POSTED 31 JAN 2002
you've got gas problems? Then consider the Atlantic ocean near Namibia,
in southern Africa. Every so often, a massive hydrogen sulfide gas eruption
from the ocean floor kills fish, exposes local people to a major rotten
egg stench, and causes ecological havoc in the ocean.
The aroma of rotten eggs is familiar to neighbors of industrial plants, but it can also arise naturally when organic stuff rots in the absence of oxygen, as at volcanic vents.
Along Namibia's coast, eruptions of hydrogen sulfide are a regular feature, although they're not featured in scratch-and-sniff tourist brochures. The eruptions are not entirely miserable from some perspectives.
Although millions of fish may die, scavenging birds feast on their carcasses.
Namibians also feast on lobsters escaping to shore from the oxygen-deprived water, where they are easy game for beachcombers.
Got my eye on you!
For hundreds of years, the Namibian Coast has been known for its hydrogen sulfide perfume. The fish kills, combined with the satellite data, have made the giant belches a pressing subject of scientific study.
The Namibian Coast has the most intense upwelling of fertile deep-ocean water in the world, says Weeks, who collaborated on the research with colleague Andrew Bakun and Bronwen Currie, a biogeochemist with Namibia's National Marine Research and Information Center.
But there may be too much of a good thing. "As a result of almost continual phytoplankton production," Weeks says, "there is massive diatom fallout." Dead diatoms fall through the water, where they are decayed by aerobic bacteria.
Anaerobic bacteria take over the decomposition task in the thick, muddy sediment, producing hydrogen sulfide. When the gas enters the water column during an eruption, it separates into hydrogen and sulfur, and the hydrogen atoms combine with oxygen to form water. That reaction removes oxygen from the water, creating deadly low-oxygen conditions.
Hydrogen sulfide that enters the atmosphere makes "this awful, noxious smell of corroding metal," says Weeks. "But other than the noxious smell and headaches, there are no known effects on the human population."
Weeks and her colleagues tracked the eruptions with the OrbView-2 SeaWiFS satellite. "The surface is milky green due to the elemental sulfur," she says. When the hydrogen sulfide oxidizes, it leaves sulfur at the surface water that reflects visible light.
The low-oxygen conditions are not simply a transient, local affair. Weeks followed one "event" out to sea, where it covered 20,000 square kilometers and lasted at least three weeks.
These hydrogen sulfide eruptions are the largest in the world, Weeks adds. "Certainly you get hydrogen sulfide outgassing in places like the Baltic Sea and the Black Sea, but the important thing is that nowhere else does this actually happen remotely to the intensity of Namibian waters, and yet it's completely natural."
Natural, perhaps, but given the stakes to the critical fishing industry, and the suspicion that the eruptions may be getting more frequent and intense, an international collaboration is starting to examine possible links to global warming or other large ecological disturbances.
-- David Tenenbaum