[meteorite-list] Ancient Crash, Epic Wave

[Darren Garrison]

http://www.nytimes.com/2006/11/14/science/14WAVE.html?_r=1&ref=science&oref=slogin

November 14, 2006
Ancient Crash, Epic Wave
By SANDRA BLAKESLEE

Correction Appended

At the southern end of Madagascar lie four enormous wedge-shaped sediment
deposits, called chevrons, that are composed of material from the ocean floor.
Each covers twice the area of Manhattan with sediment as deep as the Chrysler
Building is high.

On close inspection, the chevron deposits contain deep ocean microfossils that
are fused with a medley of metals typically formed by cosmic impacts. And all of
them point in the same direction — toward the middle of the Indian Ocean where a
newly discovered crater, 18 miles in diameter, lies 12,500 feet below the
surface.

The explanation is obvious to some scientists. A large asteroid or comet, the
kind that could kill a quarter of the world’s population, smashed into the
Indian Ocean 4,800 years ago, producing a tsunami at least 600 feet high, about
13 times as big as the one that inundated Indonesia nearly two years ago. The
wave carried the huge deposits of sediment to land.

Most astronomers doubt that any large comets or asteroids have crashed into the
Earth in the last 10,000 years. But the self-described “band of misfits” that
make up the two-year-old Holocene Impact Working Group say that astronomers
simply have not known how or where to look for evidence of such impacts along
the world’s shorelines and in the deep ocean.

Scientists in the working group say the evidence for such impacts during the
last 10,000 years, known as the Holocene epoch, is strong enough to overturn
current estimates of how often the Earth suffers a violent impact on the order
of a 10-megaton explosion. Instead of once in 500,000 to one million years, as
astronomers now calculate, catastrophic impacts could happen every 1,000 years.

The researchers, who formed the working group after finding one another through
an international conference, are based in the United States, Australia, Russia,
France and Ireland. They are established experts in geology, geophysics,
geomorphology, tsunamis, tree rings, soil science and archaeology, including the
structural analysis of myth. Their efforts are just getting under way, but they
will present some of their work at the American Geophysical Union meeting in
December in San Francisco.

This year the group started using Google Earth, a free source of satellite
images, to search around the globe for chevrons, which they interpret as
evidence of past giant tsunamis. Scores of such sites have turned up in
Australia, Africa, Europe and the United States, including the Hudson River
Valley and Long Island.

When the chevrons all point in the same direction to open water, Dallas Abbott,
an adjunct research scientist at Lamont-Doherty Earth Observatory in Palisades,
N.Y., uses a different satellite technology to look for oceanic craters. With
increasing frequency, she finds them, including an especially large one dating
back 4,800 years.

So far, astronomers are skeptical but are willing to look at the evidence, said
David Morrison, a leading authority on asteroids and comets at the NASA Ames
Research Center in Mountain View, Calif. Surveys show that as many as 185 large
asteroids or comets hit the Earth in the far distant past, although most of the
craters are on land. No one has spent much time looking for craters in the deep
ocean, Dr. Morrison said, assuming young ones don’t exist and that old ones
would be filled with sediment.

Astronomers monitor every small space object with an orbit close to the Earth.
“We know what’s out there, when they return, how close they come,” Dr. Morrison
said. Given their observations, “there is no reason to think we have had major
hits in the last 10,000 years,” he continued, adding, “But if Dallas is right
and they find 10 such events, we’ll have a real contradiction on our hands.”

Peter Bobrowski, a senior research scientist in natural hazards at the
Geological Survey of Canada, said “chevrons are fantastic features” but do not
prove that megatsunamis are real. There are other interpretations for how
chevrons are formed, including erosion and glaciation. Dr. Bobrowski said. It is
up to the working group to prove its claims, he said.

William Ryan, a marine geologist at the Lamont Observatory, compared Dr.
Abbott’s work to that of other pioneering scientists who had to change the way
their colleagues thought about a subject.

“Many of us think Dallas is really onto something,” Dr. Ryan said. “She is
building a story just like Walter Alvarez did.” Dr. Alvarez, a professor of
earth and planetary sciences at the University of California, Berkeley, spent a
decade convincing skeptics that a giant asteroid wiped out the dinosaurs 65
million years ago.

Ted Bryant, a geomorphologist at the University of Wollongong in New South
Wales, Australia, was the first person to recognize the palm prints of
mega-tsunamis. Large tsunamis of 30 feet or more are caused by volcanoes,
earthquakes and submarine landslides, he said, and their deposits have different
features.

Deposits from mega-tsunamis contain unusual rocks with marine oyster shells,
which cannot be explained by wind erosion, storm waves, volcanoes or other
natural processes, Dr. Bryant said.

