Reuters: “A thick pancake of shiny black” covered the still waters of Prince William Sound in the hours after the Exxon Valdez split open in an ecological disaster that offers lessons for any future forays for Arctic oil, eyewitness and conservationist Dennis Kelso recalled. As Alaska’s environment conservation chief 20 years ago, it was Kelso’s job to enforce clean-up standards around the supertanker on March 24, 1989, as it leaked oil into prime fishing grounds. The Valdez ultimately …