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From Greenhouse To Icehouse: New Clues On Ancient Climate Shift

"The big fall in temperature can only be explained by a decrease in carbon dioxide (CO2), the principal greenhouse gas, which has natural causes (such as volcanic eruptions) as well as man-made ones."
by Richard Ingham
Paris (AFP) Feb 07, 2007
Nearly 34 million years ago, a plunge in temperatures began to transform Antarctica, then a lush, green continent that for tens of millions of years had been bathed in warmth, into the icy wilderness more familiar to us today. Seafloor evidence taken from around Antarctica testifies to this event, known as the Oligocene glaciation. But it remains shrouded in many mysteries.

While Antarctica gradually froze and thickened into a massive icesheet, what happened in the rest of the world? And, most intriguing of all, why did the great chill happen?

In new studies published on Thursday by Nature, investigators believe they can dispel a bit of the fog surrounding the Oligocene shift, demonstrating that the freeze had a profound impact on climate and biodiversity deep inside the great continents.

Guillaume Dupont-Nivet, of Utrecht University in the Netherlands, led a team that probed layers of sedimentary rock in the Xining basin on the northeastern fringes of the arid Tibetan plateau.

Sedimentary deposits such as these came from lakes that once flooded the region on a rhythmic scale. The researchers constructed a timescale at which this flooding occurred and eventually ended by measuring a minute, residual magnetic signal from particles in the rock.

Earth's magnetic field goes through changes, and each shift can be detected in the polarity of iron particles in rock deposits, providing a calendar of geological history.

Dupont-Nivet found that the drying out of Xining occurred precisely when the Antarctic glaciation began.

That finding is a blow for the leading explanation as to why Asia became arid.

This theory suggests that Asia became drier after the Indian sub-continent collided with the Asian landmass, an impact that drove up the Tibetan plateau and created a "rain shadow" in which heavy precipitation was unable to penetrate beyond the barrier of the Himalayas.

In the continental United States, meanwhile, researchers led by Alessandro Zanazzi of the University of South Carolina measured two stable isotopes of oxygen in teeth enamel and bones of mammal fossils in the northern Great Plans.

These isotopes are indicators of the temperature record at the time the animals lived.

Zanassi's team calculate that over about 400,000 years, the temperature plummeted by 8.2 C (14.75 F), give or take 3.1 C (5.6 F).

That would explain why the fossil records show a dramatic turnover of species during this period. Many species of amphibians, reptiles and gastropods in what is now North America died out during the big chill, but most mammals were unaffected.

The likely reason: cold-blooded species cannot regulate their body temperature but mammals can.

As to the big question -- how did Antarctica's glaciation occur? -- the mainstream idea is tectonic. Antarctica, Australia and South America were glommed together in a supercontinent, which eventually pulled apart.

Antarctica became surrounded by a chill sea that left it climatically isolated, according to this thinking.

But, in a commentary also published by Nature, Gabriel Bowen of Purdue University, Indiana, says that this hypothesis, too, has to be nixed.

For one thing, computer models suggest a change in ocean circulation around Antarctica would warm, rather than cool, the northern continents. And the latest findings prove that the glaciation could not have been a purely local event, but global.

"The early Oligocene was a turning point not just for Antarctica, but for Earth as a whole," says Bowen.

He believes that the big fall in temperature can only be explained by a decrease in carbon dioxide (CO2), the principal greenhouse gas, which has natural causes (such as volcanic eruptions) as well as man-made ones.

Finding out what happened could help understand the current state of the world's climate, which is being driven by man-made warming towards a "potentially ice-free state," believes Bowen.

Source: Agence France-Presse

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Washington (AFP) Feb 07, 2007
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