As climate change opens the Arctic region to oil exploration and shipping, the findings could prove crucial in devising effective methods for cleaning up future spills, the researchers said.

Up to now, experts puzzled over why remnants of the 11 million gallons of crude that fouled some 1,300 kilometers (750 miles) of Alaskan coastline have persisted for so long.

At first it seemed that nature, with some help from technology, would soon wash away one of the worst environmental disasters in history.

The spill decimated the region's wildlife as well as the state's fishing industry.

But within a decade it became apparent that the rate at which the oil was disappearing had dramatically slowed, from 70 percent per year to about four percent.

Today, it is estimated that some 20,000 gallons remain.

Michael Boufadel and Hailong Li of Temple University in Philadelphia, Pennsylvania wanted to find out why this oil was not been broken down through biodegradation and weathering, as had been widely predicted.

Collecting field data and running computer simulations, they found the key lay in the fact that affected beaches consisted of two layers, each with different properties.

The geographically variable impact of rising and falling water tables also played a critical role.

Oil was temporarily stored in the porous upper layer, slowing the rate at which it was subject to weathering, according to the study, published in the journal Nature Geoscience.

An environment lacking the kind of nutrients needed by oil-eating micro-organisms to thrive further protected the fossil fuel.

The second layer, while composed largely of the same materials, was far less porous: on average, water moved through the top layer 1,000 times faster.

When the water level from declining tides fell below the interface between the two layers, oil seeped from the upper to the lower stratum, especially where there was little or no freshwater discharge to compensate.

"Once the oil entered the lower layer, it became entrapped by capillary forces and persisted," the authors said.

Because of the even lower oxygen content in the sub-stratum, the crude was not degraded and has remained suspended.

The study also said that oil tends to linger on gravel beaches more than on sandy ones, pointing to evidence from previous spills: the Arrow in Nova Scotia, Canada (1970), the Metula in the Strait of Magellan, Chile (1974), and the Amoco Cadiz along the French coast of Brittany (1978).

"As global warming is melting the ice cover and exposing the Arctic to oil exploration and shipping through sea routes such as the Northwest Passage, the risk of oil spills on gravel beaches in high-latitude regions will be increased," it said.

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