July 28, 2008 -- Once thought temperate, the climate on Earth 300 million years ago may have gotten far colder than scientists ever suspected.
Scientists tend to think that life on the great supercontinent Pangea enjoyed weather that was similar to today's. With an ice sheet dominating the landscape near the south pole, there was certainly an occasional cold snap, but the tropics are believed to have been hot and humid.
Gerilyn Soreghan of Oklahoma University and a team of researchers are now questioning that belief. They've found evidence that a massive glacier lived near the equator, further south and closer to sea level than thought possible.
If true, the discovery implies the climate during the late Paleozoic Era was even chillier than during the last Ice Age. It could also have been extremely volatile, swinging in wild cycles back and forth between frigid and balmy over millions of years.
Glacial deposits and changes in ancient sea level show that ice probably ebbed and flowed through a series of Paleozoic Ice Ages, much as it has in recent geologic history, with the last cold snap around 20,000 years ago.
Even during the worst of the last Ice Age, though, Earth's tropics stayed relatively mild. Any glaciers near the equator would have melted if they descended below altitudes of around 3,400 to 4,400 meters (11,145 to 14,440 feet) above sea level.
The same was thought to be true for Pangean times, until Soreghan discovered strange rocks hiding out on the floor of the remote Unaweep Canyon, high on the Colorado Pleateau.
The valley is full of deposits of loess, or silt, that has hardened into rock after millions of years of under intense pressure. Loess is commonly created when glaciers grind rocks into powder, and the wind then sweeps the powder into great heaps.
Some of the broken-up pieces of rock Soreghan and her team found are also striated, a feature that glaciers often leave as they scrape slowly across the landscape.
If Unaweep were carved by a huge tongue of ice in the late Paleozoic, it would have been too low in elevation, perhaps 500 to 1,000 meters (1,640 to 3,280 feet) above sea level. Soreghan suggests we may need to rethink what climate was like back then.
"The implication is that the magnitude of climate change between glacial and interglacial periods was either huge, or it just stayed a lot colder all of the time." Soreghan said.
But Nick Eyles of the University of Toronto said the evidence isn't conclusive: "These types of deposits can be formed in several different ways. There's an 'if' or a 'but' to every plank of their conclusions."
"It's good because it will make people think," he said of the team's work. "But they're arguing for a global climate event from one canyon, and on weak evidence. This is a radical reinterpretation of climate, and that's got to be matched by better data."
One way to prove their case, Eyles said, would be if the team went back and found striations raked across the canyon walls, a 'smoking gun' that a glacier carved the ancient Unaweep.
Related Links:
Discovery News blog: Earth Impacts
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