The rocky coastlines in the Shelikof Strait, hundreds of miles southwest of the 1989 spill, contain small remnants of oil which appear to be protected by a stable boulder and cobble "armor," Gail Irvine of the U.S. Geological Survey's Alaska Science Center said.
"To have oil there ... is remarkable," Irvine said. "We have these marked boulders whose movement we've been studying for more than 18 years. The oil itself has hardly weathered and is similar to 11-day-old oil."
Chemists have positively identified the oil as coming from the Exxon Valdez.
"Very old oil spills can be found to still have oil," said Christopher Reddy of the Woods Hole Oceanographic Institution, who specializes in investigating oil spills of all kinds -- especially old ones. "We were capable of fingerprinting that oil."
The study offers insights about where oil can persist and which chemical compounds in the oil are more and less durable, the researchers said.
"One lesson is that if you are responsible for cleaning up a spill, you want to be proactive about cleanup behind the boulders," said Reddy, who suggested response efforts should try to prevent oil from stranding in these areas where it may persist for years or decades.