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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