Pale green patterns tinted the water along the Namibian coast in late February 2012. But unlike other bright hues that occasionally show up in the ocean, these colors didn’t result from a phytoplankton bloom.
Scientists have long known that hydrogen sulfide gas is emitted periodically along the Namibian coast. Ocean currents carry oxygen-poor water to the region, and chemical and biological processes can deplete what little oxygen is available. The sediments in the local seafloor are also rich with organic matter. When organic matter decays in an oxygen-poor environment, hydrogen sulfide emissions can result.
Before the satellite era, residents of the region could detect the hydrogen sulfide emissions thanks to the pervasive rotten-egg smell. But satellites’ “eyes in the sky” have shown just how big and long-lasting the emission events can be.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured this natural-color image on February 29, 2012. The pale-hued surface waters snake along the shore of the Namib Desert, stretching roughly 150 kilometers (90 miles).
The milky-green colors along Namibia’s coast indicate high concentrations of sulfur and low concentrations of oxygen. Episodes like this aren’t just colorful, they are actually toxic to local marine organisms. Fish die in the low-oxygen water; however, what is deadly for the fish can be good for birds that feed on their carcasses. Likewise, lobsters crawling onto shore to escape the toxic seawater can make meals for locals. And some species of foraminifera—tiny shelled marine organisms—actually thrive in the oxygen-poor sea floor sediments off the Namibian coast.
Acquired March 13, 2010, this true-color image shows pale-hued surface waters along the coat of Namibia, ranging in color from peacock green to off-white. Inland, the image captures the rippling sand dunes of the Namib Desert.
People living along Namibia’s desert coast have long been familiar with the rotten-egg smell that periodically emanates from the Atlantic Ocean. With an economy that is largely based on fishing, the locals are also used to seeing millions of fish die whenever the unpleasant scent fills the air. The smell and the fish die-off are caused by hydrogen sulfide erupting from decaying plants on the sea floor. On May 12, 2004, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this image of a hydrogen sulfide eruption in progress.