ADN fossile du Groenland

Published online: 5 July 2007; | doi:10.1038/news070702-14 / http://www.nature.com/news/2007/070702/full/070702-14.html

DNA reveals a green Greenland

Old forests hint that the island has been icy for 450,000 years.

Louis Buckley

Under the ice: DNA has been found from trees such as alder, spruce and pine, along with a mass of insects. Science

Scientists have drilled through two kilometres of ice in southern Greenland and retrieved DNA from the pine forest that once existed there, buzzing with prehistoric insect life. Dated to between 450,000 and 800,000 years old, the DNA is among the oldest ever found.

Greenland is known to have once been green - plant fossils dating to 2.4 million years ago have been found in the far northeast of the country. But, surprisingly, the DNA evidence for plant life stops at 450,000 years ago. Researchers say the lack of younger DNA suggests that this portion of the land has been covered by ice ever since - and that goes against the prevailing view of Greenland's climatic history.

During the last interglacial period (130-116 thousand years ago), the climate was 5 °C warmer than it is today, says Eske Willerslev, director of the centre for ancient genetics at the University of Copenhagen in Denmark. "Sea levels were 5-6 metres higher, and most scientific models have assumed that the melting of the southern Greenland ice cap was responsible. But our data suggest that this was not the case."

"If our data are correct, then the southern Greenland ice cap is more stable than previously thought," says Willerslev. He is, however, keen to dismiss the idea that his results mean that the predicted increases in sea level will not occur. "We know that during the last interglacial period, sea levels rose, but this must have come from other sources additional to the Greenland ice cap, such as Antarctic ice," he says.

Other researchers, though, are doubtful that Greenland kept its ice through the interglacial. "To account for the rise in sea levels most models require that the southern Greenland ice sheet totally melted," says Roderik van de Wal, an expert on ice sheets at the Utrecht University in the Netherlands. "If this study shows that it didn't melt then there's a problem somewhere."

Digging deep

Willerslev and his team found the DNA in a single sample of silt taken from underneath the ice sheet, they describe in Science. DNA has never been extracted in this way before, says Willerslev. "The technique represents an important new way of reconstructing the biological history of many areas where fossils are buried under kilometres of ice, or have been scoured away by glaciers," he says.

From the samples retrieved, the team identified a wide range of plant and insect species - trees such as alder, spruce, pine and yew, and an associated fauna of beetles, flies, spiders, butterflies and moths. Similar landscapes can be seen in Eastern Canada and Swedish forests today, says Willerslev. From the plant species found, they estimate that temperatures in the forest were at least 10 °C in summer and a minimum of -17 °C minimum in winter.

How far north the forest extended or what else lived beneath its boughs, though, remains a mystery. "We found no DNA from animals besides insects [and spiders], probably due to the small amount of ice analysed," says Willerslev. "Animal DNA vanishes faster and is much harder to find than plant or insect DNA because there is less to start with."

Uncertain future

Willerslev's findings may have implications for predicting how the ice sheet will respond to future climate change. If Greenland's ice vanished during the last interglacial to raise sea levels, then, scientists have reasoned, it is likely to do so again as global temperatures increase over the course of the century. If it did not disappear in the past, this may confuse current predictions about the future.

Could inaccuracies with the techniques used to establish the age of the samples be responsible for the team's unexpected results? Willerslev doesn't think so. "I admit that there are uncertainties with the dating methods used, but what I find convincing is that we used four different independent methods and they all produced similar dates," he says. "I think it's very unlikely that it's down to chance."

Reference : Willerslev, E. et al. Science 317, 111-114 (2007).