| Subject: Re: Precambrian impact |
Author:
Blobrana
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Date Posted: 23:38:59 12/13/04 Mon
In reply to:
Blobrana
's message, "Precambrian impact" on 20:05:30 12/03/04 Fri
UPDATE:
A cataclysm 250 million years ago wiped out nearly all life in the Earth's oceans, and nearly three-quarters of the plants and animals on land vanished too. It was the greatest catastrophe the Earth has ever experienced - - but scientists who study such events are in sharp disagreement over what caused it.
Was it the crash of a giant asteroid or meteorite that killed off so much life? Or was a violent surge of volcanism from deep within the Earth the deadly factor?
The argument over sketchy evidence from the long-ago geologic time called the "End Permian" persists, and the contentious scientific debate will continue this week when the American Geophysical Union meets at Moscone Centre, with nearly 11,000 scientists in attendance.
Scientists do agree that the mass extinction was sudden: The Earth is known to be at least 4 billion years old, and "the Great Dying," as palaeontologists call it, may have lasted less than 200,000 years from start to finish, a mere moment in geologic time.
In the most recent stage of the controversy, teams of researchers have squared off in support of two opposing theories to explain what triggered the disaster.
One international research group, led by Christian Koeberl of the University of Vienna and Kenneth Farley of the California Institute of Technology, is arguing that there is no hard evidence at all to support the impact theory.
Newly discovered chemical signs in the Austrian Alps and the Italian Dolomites, where elements typical of asteroids or meteorites are almost nonexistent, is one key to their argument.
Koeberl and Farley, as well as many other geochemists, say the extinction was more likely due to an immense outpouring of lava in the northern part of a once huge super-continent known as Pangaea. The remains of that event can be found today in a vast surface region of basaltic rock in northern Russia known as the Siberian Traps.
The volcanic violence would have induced abrupt global heating, Throwing up a dark pall of hot ash, toxic gases and carbon dioxide that virtually no living plants or animals could survive, according to this theory.
The darkness of the skies then would have caused a major period of global cold.
Farley, Koeberl and their colleagues have just published their Arguments in the December issue of the journal Geology.
"Our findings support the view that evidence for an extraterrestrial impact event is weak and inconsistent."
At the same time, they suggest that widespread volcanic activity may have been the 'smoking gun,' quite literally, that wiped out much of life on Earth.
But another team, including Luann Becker of UC Santa Barbara and Asish Basu of the University of Rochester in New York, insists that ancient meteorite fragments discovered in Antarctica -- and traces of the unique isotope of helium in Antarctic rocks -- clearly show that when Pangaea was forming, a gigantic meteorite must have crashed into the Southern Hemisphere.
That impact created an enormous crater hundreds of miles wide that may be located now in the seabed beneath the Indian Ocean off Australia's north-western coast, on the edge of a seismically active desert region called the Canning Basin. Becker believes that a submerged mountain there called the Bedout High (pronounced Be-doo) could be the central peak of the long-vanished crater.
The crater-forming blast could have darkened the skies, raised a life-choking cloud of dust, rock fragments and gases, and created what the late Carl Sagan and his colleagues 20 years ago termed a "nuclear winter."
They and their colleagues published their view of the mass extinction's causes in the journal Science last year, and pinpointed the Bedout High as the best evidence for impact in another Science report last May.
The time when all life succumbed is known as the boundary between the end of the Permian period -- an epoch that lasted from 286 to 245 million years ago -- and the start of the Triassic period, when life flourished anew and the dinosaurs evolved, ultimately to rule the Earth.
The dinosaurs disappeared during the second greatest mass extinction known in Earth's history. This event took place some 65 million years ago when -- as almost all palaeontologists and geologists agree -- an
asteroid impact extinguished 70 percent of Earth's life and created a wide crater called Chixchilub that now lies under the sea just off the Yucatan Peninsula.
One of the main pieces of evidence supporting an extraterrestrial Impact for the dinosaur-killing event is a thin layer of the element iridium in rock formations around the world, discovered almost 25 years ago by Walter Alvarez of UC Berkeley. Iridium is rare on Earth but common in asteroids. Other evidence is the widespread presence of tiny grains of shocked quartz -- quartz that was subjected to some violent impact -- and the presence of an isotope of helium -- unique in nature to extraterrestrial objects like meteorites, asteroids and the moon – in the layers of iridium and other rocks.
According to Basu and Becker, iridium was rare in the rock samples Becker found in Antarctica -- but there was plenty of shocked quartz And many fragments of what could only be extremely ancient meteorites, they reported. And in many formations from the end of the Permian period, they found clear signs of the crucial evidence: the presence of the unique extraterrestrial isotope of helium, known as helium-3.
Farley, however, is an expert on the so-called noble gases, including helium, and in all his laboratory tests he insists he has been unable to find any trace of the extraterrestrial gases in the very same formations that Becker and Basu have studied.
"There's absolutely no evidence of an extraterrestrial impact in any of the formations that date from the end of the Permian period."
But "As the late Carl Sagan always said, 'The absence of evidence is not evidence of absence.' “In other words, just because Farley can't find helium-3 does not prove it isn't there.
As a kind of neutral observer, Paul R. Renne, who directs the Berkeley Geochronology Centre, and where he and his fellow scientists determine the ages of the ancient rocks and fossils that scientists find all over the world, has examined the evidence on both sides.
"The debate is a real mess scientifically, but there's massive doubt about the work in Antarctica, and there's no really consistent evidence to support it."
The argument over what in fact wiped out so much of life on Earth some 250 million years ago is bound to continue for a long, long time, and only a month ago in letters to the Science, no fewer than eight scientists who study the issue called the report by Becker and Basu a sensationalistic claim.
To which Becker replied, in an angry statement to the journal Nature,
"this is science by intimidation."
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