“We’re not talking about any tsunami you’re ever seen,” Dr. Bryant said. “Aceh
was a dimple. No tsunami in the modern world could have made these features.
End-of-the-world movies do not capture the size of these waves. Submarine
landslides can cause major tsunamis, but they are localized. These are deposited
along whole coastlines.”

For example, Dr. Bryant identified two chevrons found over four miles inland
near Carpentaria in north central Australia. Both point north. When Dr. Abbott
visited a year ago, he asked her to find the craters.

To locate craters, Dr. Abbott uses sea surface altimetry data. Satellites scan
the ocean surface and log the exact height of it. Underwater mountain ranges,
trenches and holes in the ground disturb the Earth’s gravitational field,
causing sea surface heights to vary by fractions of an inch. Within 24 hours of
searching the shallow water north of the two chevrons, Dr. Abbott found two
craters.

Not all depressions in the ocean are impact craters, Dr. Abbott said. They can
be sink holes, faults or remnant volcanoes. A check is needed. So she obtained
samples from deep sea sediment cores taken in the area by the Australian
Geological Survey.

The cores contain melted rocks and magnetic spheres with fractures and textures
characteristic of a cosmic impact. “The rock was pulverized, like it was hit
with a hammer,” Dr. Abbott said. “We found diatoms fused to tektites,” a glassy
substance formed by meteors. The molten glass and shattered rocks could not be
produced by anything other than an impact, she said.

“We think these two craters are 1,200 years old,” Dr. Abbott said. The chevrons
are well preserved and date to about the same time.

Dr. Abbott and her colleagues have located chevrons in the Caribbean, Scotland,
Vietnam and North Korea, and several in the North Sea.

Hither Hills State Park on Long Island has a chevron whose front edge points to
a crater in Long Island Sound, Dr. Abbott said. There is another, very faint
chevron in Connecticut, and it points in a different direction.

Marie-Agnès Courty, a soil scientist at the European Center for Prehistoric
Research in Tautavel, France, is studying the worldwide distribution of
cosmogenic particles from what she suspects was a major impact 4,800 years ago.

But Madagascar provides the smoking gun for geologically recent impacts. In
August, Dr. Abbott, Dr. Bryant and Slava Gusiakov, from the Novosibirsk Tsunami
Laboratory in Russia, visited the four huge chevrons to scoop up samples.

Last month, Dee Breger, director of microscopy at Drexel University in
Philadelphia, looked at the samples under a scanning electron microscope and
found benthic foraminifera, tiny fossils from the ocean floor, sprinkled
throughout. Her close-ups revealed splashes of iron, nickel and chrome fused to
the fossils.

When a chondritic meteor, the most common kind, vaporizes upon impact in the
ocean, those three metals are formed in the same relative proportions as seen in
the microfossils, Dr. Abbott said.

Ms. Breger said the microfossils appear to have melded with the condensing
metals as both were lofted up out of the sea and carried long distances.

About 900 miles southeast from the Madagascar chevrons, in deep ocean, is
Burckle crater, which Dr. Abbott discovered last year. Although its sediments
have not been directly sampled, cores from the area contain high levels of
nickel and magnetic components associated with impact ejecta.

Burckle crater has not been dated, but Dr. Abbott estimates that it is 4,500 to
5,000 years old.

It would be a great help to the cause if the National Science Foundation sent a
ship equipped with modern acoustic equipment to take a closer look at Burckle,
Dr. Ryan said. “If it had clear impact features, the nonbelievers would
believe,” he said.

But they might have more trouble believing one of the scientists, Bruce Masse,
an environmental archaeologist at the Los Alamos National Laboratory in New
Mexico. He thinks he can say precisely when the comet fell: on the morning of
May 10, 2807 B.C.

Dr. Masse analyzed 175 flood myths from around the world, and tried to relate
them to known and accurately dated natural events like solar eclipses and
volcanic eruptions. Among other evidence, he said, 14 flood myths specifically
mention a full solar eclipse, which could have been the one that occurred in May
2807 B.C.

Half the myths talk of a torrential downpour, Dr. Masse said. A third talk of a
tsunami. Worldwide they describe hurricane force winds and darkness during the
storm. All of these could come from a mega-tsunami.

Of course, extraordinary claims require extraordinary proof, Dr. Masse said,
“and we’re not there yet.”

Correction: Nov. 16, 2006

An article in Science Times on Tuesday about new research suggesting that a
comet or an asteroid may have struck the Indian Ocean 4,800 years ago included
an incorrect estimate from researchers for the frequency of such collisions. The
current estimate is one impact on the order of a 10-megaton bomb every 1,000
years, not every few thousand years. The article also misstated the name of a
state park on Long Island that has a large sand wedge called a chevron, which
may indicate that a comet or meteor landed in the ocean nearby. It is Hither
Hills, not Heather Hill.