Title: The day the dinosaurs died.   
Subject(s): EVOLUTION; DINOSAURS; EXTINCTION (Biology)
Source: Astronomy, Apr96, Vol. 24 Issue 4, p34, 8p, 6c
Author(s): Cowen, Ron
Abstract: Focuses on the theory that the planet Earth collided with an
asteroid or comet which killed the dinosaurs. Discovery of a buried
crater in the Yucatan Peninsula; Solar system impact; Ramifications of
a large impact; Chronology of the catastrophe. INSET: Tracking down
Chicxulub.
AN: 9602290206
ISSN: 0091-6358
Note: Tucson-Pima Public Library subscribes to this magazine.
Database: MasterFILE Elite

THE DAY THE DINOSAURS DIED

For more than 4 billion years, it orbited the Sun -- a pockmarked,
misshapen body roughly the size of Manhattan. Over time, it
wandered into the inner solar system, occasionally passing near
our planet as it journeyed around the Sun.

Then, one day about 65 million years ago, Earth and the roughhewn body
reached the same place at exactly the same time.

The solar system is a hazardous place. Countless fragments of rock and
ice tumble about in orbits that occasionally produce impacts on
planets. Most of the time these are minor events. Once in a great
while, they are catastrophic events, affecting planets as a whole and
leaving long-lasting effects. The most famous large impact on planet
Earth, the one the killed the dinosaurs, is now coming into clear
focus.

After 15 years of fractious debate, most scientists now agree that an
asteroid or comet collided with Earth 65 million years ago. Most of
the nonbelievers threw in the towel in 1991 when scientists discovered
the smoking gun for the impact -- a vast, buried crater in the Yucatan
Peninsula. Known as Chicxulub (CHEEK-shoe-lube), a Mayan word that
ironically means "devil's tail," this huge hole was formed during the
interval of geologic time known as the K-T boundary, which marks the
end of the Cretaceous (K) period and the beginning of the Tertiary
(T). (See "Tracking Down Chicxulub," p. 35.)

But if there's consensus that a huge object smacked into Earth,
critics have found another battleground. A catastrophic solar system
impact has leaked into other fields as geologists, paleontologists,
astronomers, and atmospheric scientists debate exactly how lethal the
long-ago collision was, and which of its many aftereffects proved most
deadly.

Before the impact, dinosaurs still roamed Earth and numerous species
of phytoplankton -- which serve as food for tiny sea animals -- filled
the oceans. A geologically short time later, the last dinosaurs had
gone belly up, along with two-thirds of all living species.

Was the K-T impact entirely responsible? And if so, exactly how did it
exert its lethal influence?

To understand the K-T impact, scientists are drawing on their
knowledge of all solar system impacts. "The ramifications of a large
impact are still poorly understood," note planetary scientists Owen
Toon and Kevin Zahnle of NASA's Ames Research Center in Mountain View,
Calif.

Says Buck Sharpton of the Ltinar and Planetary Institute in Houston:
"The focus here is to try to understand how you can have an impact
event [that was so catastrophic]. Bear in mind that we always thought
that a [comet or asteroid] would deposit most of its energy right at
ground zero. So how can it, as energetic as it was, cause a global
extinction? It had to wipe out just about everything."

"It's not so much that the impact itself was so lethal," agrees
paleontologist Peter Ward of the University of Washington in Seattle.
"It squashed a few moths, a few this and a few that. That's not what
killed everything."

"My own philosophy about the K-T impact is that everybody wants to
discover the one thing that killed the dinosaurs," says Toon. "But the
fact is it's a very complicated situation, in which multiple things
happened. It's not like everything died from one cause."

Chronology of a Catastrophe

A grand planetary impact is an amazingly destructive thing. Imagine
the energy unleashed if the entire world's arsenal of nuclear weapons
suddenly exploded. Now multiply that by 10,000. The blast, equivalent
to 10 trillion megatons of TNT, is only a conservative estimate for
the destructive force generated by the smash. Here's the chronology of
the catastrophe, according to Toon and several other scientists.

Time zero: As the object cannonballed through Earth's atmosphere, it
generated a shock wave that blew a hole in the air.

