Methane dead zone found just offshore “linked to oil”; Stetches entire 1500 sq. miles of sample area

Dead Zone in Gulf Linked to Oil, Discovery.com, July 9, 2010:

An unusual low oxygen zone in Gulf of Mexico waters off the Alabama shore has persisted for more than a month, and evidence points to the ongoing Deepwater Horizon oil spill as the cause. …

The researchers measured low oxygen levels along the entire 40-mile stretch they sampled around Dauphin Island, Ala., from about 40 miles offshore to within a mile or two of the shoreline. The bottom layer of water was oxygen-depleted at depths of about 30 feet close to shore to 100 feet further out, along the continental shelf — a rim of shallow water tracing the coast from Mississippi to Florida.

“It’s not little local pockets,” said Monty Graham of the Dauphin Island Sea Lab, who is tracking the zone. “It’s over a regional scale. It wouldn’t surprise me if there were a band of low oxygen over that entire area between the Mississippi River and Apalachicola, Florida.”

“The low oxygen was pushing up very close to the shore,” he added. …

[T]he low-oxygen waters seem to be arriving via a tongue of cold water that has pushed up from offshore carrying elevated levels of methane, Graham said. …

“The effects off the shelf might be longer,” Graham said. “If you drive the oxygen down at 1200 meters (3600 feet), there is nothing to replace that oxygen rapidly. You might see a low oxygen signature for years, maybe even decades.”