On March 18, 2014, the Terra satellite flew above the coast of Namibia, allowing the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard to capture a stunning true-color image of hydrogen sulfide eruptions in the South Atlantic Ocean. The eruptions appear as bright green tendrils and milky green clouds just off the brilliantly tan coast of Namibia.
While these eruptions are not common, they are a less-than-rare occurrence off the Namibian coast. Here the ocean is shallow, and the bottom is thickly covered with layers of decomposing material. This sea-bottom ooze can trace its beginning to the strong upwelling of deep ocean waters in this region, which pours nutrients into the shallow, well-lit, warm-water environment. This combination spurs lush growth of phytoplankton – plant-like organisms that form the base of the marine food chain. Each organism lives only days, then drops to the bottom to add to the fertile sea-bottom when it dies. Anaerobic bacteria feast on such dead material, and releases hydrogen sulfide as a natural result of decomposition. As pockets of the toxic gas form, they build up pressure in the ooze, eventually breaking free as bubbles, then rise to the surface. The oxidation of hydrogen sulfide in ocean water leaves elemental sulfur floating at the surface, and as it reflects visible light it appears green.
While natural, the hydrogen sulfur release is not benign. For starters, the odor of sulfur has been described as a “stench of rotten eggs”. Such smells wafting onto a beach can drive beachgoers – and tourist dollars – away. Worse, the gas is a respiratory toxin harmful to marine life as well as to humans. In addition, massive decomposition often causes low-oxygen events in the ocean, which also can be deadly to marine life. Hydrogen sulfide emissions happen in many parts of the ocean – but rarely on the scale and intensity as off the coast of Namibia.