London, September 19 : A new study has determined that a 740,000-year-old wedge of ice discovered in central Yukon Territory, Canada, is the oldest known ice in North America, thus suggesting that permafrost has survived climates warmer than today's.
"Previously, it was thought that the permafrost had completely disappeared from the interior about 120,000 years ago," said Duane Froese, an earth scientist at the University of Alberta in Edmonton, Canada, who is the author of the study.
"This deep permafrost appears to have been stable for more than 700,000 years, including several periods that were warmer and wetter," he added.
According to a report in Nature News, the relict ground ice is located in the Klondike region of the Yukon and was exposed by gold mining in the late 1990s.
Froese first identified the site in 2000, but assumed at the time that the ice and surrounding permafrost were relatively young.
It wasn't until 2005, when a large rainstorm uncovered a layer of volcanic ash, or tephra, on top of the ice that he and his colleagues were able to estimate its age.
Fission-track dating, which quantified the damage done to the tephra's glassy particles by the decay of the uranium and other radioactive materials they contain, put the ash's age between 680,000 and 800,000 years.
"Based on the exposure, the ash is above the ice and it seems to be a valid date," said Jim Beget, a tephra expert at the University of Alaska in Fairbanks, who visited the site in 2005.
If the finding is corroborated and other similar sites are identified, climate modellers could use the information to improve their understanding of permafrost dynamics under warmer scenarios.
Earlier calculations estimated that 48 billion tonnes of carbon could be released from Canadian permafrost over the twenty-first century if the mean annual temperature increased by 4 degree Celsius.
The old-ice finding suggests that some pockets of permafrost may be more resilient to climate change.
According to Vladimir Romanovsky, a geophysicist who is also at the University of Alaska in Fairbanks, "But it doesn't mean that we shouldn't expect very severe changes in permafrost if this predicted warming does happen."