Cenozoic World

An international scientific team which has been drilling beneath the bed of the Arctic Ocean says the region enjoyed a sub-tropical climate 55 million years ago.

The Arctic Coring Expedition (Acex) has recovered sediment cores from nearly 400m (1,300ft) below the sea floor.

It says fossilised algae in the cores show the sea temperature was once about 20C, instead of the average now, -1.5C.

The expedition, which has relied on three icebreakers during its work, is now heading back to Tromso in Norway.

Unlocking the Arctic's history

The scientists, from eight nations, recovered the cores from below the sea floor in waters 1,300m (4,260ft) deep.

Acex has been taking cores from the Lomonosov Ridge between Siberia and Greenland. The ridge, 1,500km (930 miles) long, rises to 800m (2,625ft) below sea level and is topped by 450m (1,475ft) of layered sediments.

The scientists said before they set sail from Tromso last month their findings would help science to work out how long the Arctic sea ice, now in retreat, had persisted.

The cores they have extracted show the Arctic Ocean was once a subtropical, shallow sea. The evidence, Acex says, is in the form of tiny algal fossils found in the cores, which were once marine plants and animals.

They date back to a period known as the Palaeocene-Eocene thermal maximum, a brief period that occurred around 55m years ago.

Huge die-off

It was characterized by an extremely warm climate that created a natural greenhouse effect, which caused massive amounts of carbon to be deposited in both sea and air.

Atmospheric carbon levels then are thought to have been about 2-3,000 parts per million (ppm), compared with almost 380 ppm today.

The algae found in the Lomonosov cores, which lived only in subtropical conditions, prove how warm the Arctic once was, Acex says. It says the ocean's temperature was once similar to the waters off New York in August.

Dr Michael Kaminski, a palaeontologist from University College London, UK, said: "We're seeing a mass extinction of sea-bottom-living organisms caused by these conditions.

"Moving forward in time, we see many species disappear. Only a few hardy survivors endure the thermal maximum."

There is also evidence that part of the Arctic Ocean was once a freshwater lake, probably when the Lomonosov Ridge was part of what is now Siberia.

The last 250,000 years of Arctic history were known already in some detail thanks to cores taken from the Greenland ice cap.

Coping with Nature

But Professor Jan Backman of Stockholm University, one of the two chief scientists of Acex, said: "We now have sediment records going back to 56m years, which are resting on 80-million-year-old bedrock.

"The early history of the Arctic Basin will be re-evaluated based on the scientific results collected on this expedition."

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