Seconds to first hour: Reaching the ground about three seconds later,
the bolide plowed through a 1 kilometer-deep ocean and into Earth's
crust, generating earthquakes at every exposed fault line over the
next few hours. Within the first hour of impact, 100-foot-high walls
of water -- ocean waves from what is now the Gulf of Mexico -- flooded
coastal plains. But these were only the local effects, devastating
just a few percent of Earth's surface. Minutes after the impact,
billions of tons of debris blasted into space, three to four times the
mass of the bolide, Rocketing upward and outward at speeds nearly
great enough to escape Earth's gravity, the dusty debris, reentered
Earth's atmosphere far from the site of the impact with a vengeance,
reheating and glowing red-hot.

For 30 minutes to an hour after the impact, "the entire sky was like a
big, glowing sheet of rock," says Toon.

"Imagine the effects of a thousand shooting stars suddenly entering
the atmosphere and ablating at an altitude above 60 km. The sky would
turn from its normal transparent blue to a brilliant red sheet of
glowing lava," note Toon and Zahnle.

Toon adds, "The shooting stars wouldn't have hit you; they stop at 60
km or so above the ground. But there's so many of them that you're
standing there in this radiation bath, hot enough to set paper on
fire.

The collision of Comet Shoemaker-Levy 9 with Jupiter drove this point
home, several scientists assert. Fragments of the comet no bigger than
half a kilometer in diameter -- far smaller than the size of the
impactor that created Chicxulub -- produced a plume of debris that
reached 5,000 kelvins.

More to the point, the hot plume splashed down over an area on Jupiter
that exceeded that of Earth.

"Comet Shoemaker-Levy 9 provided proof that the K-T impact ignited
global fires and scorched Earth," says Jay Melosh of the University of
Arizona in Tucson.

"The bigger animals would have been broiled in their tracks," he
notes. But, says Toon, some organisms that had natural shelter might
still survive. "If you're a crocodile underwater -- no problem. If
you're a mouse in your hole -- no problem."

From the first hour to six months: After an hour or so, according to
Toon and Zahnle, the red sky would cool and blacken. By the end of the
first day, soot from charred debris would block out the Sun, plunging
rite world into darkness for as long as a year.

Six months to a decade: Sulfur ejected into the stratosphere by the
impact and slowly converted to sulfuric acid, a highly efficient Sun
blocker, may have prolonged the blackout for as long as a decade.

"The fires themselves wouldn't have killed ocean life, but if you turn
the lights out, phytoplankton stops reproducing and you trigger the
collapse of the ocean ecosystem," Toon notes.

Without sunlight, temperatures on land would take a nosedive -- a far
larger drop than Earth endured during the Ice Age. Global cooling may
have lasted for a decade.

A decade after the hit: Following the cooling came the warming. The
collision may have squeezed carbon-containing material out of rock at
the impact site, creating a blanket of carbon dioxide in the
atmosphere. This greenhouse gas would have heated Earth for 50 to 100
years, according to Toon. At first, he notes, the oceans would have
soaked up some of the excess carbon dioxide, but their capacity would
soon have been overwhelmed by the concentration in the atmosphere.
Temperatures would rise for up to a century, Toon conjectures.
"Temperatures don't just go back to normal after the global cooling
but rise considerably higher," notes Sharpton. "There's a shift in
temperature and it stresses the heck out of things. You might just be
getting used to the cold and all of a sudden you've got this hot dry
spell."

However, not all scientists agree on this warming. Kevin Pope of Geo
Eco Arc Research in La Canada, Calif., feels the warming would be
negligible. "Recent research has shown that climate warming due to
greenhouse effects from carbon dioxide released by the impact was
extremely minor," says Pope. He is not alone: Studies by Thomas Ahrens
of Caltech suggest the same result.

The Rock Hits the Powder Keg

The sheer size of the impactor -- estimated to be 10 km in diameter
--can't by itself account for the devastation. Haraldur Sigurdsson of
the University of Rhode Island and other researchers believe that the
impact sounded the death knell for so many species because of the
unique composition of the crash site.

Several lines of evidence reveal that the bolide hit a sulfur-rich
region of the Yucatan Peninsula, kicking up billions of tons of the
element. "We think the severity of the event relates to the unusual
chemistry of the terrain where the impact occurred," notes Sigurdsson.
"The bolide hit the powder keg when it struck this chemical sediment."

