Twice an Exxon Valdez spill worth of oil seeps into the Gulf of Mexico every year, according to a new study that will be presented January 27 at the Ocean Sciences Meeting in San Antonio, Texas.
But the oil isn't destroying habitats or wiping out ocean life. The ooze is a natural phenomena that's been going on for many thousands of years, according to Roger Mitchell, Vice President of Program Development at the Earth Satellite Corporation (EarthSat) in Rockville Md. "The wildlife have adapted and evolved and have no problem dealing with the oil," he said.
Oil that finds its way to the surface from natural seeps gets broken down by bacteria and ends up as carbon dioxide, a greenhouse gas. So knowing the amount of fossil fuel that turns to carbon dioxide naturally is important for understanding how much humans may be changing the climate by burning oil and gas.
Using a technique they developed in the early 1990s to help explore for oil in the deep ocean, Earth Satellite Corporation scientists found that there are over 600 different areas where oil oozes from rocks underlying the Gulf of Mexico. The oil bubbles up from a cracks in ocean bottom sediments and spreads out with the wind to an to an area covering about 4 square miles.
"On water, oil has this wonderful property of spreading out really thin," said Mitchell. "A gallon of oil can spread over a square mile very quickly." So what ends up on the surface is an incredibly thin slick, impossible to see with the human eye and harmless to marine animals.
When oil spreads out over water, surface tension causes it to act like a super-thin sheet of Saran Wrap, flattening down small waves on the ocean surface. To spot the oil slicks, EarthSat scientists use radar data from Canadian and European satellites. The oil slicks stand out in the radar image because they return less of the radar signal than the wavy surfaces.
To get an estimate of how much oil seeps into the Gulf each year, the researchers took into account the thickness of the oil-only a hundredth of a millimeter, the area of ocean surface covered by slicks, and how long the oil remains on the surface before it's consumed by bacteria or churned up by waves. "The number is twice the Exxon Valdez's spill per year, and that's a conservative estimate," said Mitchell.
With funding from NASA, EarthSat researchers began this work in the early 1990s using Landsat satellite and radar data to identify marine oil seeps for petroleum exploration. The method has had amazing success. Drilling for oil in the ocean is extremely expensive, and with radar data, oil companies have a much better shot at finding oil deposits.
In the future, EarthSat hopes to refine this method using data from NASA's new EO-1 satellite, set for launch in June 2000. A sensor aboard EO-1 may be able to tell gas from oil and better pinpoint the source of the slick.