Aisling Irwin reports on a research ship which scientists hope will unlock some secrets from the ocean floor. The strangely shaped JOIDES Resolution, a floating laboratory and ocean drilling rig in one, left Edinburgh last month for the Arctic, where it will drill up to five miles into the ocean floor to look for evidence that global warming might trigger another ice age.

The ship, which carries out the international Ocean Drilling Programme, had been on a rare visit to the United Kingdom where it stocked up on provisions and a fresh lot of scientists before starting its next, 50-day expedition.

Its 62-foot-high derrick, startling even when viewed from Edinburgh's Arthur's Seat, enables it to drill up to five miles down. It can keep going even in 40-foot waves. It produces a succession of slender columns of sediment which are examined for their density, radioactivity, magnetism and composition. The scientists, working in high-tech laboratories around the seven-storey ship, then slice up the sediment for more detailed analysis, using techniques including X-ray diffraction and X-ray fluorescence.

The bits of ocean detritus that have fallen to the floor to form each layer of sediment hold information about the conditions of the ocean and the climate at the time. Certain species of marine organism, for example, could have only lived if the ocean was a certain temperature; there are more clues from their skeletons.

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Techniques have become so precise, says Robert Kidd, head of the ODP planning office and head of marine geoscience at the University of Wales, Cardiff, that the scientists can read the record of the sediment to the nearest 100 years and sometimes to the nearest ten.

The JOIDES Resolution has just returned from the western Mediterranean, where it has been investigating aspects of the centimetre-a-year collision between Europe and Africa.

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In the Arctic it will drill deep enough to look back three million years - when the climate was the same as it could soon become as a result of global warming. The sedimentation rates have been high where the scientists are drilling - and Kidd says that they should be able to pinpoint activity to within a time span of ten years.

The Arctic is important for studying climate change because of the serious implications of any instability that might be induced in the North Atlantic drift. In the cold atmosphere, Arctic water cools and sinks; warm water from the tropics rushes north to fill the gap and the deep, cold Arctic water travels south.

Computer models predict that, if the atmosphere were to heat up, the accompanying increased rainfall will cause this conveyor belt to shut down. Maureen Raymo, professor of marine geology at MIT, who is leading the expedition, says: "Some threshold is crossed and suddenly the water is not dense enough to sink. A gradual warming could flip you into some new mode of ocean circulation. If this happened then this whole region would be plunged into colder conditions."

It is vital, therefore, to know how sensitive this conveyor belt has been to small fluctuations in temperature in the past.

"We know from very short ice cores that this kind of climate instability occurred 30,000 to 120,000 years ago," says Professor Raymo. Now she can look back further.

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At the end of this year the JOIDES Resolution will drill in the Caribbean to investigate one of the key events in the earth's geological history: the meteor that hit the Yucatan Peninsula in Mexico. The impact was followed by major climate change and the extinction of the dinosaurs. At the same time as the dinosaurs became extinct, says Kidd, there were major extinctions in the ocean. The scientists will search for spreads of minerals that indicate the impact of a meteorite.

Before that expedition the ship will have done a 50-day study of gas hydrates - frozen gases in the ocean sediments. Gas hydrates have an upside and a downside: they are a promising energy source but, if the globe warms up, they could release a lot of methane, their main component, into the atmosphere.

One of the most famous achievements of the ocean drilling rig was the discovery that bacteria exist 500 metres below the ocean floor. This discovery suddenly extended the known volume of our biosphere by another 10 per cent. Professor Kidd argues that the rig's work is essential for understanding human impact on the ocean before we damage it irretrievably.

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At a time when underwater nuclear testing is controversial and feelings run high on dumping in the North Sea, he says that finding out about the consequences of our actions is becoming a race against time. In the Arctic, the JOIDES Resolution will be capable of such fine detail that it will document the beginning of nuclear testing from radionuclides in the sediment.

Professor Kidd hopes that the knowledge gained from ocean drilling will release us from our "Victorian idea that the ocean is somewhere to dump things". But he says that it could also reveal sustainable ways in which we can exploit the oceans.

"If we can get a feeling for the rates at which man is changing things we may be able to put waste down there," he says. "Perhaps, for example, we could spread sewage on the sea floor."

The ship is owned by 19 countries. The UK, through the Natural Environment Research Council, has 7 per cent, at a cost of about Pounds 2 million. The United States is the majority holder.

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