." She argues that the blind test
   has limited value because the four investigators disagreed over the
   names they gave to individual species, muddling the extinction
   record.
   
   In theory, the four testers can resolve the debate over early
   extinctions by agreeing on the names of the species in question,
   Ginsburg says. They could do that by meeting in person or by
   sending around micrographs of the disputed forams. Although either
   method involves considerable work, "it looks like we have to do
   that," says Ginsburg.
   
   Whatever the results of the test, Keller and Smit do agree on one
   issue-the need for more studies of extinction patterns. By
   analyzing which regions and ecosystems suffered and which escaped
   unharmed, researchers can better decipher what happened to life at
   the end of the Cretaceous.
   
   While paleontologists have concentrated on counting bodies,
   geophysicist shave attempted to explore the anatomy of the crater
   and how the crash altered Earth's environment.
   
   Early versions of the impact theory concentrated on the devastating
   nature of the resulting dust cloud, which could have blocked out
   sunlight, halted photosynthesis, and killed the plants that sustain
   larger organisms. Since then, investigators have proposed myriad
   different ways in which a tremendous impact could have disrupted
   life.  Global wildfires, rainfall as poisonous as battery acid, and
   a shredded ozone layer might all follow such a collision,
   
   While not denying the importance of such effects, several
   researchers are now convinced that the bolide may also have knocked
   Earth's climate off balance. Because the Yucatan Peninsula has a
   thick cover of sulfur-rich rock, a hit there could have released as
   much as a trillion tons of sulfur dioxide gas, which reacts in the
   atmosphere to form droplets of sulfuric acid. Like tiny parasols,
   these droplets block out sunlight and cool the planet's
   surface. The most recent demonstration of sulfur cooling occurred
   courtesy of Mt. Pinatubo, which lofted some 20 million tons of
   sulfur dioxide into the air and depressed global temperatures by
   0.25'C.
   
   Richard Turco of the University of California, Los Angeles, used a
   simple one-dimensional model of the atmosphere to study the effects
   of sulfuric acid from an impact. Assuming that the strike liberated
   roughly 50,000 times more sulfuric acid than did Pinatubo, he found
   that Earth's temperature could have dropped by 5'C to 10'C for a
   decade - a staggering cooling when compared to the 4'C drop during
   the last ice age.
   
   Turco says such simulations are crude and require more information
   about the actual physical and biological effects of the impact.
   Geologists will tap a major new source of data when they drill into
   the Chicxulub structure, a project slated to begin possibly this
   year.  Direct measurements of rock in and under the crater should
   answer questions about the impact's size, as well as resolve
   lingering doubts about the age of the crater. Whereas most evidence
   suggests that the feature formed precisely at the end of the
   Cretaceous, a few skeptics point to puzzling data that hint of a
   different age. This discrepancy has kept some conservatives from
   accepting Chicxulub as the K-T impact crater.
   
   Together with the ongoing arguments over extinctions, debates about
   the age of the crater and other issues demonstrate the deep rift
   that continues to divide scientists studying the K-T
   event. Fundamentally, the controversy centers on philosophical
   differences about how the world works.
   
   William Glen, a geoscientist and historian with the U.S. Geological
   Survey in Menlo Park, Calif., has studied the debate for the last
   10 years. According to his analysis, the impact theory met such
   strong resistance, in part, because it challenged the reigning
   doctrine of uniformitarianism, which includes the idea that changes
   on Earth occur gradually As increasing numbers of geoscientists
   subscribe to the impact hypothesis, they are stretching the
   boundaries of the old doctrine to include catastrophic processes
   that were formerly ignored.  "We're seeing a recasting of the
   definition of uniformitarianism," says Glen.
   
   How long will the debate rage? Glen answers by referring to German
   physicist Max Planck, who started the quantum mechanics revolution
   at the turn of the century "In the case of an upheaval in science
   in which a new thesis or paradigm is offered up, the last vestiges
   of debate cease only with the death of the old guard," says Glen.
   
   PHOTO: Evidence suggests that the impact spawned a huge tsunami in
   the Gulf of Mexico. The dust cloud and other effects, however,
   would have caused truly global damage. (Ron Miller)
   
   MAP: The Chicxulub crater measures at least 180 kilometers in
   diameter, making it the largest known on Earth. Some evidence hints
   that the crater may actually be twice as large.
   
   ~~~~~~~~
   
   By RICHARD MONASTERSKY
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   Source: Science News, 3/5/94, Vol. 145 Issue 10, p156, 2p, 1c.
   Item Number: 9403087778