Oldest Ice in North America Hints at Hardy Tundra

Kimberly Johnson
for National Geographic News
September 22, 2008

The oldest ice ever found in North America shows that ancient permafrost withstood periods of warming, a new study says.

Scientists fear that modern permafrost—soil that remains frozen in the polar regions—may melt and release potentially huge reservoirs of carbon that would speed global warming, scientists say. (See story.)

But the new study suggests that such a thaw could take much longer than previously believed, according to study leader Duane Froese, a geology professor at the University of Alberta in Edmonton, Canada.

Estimated to be at least 740,000 years old, the wedges of Canadian ice illustrate the longevity and resiliency of deeper permafrost during warmer climates of the past, they say.

The findings counter previously held theories that permafrost in Alaska and in Canada's central Yukon Territory thawed about 120,000 years ago, during a period warmer than today.

The study appears tomorrow in the journal Science.

Ash and Ice

Ice wedges are formed in frigid dry areas when temperatures get so cold that the ground cracks open. Water runoff from spring thaws fills the vertical cracks in the earth and then freezes, creating a vein of ice that builds outward with each passing year.

The ancient ice wedge studied by Froese and his team was found buried under layers of volcanic ash and sediment in a mining area in Canada's central Yukon Territory.

When gold miners exposed the ancient ice vein, they also uncovered a layer of volcanic ash immediately covering the ice wedge, the researcher explained.

"What was unique about this situation is we had volcanic ash we could date," Froese said.

Volcanic ash can help scientists determine the age of ice that is older than the range of radiocarbon dating, which spans about 50,000 years, Froese explained. It's a strategy often used in volcanic regions, such as New Zealand, Alaska, and Iceland, he added.

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