[meteorite-list] When Earth Was Gasping For Breath 250 Million Years Ago

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 14 20:21:57 2005
Message-ID: <200504150021.j3F0LS502666_at_zagami.jpl.nasa.gov>

http://www.sitnews.us/0405news/041405/041405_shns_greatdying.html

When Earth was gasping for breath
By LEE BOWMAN
Scripps Howard News Service
April 14, 2005

The world nearly suffocated about 250 million years ago, according to a
new study of oxygen levels drawn from sediments laid down around the
time of the biggest mass extinction in Earth's history.

Not only did plummeting oxygen levels over a 20 million-year span
directly contribute to an event called the "Great Dying," but the
changes also made most dry land above sea level uninhabitable for many
animals and plants for millions of years.

"Oxygen dropped from its highest level to its lowest level ever (during
the time Earth has supported life) in only 20 million years, which is
quite rapid, and animals that once were able to cross mountain passes
easily suddenly had their movements severely restricted," said Raymond
Huey, a University of Washington biology professor and co-author of the
report. It is being published Friday in the journal Science.

Scientists calculate that 90 percent of all marine life and
three-quarters of all land plants and animals became extinct during the
episode of low oxygen, greenhouse conditions and lowered sea levels at
the boundary of the Permian and Triassic geological periods.

Huey and co-author Peter Ward, a paleontologist at the university who
specializes in extinction events, say that global warming - triggered by
massive volcanic activity and a lower sea level - was the biggest
contributor to the Great Dying.

Ward and other researchers, in another report published by Science in
January, reported that fossil evidence collected in South Africa showed
that the die-off was gradual over about 10 million years, then
accelerated for another 5 million years. Oxygen levels - and ecological
diversity - didn't fully recover for another 100 million years.

Ward doesn't believe an asteroid impact played a part at the time. He
says that evidence of the gradual decline is very different from the
evidence of the sudden demise of most dinosaurs and many other life
forms following an asteroid impact 66 million years ago. Other
scientists still believe that an asteroid impact contributed to climate
change 250 million years ago.

What Huey and Ward did in the new study was calculate just how
unpleasant a world the low oxygen conditions created. During most of the
Permian period, all continents were clumped into a single landmass,
called Pangea.

Sediment analysis from early in the period shows oxygen saturation of
the air at sea level was about 30 percent, lung candy even compared to
today's 21 percent. By the end of the Permian period, the air was only
about 16 percent oxygen, falling to a low of 12 percent -the ratio now
found atop some of the world's highest mountains - about 10 million
years into the Triassic.

Ward said researchers have previously assumed that Pangea was not only a
supercontinent but also a great highway of migration for most species
during the early Permian. Fossils of the same animals from the period
had turned up in today's South Africa, China and Russia.

But now it seems that thinner air not only starved many animals and
plants into extinction, it forced survivors into low-lying niches where
they could breathe more easily. And those survivors not only could live
with less oxygen but also adapted to live in new terrain.

"Declining oxygen levels and warming temperatures would have been doubly
stressful for late Permian animals," Huey said. "As the body warms, body
temperatures and metabolic rates go up. That means oxygen demand is
going up, so animals would have faced an increased oxygen demand and a
reduced supply."

The researchers calculate that about half of all land on the planet -
any spot more than about 1,500 feet above sea level - would have had air
too thin to sustain life.

"I think we have to go back and look at oxygen and its role in evolution
and how different species developed," Ward said. "You can go without
food for a couple of weeks. You can go without water for a few days. But
how long can you go without oxygen? A couple of minutes? There's nothing
with a greater evolutionary effect."

A few mammal-like animals that lived in burrows and had lungs adapted to
low-oxygen conditions survived in the thin air, as did the ancestors of
dinosaurs and birds.

"We know that birds can live at much lower oxygen concentrations than we
do, and we think there were similar lung adaptations in dinosaurs," Ward
said.
Received on Thu 14 Apr 2005 08:21:27 PM PDT