Papillons au groenland : Casse-tête pour Al Gore…
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 d'années.

Les températures du Groenland quand la forêt boréale recouvrait son territoire allaient de 10 degrés Celsius en été à moins 17 degrés Celsius en hiver.

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 alors
montés de 5 à 6 mètres au-dessus de leur niveau d'aujourd'hui.

" 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 (Danemark), le
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 Science paper.


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 1,500m thick.

"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 ice cores.

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 years ago.

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.






- free of copyrights -
Veuillez simplement citer la source de ces documents si vous les utilisez, Merci !!