[meteorite-list] New Theory On Dinosaurs: Multiple Impacts Did Them In

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:14 2004
Message-ID: <200211062225.OAA03497_at_zagami.jpl.nasa.gov>

New Theory On Dinosaurs: Multiple Impacts Did Them In
By WILLIAM J. BROAD
New York Times
November 5, 2002

For more than a decade, most scientists have believed that the extinction of
the dinosaurs was caused by a single event: the crash of an immense body
from outer space, its explosive force like a hundred million hydrogen bombs,
igniting firestorms and shrouding the earth in a dense cloud of dust that
blocked sunlight and sent worldwide temperatures plummeting.

The theory gained wide acceptance in 1991, after the discovery of a crater
buried under the tip of the Yucatán Peninsula. The giant gash stretched 110
miles from rim to rim, and its age was found to be 65 million years, the
same time as the death of the dinosaurs.

Now, however, scientists working in Ukraine have discovered that a
well-known but smaller crater, some 15 miles wide, had been inaccurately
dated and is actually 65 million years old, making the blast that created it
a likely contributor to the end of the dinosaurs.

So too, a British team has recently found a crater at the bottom of the
North Sea dating to the same era and stretching over 12 miles in a series of
concentric rings.

The discoveries are giving new support to the idea that killer objects from
outer space may have sometimes arrived in pairs or even swarms, perhaps
explaining why the extinctions seen in the fossil record can be messy
affairs, with species reeling before a final punch finishes them off.

"It's so clear," said Dr. Gerta Keller, a geologist and paleontologist at
Princeton, who studies the links between cosmic bombardments and life
upheavals. "A tremendous amount of new data has been accumulated over the
past few years that points in the direction of multiple impacts."

But Dr. Keller added that many scholars had staked their reputations on the
idea of a single dinosaur-ending disaster and were reluctant to consider the
new evidence. "Old ideas," she said, "die hard."

Her own research, Dr. Keller added, suggests the reality of multiple strikes
and raises doubts that the Yucatán rock, whose crater is known as Chicxulub,
was the event that sealed the dinosaurs' fate. Instead, she said, the main
killer "has yet to be found."

The ferment is prompting scientists around the globe to look for new craters
and to reassess the ages of old ones in search of clues to the wave of
global extinction that did in thousands of species - not only the dinosaurs
but many plants, fish and plankton - at the end of the Cretaceous period.

"There are over 170 confirmed craters on earth and we know the precise
impact age of only around half," said Dr. Simon P. Kelley of the Open
University in Britain, who found the dating error on the Ukraine crater,
along with Dr. Eugene P. Gurov of the Institute of Geological Sciences in
Ukraine. Even in the United States, he added, several craters are poorly
dated.

"In the U.K., we have a phrase, `You wait an hour for a bus, then three come
along all at once,' " he remarked in an interview. "Maybe impacts are like
that."

The idea that a giant intruder from outer space killed off the dinosaurs was
proposed in 1980 by Dr. Luis W. Alvarez; his son, Dr. Walter Alvarez; and
their colleagues at the University of California at Berkeley. It was met
with great skepticism at first, but in time became the standard belief.

In his 1997 book, "T. Rex and the Crater of Doom," Dr. Walter Alvarez, a
geologist, said he had considered the possibility of multiple impacts until
1991 and the discovery of the huge Yucatán crater, which seemed big enough
to solve the mystery on its own.

Dr. Kelley and Dr. Gurov presented their findings from Ukraine in the August
issue of the journal Meteoritics & Planetary Science. In geologic time, the
twin birth throes of the Ukraine and Yucatán craters, they note, suggest
rather than prove "that they combined to lead to the mass extinction" at the
end of the Cretaceous period and raise questions of other possible cosmic
killers.

Known as Boltysh, the newly dated crater lies in eastern Ukraine in the
basin of the Tyasmin River, a tributary of the Dnieper. Though just 15 miles
wide, the buried crater, whose presence is revealed by deep jumbled masses
of melted and broken rocks, is surrounded by a ring of rocky debris that
extends over many hundreds of square miles, conjuring up a fiery cataclysm.
The two scientists say in their report that this kind of crash today would
have devastated a densely populated nation.

Over the years, scientists had analyzed rocky samples from the Boltysh
crater and found ages ranging from 88 million to 105 million years.

