|Groënland: de l'ADN dans la glace profonde
révèle une flore passée riche
Source : http://news.bbc.co.uk:80/2/hi/science/nature/6276576.stm
Groënland: de l'ADN dans la glace profonde révèle
une flore passée riche
AFP - Jeudi 5 juillet, 21h23
WASHINGTON (AFP) - Des échantillons d'ADN de plantes et
d'insectes extraits des parties les plus profondes de la calotte
glaciaire du Groënland témoignent de l'existence d'une
flore et faune riches au cours du million d'années passées,
selon une étude publiée jeudi dans la revue Science.
Les auteurs de l'étude ont prélevé ces fragments
d'ADN, provenant de différents arbres et insectes, à la
base d'une carotte glaciaire de deux kilomètres dans le
sud du Groenland. A leur grande surprise, l'analyse de ces échantillons
vieux de 450.000 à 900.000 ans a révélé une
grande variété de plantes et d'arbres, comme les
conifères, ainsi que des insectes dont des papillons, des
araignées et des puces.
Dye 3: 2km long ice core
Greenland Ice Core Project (GRIP): 3km long ice core
John Evans Glacier (JEG): Control site
Kap Kobenhavn: Previously youngest known Greenland forest
Les précédentes estimations dataient la forêt
boréale au Groenland avant la glaciation à 2,4 millions
Les températures du Groenland quand la forêt boréale
recouvrait son territoire allaient de 10 degrés Celsius
en été à moins 17 degrés Celsius en
Cette recherche indique aussi que la glace du Groenland, dont l'épaisseur
actuelle dépasse deux kilomètres, s'est largement
maintenue durant la période interglaciaire, il y a 116.000 à 130.000
ans, quand les températures étaient 5 degrés
Celsius plus chaudes qu'aujourd'hui. Les océans était
montés de 5 à 6 mètres au-dessus de leur niveau
Si nos calculs sont exacts, cela veut dire que la couche de glace
recouvrant le sud du Groenland est plus stable qu'on ne le pensait",
souligne Eske Willerslev, de l'université de Copenhague
principal auteur de l'étude conduite par une équipe
internationale de chercheurs.
DNA reveals Greenland's lush past
Armies of insects once crawled through lush forests in a region
of Greenland now covered by more than 2,000m of ice.
DNA extracted from ice cores shows that moths and butterflies
were living in forests of spruce and pine in the area between 450,000
and 800,000 years ago.
Researchers writing in Science magazine say the specimens could
represent the oldest pure DNA samples ever obtained.
The ice cores also suggest that the ice sheet is more resistant
to warming than previously thought, the scientists say.
"We have shown for the first time that southern Greenland,
which is currently hidden under more than 2km of ice, was once
very different to the Greenland we see today," said Professor
Eske Willerslev from the University of Copenhagen, Denmark, and
one of the authors of the paper.
"What we've learned is that this part of the world was significantly
warmer than most people thought," added Professor Martin Sharp
from the University of Alberta, Canada, and a co-author of the
The ancient boreal forests were thought to cover southern Greenland
during a period of increased global temperatures, known as an interglacial.
Temperatures at the time were probably between 10C in summer and
-17C in winter.
When the temperatures dropped again 450,000 years ago, the forests
and their inhabitants were covered by the advancing ice, effectively
freezing them in time.
Studies suggest that even during the last interglacial (116,000-130,000
years ago), when temperatures were thought to be 5C warmer than
today, the ice persevered, keeping the delicate samples entombed
and free from contamination and decay.
At the time the ice is estimated to have been between 1,000 and
"If our data is correct, then this means that the southern
Greenland ice cap is more stable than previously thought," said
Professor Willerslev. "This may have implications for how
the ice sheets respond to global warming."
Research by Australian scientists has suggested that a 3C rise
in global temperatures would be enough to trigger the melting of
the Greenland ice sheet.
In 2006, research conducted by researchers at Nasa suggested that
the rate of melting of the giant ice sheet had tripled since 2004.
While in February 2006, researchers found that Greenland's glaciers
were moving much faster than before, meaning that more of its ice
was entering the sea.
And in 1996, Greenland was losing about 100 cubic km per year
in mass from its ice sheet; by 2005, this had increased to about
220 cubic km.
A complete melt of the ice sheet would cause a global sea level
rise of about 7m; but the current picture indicates that while
some regions are thinning, others are apparently getting thicker.
The new results were obtained from the sediment rich bottom of
The 2km-long Dye 3 core was drilled in south-central Greenland,
whilst the 3km-long Greenland Ice Core Project (GRIP) core was
taken from the summit of the Greenland ice sheet.
Samples from other glaciers, such as the John Evans Glacier on
Ellesmere Island, in northern Canada, were used as a control, to
verify the age of the samples and to confirm that the DNA was from
plants that grew in southern Greenland, rather than from plant
matter carried by wind or water from elsewhere in the world.
Although the ice contained only a handful of pollen grains and
no fossils, the researchers were able to extract DNA from the organic
matter held in the silt.
Comparisons with modern species show that the area was populated
by diverse forests made up of alders, spruce, pine and members
of the yew family.
Living in the trees and on the forest floor was a wide variety
of life including beetles, flies, spiders, butterflies and moths.
The discovery pushes forward the date when the last forests were
known to exist in Greenland by nearly two million years.
Previously, the youngest fossil evidence of a native forest in
the region came from fossils found in the Kap Kobenhavn Formation
in northern Greenland. There, the fossils date from around 2.4m
The study paves the way for scientists to probe beneath the ice
in other parts of the world.
"Given that 10% of the Earth's terrestrial surface is covered
by thick ice sheets, it could open up a world of new discoveries," said
Dr Enrico Cappellini of the University of York, UK.