Oldest frozen DNA reveals a greener Greenland

19:00 05 July 2007
NewScientist.com news service
Catherine Brahic
PrintSendFeeds

Researchers say southern Greenland was covered in conifer forest over 450,000 years ago (Image: Science)Enlarge
Tools
Advertisement


Picture this: sweeping conifer forests, with pine, alder, spruce and yew trees, crawling with beetles, flies and spiders, and with butterflies fluttering through the sun-dappled branches.
It does not sound like a description of Greenland, but scientists say this is what the island looked like half a million years ago. They were able to paint the picture by extracting what is probably the oldest-known DNA from the ice at the base of the Greenland ice sheet.
The approach fills a significant gap in knowledge, says lead researcher Eske Willerslev at the University of Copenhagen in Denmark: "About 10% of Earth is covered in ice, and we have really no idea of what the environment looked like in these regions before the ice was formed."
Willerslev and his colleagues extracted hundreds of DNA samples from the ice 2000 metres down at the very base of the southern Greenland ice sheet, in a location known as Dye 3.
Comparing the recovered fragments with the DNA sequences of modern plant and animal species revealed the presence of alder, spruce, pine, and yew trees. They also found DNA from beetles, spiders, flies and butterflies. The best modern-day analogue is the forest in eastern Canada.
Mammals possible
Willerslev does not exclude finding frozen mammal DNA, if researchers were to process larger volumes of ice. "We know from previous experiments to extract DNA from permafrost that plant DNA survives better, probably because there is much more to start with," he told New Scientist.
In 2003, Willerslev extracted horse DNA from permafrost.
Once the researchers extracted the frozen DNA, they set about dating it – and more surprises followed.
A well-regarded modelling study from 2006 by Bette Otto-Bliesner of the US National Center for Atmospheric Research suggested that the Dye 3 site was not covered in ice during the last interglacial period – about 120,000 years ago (Science, vol 311 p 1751).
Disputed dates
But when Willerslev's team dated their recovered DNA, they found that it was at least 450,000 years old. This would mean that the Greenland site must have been completely covered in ice 120,000 years ago.
"We used four different dating techniques," says Willerslev, which returned overlapping date ranges. He adds that the samples were also sent to other labs, which returned the same results. "All the dating experiments suggest the DNA is older than the last interglacial period," he says.
The researchers conclude that the ice at the bottom of the ice sheet at Dye 3 is between 450,000 and 800,000 years old. This implies the 2006 modelling study is incorrect and Dye 3 was not ice-free 120,000 years ago.
This suggests that the Greenland ice sheet is more stable than currently thought. Temperatures in the Arctic during the last interglacial period were significantly warmer than they are today – between 3°C and 5°C higher, according to the latest Intergovernmental Panel on Climate Change report.
The IPCC also says that sea levels are likely to have been between 4 and 6 metres higher than today.
Sea-level clues
"We know the sea level was higher in past and we know [that the extra water] had to come from the ice sheets," says David Vaughan of the British Antarctic Survey. Many scientists think that this water probably came from Greenland, a hypothesis that was strengthened by Otto-Bliesner's 2006 study.
But if Willerslev's dates are correct, and the Greenland ice cap extended all the way to southern Greenland during the last interglacial period, then the sea-level rise could not have come from melting ice in Greenland, at least not in its entirety. "If it wasn't Greenland then it had to be Antarctica," says Vaughan.
Eric Wolf, also at the British Antarctic Survey says this may mean scientists have underestimated the stability of Greenland's ice sheet.
"It may mean that we have been too dramatic about what is happening to Greenland," he told New Scientist. "But the flipside is that if this is true, then we have not been dramatic enough about what is happening to Antarctica."
Journal reference: Science (vol 317, p 111)
Climate Change – Want to know more about global warming: the science, impacts and political debate? Visit