He and his colleagues had found that the target rock in the Yucatan
includes a 3-km thick sequence of carbonites and evaporites.
Evaporites include gypsum and anhydrites, forms of calcium sulfate
that contain large concentrations of sulfur.

Unlike soot and smoke, which probably washed out of the lower
atmosphere in six months, the sulfur dioxide spewed into the upper
atmosphere and lingered there for several years. The globally
dispersed sulfur dioxide does not readily undergo chemical reactions
and, because it is a gas, it does not settle out quickly like soot and
dust.

By itself, sulfur dioxide doesn't do much damage. But ultraviolet
sunlight and the presence of water eventually transform it into
sulfuric acid. This falls to Earth as a toxic rain as corrosive as
battery acid. "Unlike the aftermath of typical impacts, the skies
remained murky for at least a decade due to chemically generated
clouds of sulfuric acid high in the stratosphere," says Kevin Baines
of NASA's Jet Propulsion Laboratory.

Baines and other team members led by Kevin Pope comprise an
interdisciplinary group that is combining computer models of the
Chicxulub impact and its atmospheric effects with geological studies
of the crater.

Baines notes these same atmospheric conditions occur on Venus, which
is perpetually cloaked in sulfur clouds. "The entire ecosystem of
Earth, including plants and animals, was subjected to extreme
environmental conditions for more than a decade. This Mother of all
Environmental Disasters was simply too much for a large number of
well-established species, such as the dinosaurs, to cope with," he
says.

But Toon strikes a cautionary note. "The soot is in the K-T boundary
layer, so you know that happened; we had to burn the entire world's
biomass to have the amount of soot observed in the boundary layer," he
says. In contrast, "We don't actually have any direct evidence that
the sulfur did anything in the boundary layer."

"I would not bet my life on the contribution of sulfur to this whole
business," says Peter Ward. "Sulfur is the least reliable kill
mechanism we have in the sense that there is so much leeway, a lot of
sway in the numbers. Dust going up in the atmosphere could have been
every bit as catastrophic as the sulfur emissions. Several analyses
suggest that when dust goes up, it perturbs rainfall cycles.
Immediately after the K-T impact, places that had been previously wet
became dry and those that were very dry became wet."

Despite the feelings of Toon and Ward, many scientists remain
skeptical of the sulfur story and research continues, but evidence is
mounting that a decade of cooling may have contributed to the
dinosaurs' demise.

To Kill the Dinos or Not to Kill?

Some researchers consider micropaleontologist Gerta Keller of
Princeton University a stubborn holdout. She has coined her own phrase
for the true believers in the K-T catastrophe: "impact diehards."

It's largely physicists, astrophysicists, and chemists, she contends,
who support the theory. "Relatively few paleontologists believe the
impact can snuff out life; they just don't see that in the data.
Dinosaurs were already in decline the last few million years of the
Cretaceous period," Keller adds.

Keller says she does agree with other scientists that because dinosaur
bones are scarce these fossils can't provide a detailed record of what
happened 65 million years ago. "The best information is from the
bottom of the food chain," she says, in part because such primitive
organisms as pollen grains and fungal spores are plentiful and well
preserved in sedimentary rocks.

Among the most abundant inhabitants of the sea is foraminifera, a type
of plankton supposed to have suffered nearly complete extinction
according to the impact theory, Keller says. These single-celled
organisms live in the upper 100 to 400 meters of the ocean, and in the
tropics they did indeed disappear quickly at the K-T boundary, she
adds. But at more northerly and more southerly latitudes the die-off
is considerably less severe. "Basically what paleontologists see is a
differential pattern of extinction," Keller notes.

"The impact diehards don't like this," she asserts. "They predict that
effects of such an impact must be global, extinction must be global,
and there must be no differential across latitudes."

Pope, however, replies, "The fact is, such a latitudinal gradient,
with most extinctions occurring in the tropics, is exactly what is to
be expected from impact-induced global cooling and blackout because
high-latitude species are better adapted to cold and low light
levels."

Paleontologist Ward concurs that extinctions were not as great at the
poles. "The high latitudes in all probability served as a refuge;
plant extinctions were definitely not as severe at high latitudes."
However, he adds, such limited protection in no way mitigates the
global influence of the K-T impact.