The new dating of the crater by Dr. Kelley and Dr. Gurov used a highly
accurate method that carefully measures the ratio of two isotopes of the
element argon, a colorless, odorless gas that makes up about 1 percent of
the earth's atmosphere. Argon-argon dating works because the isotopes decay
at different rates. By measuring the ratio, it is possible to estimate how
long ago the sample melted to trap atmospheric argon.

Dr. Kelley and Dr. Gurov report that seven samples of melted rock from the
depths of the Boltysh crater yielded an average age of 65.2 million years,
with an accuracy of plus or minus 600,000 years.

By contrast, Chicxulub (pronounced CHEEK-soo-loob) has been dated to 65.5
million years, plus or minus 600,000. Given the range of dating uncertainty,
the two impacts that made the craters may have occurred simultaneously or
been separated by thousands of years.

Scientists have recently looked more favorably at the idea that comets can
travel in packs. In the 1980's, a few speculated that comet showers might
produce strikes on the earth over a period of a million years or so to bring
on extinctions. The idea gained support in 1994 when the comet
Shoemaker-Levy 9 was fractured by the gravitational pull of Jupiter into 21
discernible pieces that then, one by one, bombarded the planet.

Dr. Kelley and colleagues at the University of Chicago and the University of
New Brunswick, writing in the journal Nature in 1998, gave precise dating
evidence to argue that a similar kind of celestial barrage hit the earth 214
million years ago. Spread over Europe and North America, the chain of five
craters, they wrote, indicated that a large comet or asteroid had broken up
and struck the earth in a synchronized assault.

Today, Dr. Kelley said, the odds of the Boltysh and Chicxulub craters'
having formed simultaneously, like the chain, are not great. Still, even if
their times of impact prove to have been only close, experts say, the
one-two punch could still have added to the global turmoil that did in the
dinosaurs and other creatures.

Beneath the North Sea, two British oil geologists have found another crater,
buried under hundreds of feet of ooze, that may have contributed to the
chaos. Writing in the Aug. 1 issue of Nature, Simon A. Stewart and Philip J.
Allen said they were able to date the 12-mile structure to a period 60
million to 65 million years ago. They named it Silverpit, after a nearby
sea-floor channel.

Experts say the new finds may answer an old criticism of the single-impact
theory. Critics, especially the paleontologists who specialize in dinosaur
extinction rates, had long noted that the fossil record of the late
Cretaceous shows a slow decline of many life forms rather than a single vast
die-off. That seemed inconsistent with a cosmic catastrophe.

But now, the emerging family ties among the Boltysh, Silverpit and Chicxulub
craters suggest that a series of impacts may have driven or contributed to
this slow decline.

Dr. Keller of Princeton and her colleagues have found signs of other
intruders from outer space that hit at slightly different times about 65
million years ago, strengthening the gradualist idea.

Working in northeastern Mexico, they discovered that glass spheres of melted
rock once thought to have been thrown out by the Chicxulub impactor were
more likely the result of at least two separate disasters, about 300,000
years apart. They recently presented their findings in a paper for the
Geological Society of America.

Moreover, Dr. Keller said, the evidence suggests that the earlier of the two
cataclysms formed the Chicxulub crater, making its arrival too early to
account for the killer punch of the dinosaur extinction.

Geologic clues that she and her colleagues are collecting from Mexico,
Guatemala, Haiti and Belize, Dr. Keller said, suggest that a barrage of
cosmic bodies hit the earth over the course of 400,000 years. The first was
the Chicxulub event, the second the unlocated impactor at the end of the
Cretaceous period and then a straggler some 100,000 years later.

Strong evidence exists for three impacts at the end of the Cretaceous era,
Dr. Keller said, followed by wide climate shifts that lasted through the
turbulent period.

While geologists hunt for other craters and impact events, they say the most
compelling evidence of all may have vanished. Since the earth's surface is
more than 70 percent water, it is likely that most signs of speeding rocks
from space disappeared long ago in the churning geological processes that
constantly renew the seabed. The North Sea, being relatively shallow, is an
exception.

Despite the inherent difficulties of the research, Dr. Kelley of the Open
University said he planned to redouble his hunt to "try to solve this
problem."
Received on Wed 06 Nov 2002 05:25:15 PM PST