Melting ice caps reveal polar secrets

Gerald Dickens of Rice University

The Arctic was once a home for tropical plants, according to US scientist Gerald Dickens.

An international team of researchers working for the Arctic Core Expedition (ACEX) came to the conclusion after analysing frozen sedimentary cores drilled from the seabed, which show how the climate has altered over millions of years.

The last time massive amounts of greenhouse gases were released into the atmosphere, the North Pole was an ice-free expanse of open ocean that was teeming with tropical organisms, a study has found.

Scientists have discovered that the complete disappearance of the Arctic sea ice 55 million years ago coincided with a dramatic increase in greenhouse gases in the atmosphere which must have caused global warming.

An analysis of the sediments on the floor of the Arctic Ocean has revealed that the region was once extremely warm, unusually wet and ice-free during a period when there was also a dramatic increase in concentrations of carbon dioxide or methane.

The work is important because scientists have found that the Arctic is currently experiencing one of the fastest temperature rises on record with more sea ice melting each summer than at any time in hundreds and possibly thousands of years.

The ACEX research team now has sedimentary cores that can be dated to 55 million years ago - a time known as the palaeocene-eocene thermal maximum (PETM) - when surface temperatures in some parts of the world were some 8C (15F) higher than they are now.

"Building a picture of ancient climatic events is a lot like putting together a jigsaw puzzle, and what ACEX allowed us to do was fill in a black section of the PETM picture," said Dickens, a geochemist at Rice University in Houston, Texas.

"The ACEX cores clearly show that the Arctic got very warm and wet during the PETM. Even tropical marine plants thrived in the balmy conditions," Dr Dickens said.

The ice cores were drilled from the Lomosov Ridge, a sub-sea mountain range that stretches from northern Greenland, across the Polar Sea, to Siberia in northern Russia.

The task involved three icebreaker ships: a drilling vessel and two further icebreakers - one of which was nuclear powered - which protected the drilling operation from being crushed by drifting ice floes and metre-thick pack ice.

This was the first time that scientists have had access to sedimentary cores drilled deep from under the Arctic sea, and their recovery can now be compared to similar seabed cores drilled in other parts of the world.

Dr Dickens said the fossils of certain species of microscopic plants, which can trigger algal blooms, suddenly become commonplace in the parts of the cores that are about 55 million years old.

These organisms were known only from the tropics prior to the PETM warm period, suggesting the Arctic was also warm and ice-free during this period.

Scientists are not sure what caused the warming, which occurred over a relatively rapid geological period of 100,000 years.

But they think it may have been due either to the massive release of carbon dioxide from volcanic eruptions, or the release of vast deposits of carbon-containing methane stored on the sea floor.

"The magnitude of the carbon input at the PETM outset is truly enormous. If it were all volcanic, you'd need something like a Vesuvius-sized eruption each day for centuries, which seems very unlikely," Dr Dickens said.

Another possibility is that there was a sudden release of massive amounts of methane and carbon dioxide that had been locked away in the deep ocean in a frozen form known as ocean gas hydrates.

Even today, it is estimated that there is more carbon locked away as ocean gas hydrates than all of the oil and gas reserves of the world combined.

What the latest study suggests is that this huge reservoir of carbon has been released in the past with devastating effects on the climate.

"It's difficult to overestimate the importance of this kind of experimental evidence. It's like opening a door to a room you've seen on a blueprint but never stepped foot inside," he said.

- INDEPENDENT