Says Ward: "Even in a severe forest fire, there are always pockets of
unburned material and pockets of survivors; it's very difficult to
eradicate everything. But all the evidence points to a catastrophe."

Ward notes that for several years he, too, didn't buy the impact
story. After all, the marine mollusks he studied appeared to have died
off gradually rather than suddenly, in direct contrast to the effect
of a large impact. Working with Charles Marshall of the University of
California, Los Angeles, Ward subjected fossils found at various
layers in the Cretaceous and Tertiary periods to a new statistical
analysis. The researchers found that "in almost all cases, the big
fossils all show a sudden extinction at the K-T boundary -- even
though the actual pattern in the rock record looks anything but
sudden," says Ward.

"The bigger the fossil, the rarer they are, and the less chance you
have of finding the last one right at the K-T boundary. So the dieoffs
look gradual, but they're really not. A catastrophic extinction will
always look gradual for big fossils because there are so few of them
and because paleontologists have trouble finding them."

Ward says that several paleontological mysteries remain. "We want to
know why some plankton go extinct and why others do not. The easy
answer is that deep plankton survive the blast best, overwintering
deeper in the ocean until the atmosphere warms."'

Subtle Evidence for the Big Smack

But that type of easy answer isn't what Jablonski and University of
Chicago paleontologist David Raup discovered when they examined the
survival of 350 evolutionary lineages of marine bivalves -- clams and
other two-shelled ocean dwellers -- at the time of the Cretaceous
period. Based on the rich, well preserved fossil record of these at
the K-T boundary, the researchers find that the normal rules of
evolution simply didn't apply. Mass extinctions wipe out all types of
bivalves, without regard to special survival strategies developed
during other, less violent times, the paleontologists conclude.

Raup and Jablonski also uncovered one factor that seemed to enhance
survival. Those organisms distributed over many continents had a
higher survival rate than those that had a narrow geographic
existence. One intriguing notion, he muses, is that "maybe this change
in the rules for extinction or survival helps explain why the
dinosaurs are gone and mammals weathered the extinction at the end of
the Cretaceous." This could "open the door to a whole new way of
looking at mass extinction as an evolutionary force."

Comet or Asteroid?

Scientists continue to debate whether the huge impactor was an
asteroid or a comet. Sharpton argues that it was probably a comet.
"The materials in the KT boundary suggest that the impactor is
primitive, undifferentiated material," Sharpton says. This suggests
the possibility that it was a comet." He adds that the estimated size
of the bolide, about 10 km, also makes a cometary source more likely.

On the other hand, he notes, "we have a lot of these objects in our
vicinity called near-earth asteroids. So we can't eliminate the
possibility that it was an asteroid." (See "Far Journey to a NEAR
Asteroid," March 1996.) Geologist Eugene Shoemaker of Lowell
Observatory in Flagstaff, Arizona, says he's puzzled by another
mystery. According to Shoemaker, two distinct layers of material are
associated with the K-T impact -- created apparently by two distinct
episodes of deposition, separated perhaps by a growing season.

"The serious problem is that we do not understand how those two layers
formed. Along with shocked quartz, there is shocked zircon in the
upper layer. Other people have glibly said, 'Oh well, the lower layer
is the fallout layer, the upper layer is the fireball layer.' Well,
that's bull. The lower layer is just as much the fireball layer as the
upper."

Says Pope: "The best explanation for the two-part stratigraphy is that
the lower unit, which contains very little shocked quartz (tiny
crystal grains fractured by the impact) and lots of microtektites, was
deposited ballistically on relatively short trajectories that did not
leave the atmosphere. The upper unit, with abundant shocked quartz, as
well as most of the iridium, contains material that was initially
ejected much higher, perhaps above the atmosphere, and settled to the
ground more slowly, thus landing on top of the tektite layer."

Additionally, Adriana Ocampo of JPL and Pope recently discovered a
Chicxulub ejecta deposit in Belize, only 360 km from the center of the
impact. This deposit also contains two layers, the lower one with
tektites and carbonate spherules, and the upper one with boulders up
to 7 m across floating in mud.

And then there's a more basic puzzle. After several years of taking
gravity maps, drilling at the site, and evaluating seismic data from
Chicxulub, geologists still don't agree on the size of the crater.
Sharpton and his colleagues assert that it has a diameter of about 280
km. "Any estimate less than 200 kilometers is bordering on nonsense,"
Sharpton declares. A recent study of the topography of the northern
Yucatan by Pope and his colleagues found several concentric troughs
and ridges that also indicate that the crater is larger than 200 km in
diameter (they suggest 240 km).

