Oldest Ice in North America Hints at Hardy Tundra
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.
The team recovered 740,000-year-old ash from the 4-inch (10-centimeter) thick layer covering the ice wedge over a 160-foot (50-meter) area. The layer of ash was made up of small glass grains that the team then dated used a radiometric technique.
"We cannot directly determine the age of the ground ice, but we can determine the age of the overlying volcanic ash, providing a minimum age for the ground ice," said John Westgate, a geologist at the University of Toronto and co-author of the study.
Froese said this means the permafrost under the ash has not disappeared since at least that time.
"What it tells us is that the deeper part of permafrost has been stable for a long time," he said.
It's not surprising that permafrost dating back more than 700,000 years has been found, said Larry Hinzman, director of the International Arctic Research Center at the University of Alaska in Fairbanks.
"It has been assumed there was very old, ancient permafrost. This is the first evidence," Hinzman said.
Ice-rich permafrost holds together Arctic ground that would turn to "soup" if thawed.
That melting threatens basic infrastructure in inhabited polar regions, such as pipelines, roads, and airport runways.
Lead study author Froese emphasized that the study's findings do not suggest permafrost melting is not occurring. But the thaw may take much longer than in other areas of the Arctic that are rapidly crumbling, such as glaciers and sea ice.
Although deep permafrost may thaw slowly, shallow layers is "where the action will take place" by melting faster and releasing large stores of carbon trapped in the ice, Froese said.
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