[meteorite-list] Mass Extinction Comes Every 62 Million Years, UC Physicists Discover

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:55 2005
Message-ID: <200503101741.j2AHfnC19661_at_zagami.jpl.nasa.gov>

http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2005/03/10/MNGFIBN6PO1.DTL

Mass extinction comes every 62 million years, UC physicists discover
David Perlman,
San Francisco Chronicle
March 10, 2005

With surprising and mysterious regularity, life on Earth has flourished
and vanished in cycles of mass extinction every 62 million years, say
two UC Berkeley scientists who discovered the pattern after a
painstaking computer study of fossil records going back for more than
500 million years.

Their findings are certain to generate a renewed burst of speculation
among scientists who study the history and evolution of life. Each
period of abundant life and each mass extinction has itself covered at
least a few million years -- and the trend of biodiversity has been
rising steadily ever since the last mass extinction, when dinosaurs and
millions of other life forms went extinct about 65 million years ago.

The Berkeley researchers are physicists, not biologists or geologists or
paleontologists, but they have analyzed the most exhaustive compendium
of fossil records that exists -- data that cover the first and last
known appearances of no fewer than 36,380 separate marine genera,
including millions of species that once thrived in the world's seas,
later virtually disappeared, and in many cases returned.

Richard Muller and his graduate student, Robert Rohde, are publishing a
report on their exhaustive study in the journal Nature today, and in
interviews this week, the two men said they have been working on the
surprising evidence for about four years.

"We've tried everything we can think of to find an explanation for these
weird cycles of biodiversity and extinction," Muller said, "and so far,
we've failed."

But the cycles are so clear that the evidence "simply jumps out of the
data," said James Kirchner, a professor of earth and planetary sciences
on the Berkeley campus who was not involved in the research but who has
written a commentary on the report that is also appearing in Nature today.

"Their discovery is exciting, it's unexpected and it's unexplained,"
Kirchner said. And it is certain, he added, to send other scientists in
many disciplines seeking explanations for the strange cycles. "Everyone
and his brother will be proposing an explanation -- and eventually, at
least one or two will turn out to be right while all the others will be
wrong."

Muller and Rohde conceded that they have puzzled through every
conceivable phenomenon in nature in search of an explanation: "We've had
to think about solar system dynamics, about the causes of comet showers,
about how the galaxy works, and how volcanoes work, but nothing explains
what we've discovered," Muller said.

The evidence of strange extinction cycles that first drew Rohde's
attention emerged from an elaborate computer database he developed from
the largest compendium of fossil data ever created. It was a 560-page
list of marine organisms developed 14 years ago by the late J. John
Sepkoski Jr., a famed paleobiologist at the University of Chicago who
died at the age of 50 nearly five years ago.

Sepkoski himself had suggested that marine life appeared to have its ups
and downs in cycles every 26 million years, but to Rohde and Muller, the
longer cycle is strikingly more evident, although they have also seen
the suggestion of even longer cycles that seem to recur every 140
million years.

Sepkoski's fossil record of marine life extends back for 540 million
years to the time of the great "Cambrian Explosion," when almost all the
ancestral forms of multicellular life emerged, and Muller and Rohde
built on it for their computer version.

Muller has long been known as an unconventional and imaginative
physicist on the Berkeley campus and at the Lawrence Berkeley
Laboratory. It was he, for example, who suggested more than 20 years ago
that an undiscovered faraway dwarf star -- which he named "Nemesis" --
was orbiting the sun and might have steered a huge asteroid into the
collision with Earth that drove the dinosaurs to extinction.

"I've given up on Nemesis," Muller said this week, "but then I thought
there might be two stars somewhere out there, but I've given them both
up now."

He and Rohde have considered many other possible causes for the 62-
million-year cycles, they said.

Perhaps, they suggested, there's an unknown "Planet X" somewhere far out
beyond the solar system that's disturbing the comets in the distant
region called the Oort Cloud -- where they exist by the millions -- to
the point that they shower the Earth and cause extinctions in regular
cycles. Daniel Whitmire and John Matese of the University of Louisiana
at Lafayette proposed that idea as a cause of major comet showers in
1985, but no one except UFO believers has ever discovered a sign of it.

Or perhaps there's some kind of "natural timetable" deep inside the
Earth that triggers cycles of massive volcanism, Rohde has thought.
There's even a bit of evidence: A huge slab of volcanic basalt known as
the Deccan Traps in India has been dated to 65 million years ago -- just
when the dinosaurs died, he noted. And the similar basaltic Siberian
Traps were formed by volcanism about 250 million years ago, at the end
of the Permian period, when the greatest of all mass extinctions drove
more than 70 percent of all the world's marine life to death, Rohde said.

The two scientists proposed more far-out ideas in their report in
Nature, but only to indicate the possibilities they considered.

Muller's favorite explanation, he said informally, is that the solar
system passes through an exceptionally massive arm of our own spiral
Milky Way galaxy every 62 million years, and that that increase in
galactic gravity might set off a hugely destructive comet shower that
would drive cycles of mass extinction on Earth.

Rohde, however, prefers periodic surges of volcanism on Earth as the
least implausible explanation for the cycles, he said -- although it's
only a tentative one, he conceded.

Said Muller: "We're getting frustrated and we need help. All I can say
is that we're confident the cycles exist, and I cannot come up with any
possible explanation that won't turn out to be fascinating. There's
something going on in the fossil record, and we just don't know what it
is."
Received on Thu 10 Mar 2005 12:41:49 PM PST