BioEd Online

The Arctic Ocean used to be so warm it was practically Mediterranean, an international drilling team has found. Although the Earth was known to have warmed rapidly 55 million years ago, no one had expected to find evidence of such high temperatures so close to the North Pole.

The brief period of warming at the end of the Palaeocene epoch was probably sparked by a catastrophic release of greenhouse gases, and seems to have triggered a mass extinction unprecedented on the planet's more recent past.

The warming episode was first noticed in the 1980s, by oceanographers analysing Antarctic sediment cores. Around the world, temperatures in an already warm climate shot up by a few degrees, a rise that was probably caused by the release of millions of tons of methane gas from the sea floor or a series of huge volcanic eruptions. As a result, the oceans heated up so fast that many species were unable to adjust and became extinct.

But in some parts of the planet, the warming seems to have been even more pronounced than scientists had believed. "The exciting discovery is the extreme warmth at this time at such a high latitude," says Andrew Kingdon, a spokesman for the British Geological Survey, which coordinates Europe's ocean drilling operations.

Algae aboard

The team on board the Swedish drilling vessel Oden made the discovery after recovering sediment cores from more than 400 metres beneath the sea floor, in waters 1,300 metres deep.

Oden, drilling at a site covered by thick sea ice around 200 kilometres south of the North Pole, was assisted by a Russian nuclear ice-breaker.

The expedition will return next week to Tromsø in Norway. But the scientists have already found fossils of certain algae in the 55-million-year-old sediment that can only survive in subtropical conditions. These suggest that the Arctic Ocean temporarily warmed to around 20°C.

The researchers also found evidence that many bottom-dwelling species that had been living in the Arctic Ocean before the warming completely disappeared with its onset.

A glimpse of the future

Over the next few months, the cores will be examined and analysed in greater detail. Scientists hope the new Arctic sediment record, which stretches back 56 million years, will help them understand how the Earth's whole climate system works.

In particular, says Kingdon, researchers hope to gain a clearer idea of how the most prominent climate change in our planet's earlier history happened, and how the Earth eventually returned to its original state. This may allow us a glimpse into the likely future of our own greenhouse world, he adds.