Yet Alan Hildebrand of the Commission Geologique du Canada and his
team claim just as loudly that the crater is no more than 180 km
across. At stake are estimates of the energy and mass of the impactor,
properties directly linked to the diameter of the crater it gouges.

"It may seem strange that experts cannot agree on something so basic
as the size of an impact crater," concedes Melosh in a commentary in
the August 3 Nature. However, he notes, "the problem is that the
structure of all craters is not the same and the relation of the final
crater form to the impact that created it is often unclear."

The initial cavity gouged by an impactor, dubbed the "transient
crater," collapses immediately under the influence of gravity to form
one of a variety of crater types. And therein lies the challenge. "It
is really the transient crater size that matters in deducing impact
energy, and so the problem is to relate the measures of crater
diameter to the crater size," Melosh says.

Determining the diameter of the Chicxulub crater poses further
difficulties, he notes, because the crater lies buried under as much
as a kilometer of sediment.

In recent studies, Sharpton and his colleagues have detected a
ring-shaped feature surrounding the Chicxulub site that has a diameter
of 280 km. Sharpton maintains that the ring indicates the edge of the
crater and deduces that the transient crater must have had a diameter
between 145 and 205 km. But using the same data set, supplemented by
five new gravity maps, Hildebrand and his colleagues see no hint of
Sharpton's outer ring.

With the debate about the gravity data likely to continue, Melosh
suggests that geologists might rely on an alternative way of
estimating the diameter of Chicxulub. In this method, researchers use
the quantity of iridium scattered over Earth's surface by the impactor
to determine the diameter of the body that slammed into Earth. Current
iridium estimates indicate a 10-km projectile, which would gouge a
transient cavity of about 70 km in diameter.

"Many opinions have been changed by the impact of Comet Shoemaker-Levy
9 on Jupiter, especially as it now seems that the largest individual
fragments were only 700 meters in diameter," says Melosh. Although
Jupiter's gravity caused the fragments to slam into the giant planet
with a velocity of 60 km per second -much higher than in the
terrestrial collision -- the smaller size of the Shoemaker-Levy 9
projectiles means that the fragments carried at most one-hundredth the
energy of the K-T impactor.

"Was the impact of an asteroid or comet big enough to gouge a
180-kilometer-diameter crater in the Earth also big enough to wipe out
the dinosaurs?," asks Melosh. "Many people would now answer with a
resounding 'yes'."

PHOTO (COLOR): All artwork by Ron Miller

PHOTO (COLOR): The Impact: Asteroid burrows a tunnel through Earth's
atmosphere and smashes into what is now the Yucatan Peninsula.
Everything in the immediate area is devastated; all other life on
Earth will shortly feel the impact in other ways.

PHOTO (COLOR): Seconds to an Hour: Earthquakes and walls of water
swell from the impact point, but the real devastation comes from fiery
hot debris falling back into Earth's atmosphere, igniting the sky into
a sheet of opaque "lava." Large animals, are broiled.

PHOTO (COLOR): An Hour to Six Months: After an hour the red sky
darkens into blackness, and floating soot from charred debris begins a
year-long night on Earth.

PHOTO (COLOR): Six Months to a Decade: Sulfur thrown into Earth's
atmosphere converts to a sulfuric acid rain, blocking out the Sun for
several years and poisoning many small living creatures. Ocean life
now feels the hit as strongly as land creatures. Temperatures plummet,
creating a micro Ice Age.

PHOTO (COLOR): A Decade and Beyond: Now carbon dioxide in the
atmosphere takes over and reverses the cold, rather suddenly warming
the planet for 50 to 100 years, reaching a much hotter than normal
peak. If this controversial reverse in temperature occurred, the shock
would have been too much for many of the living beings that survived
the earlier catastrophes.

~~~~~~~~

By Ron Cowen

Ron Cowen is a frequent contributor to ASTRONOMY. He is an editor at
Science News.
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Source: Astronomy, Apr96, Vol. 24 Issue 4, p34, 8p, 6c.
Item Number: 9602290206