With little hard evidence to go on, U.S. geologists said Monday that they believe the deadly gases in Cameroon arose from one of two sources: the decomposition of organic matter in the sediments of Lake Nios or the sudden release of such gases from molten rock far below the Earth's surface.
Many scientists lean toward the latter scenario because Lake Nios lies in the crater of a near-dormant volcano, and it is now known that lethal gases released in a similar incident at nearby Lake Monoun in 1984 were of volcanic origin.
The gases that killed at least 1,200 people in northwestern Cameroon were probably hydrogen sulfide, carbon monoxide, carbon dioxide or some combination of these gases, the experts said.
Large quantities of such gases are dissolved in the molten rock of the Earth's core, placed there when the planet coalesced from gaseous clouds 4.5 billion years ago. The gas remains dissolved as long as the molten rock is under high pressure from above.
If the pressure is released suddenly, such as in a volcanic eruption, according to UCLA geochemist Arthur Boettcher, the gas is explosively released. Boettcher compared the phenomenon to removing the cap from a bottle of soda pop that has been shaken.
This explosive gas release produces most of the violence associated with volcanic eruptions such as that at Mt. St. Helens. "If there were no dissolved gases, the lava would just bubble out or form a plume like a water fountain," Boettcher said.
More often, however, the pressure on the molten rock is released slowly, so that the gases escape slowly and accumulate in underground pockets. Boettcher compared this phenomenon to the slow loss of carbonation by soda in an opened bottle.
Normally, the released gases--in small, harmless amounts--migrate to the surface through cracks and fissures and dissipate.
Such gases bubbling up under Lake Nios, however, would have been trapped in the sediments at the lake bottom; the high pressure at the bottom of the deep lake also causes the gases to be dissolved in the water in high concentrations, according to hydrologist William Evans of the U.S. Geological Survey. Under such conditions, a disturbance of the lake bottom--such as even a minor earthquake or landslide--would cause the gases to be released abruptly.
That is exactly what happened Aug. 15, 1984 at Lake Monoun, after a landslide into the lake, Evans said. After the incident, he found that water from the bottom of the lake was supersaturated with high quantities of carbon dioxide of volcanic origin.
The second, less-favored scenario being postulated by scientists Monday is that the same gases were formed on Lake Nios's bottom through the bacterial decomposition of organic matter. When bacteria consume organic matter, they release carbon dioxide and other gases. Most of such gases would also be trapped in the sediments and dissolved in water.
Such gases also could be released abruptly if the sediments are disturbed. Because they are heavier than air, these gases can form a fog-like cloud that lingers until it is dispersed by winds, according to volcanologist Don Peterson of the Geological Survey's Cascades Volcano Observatory in Washington.
Reports from Cameroon indicate that just such a cloud from Lake Nios was blown over nearby villages.
Carbon Dioxide Cloud
The health effects of the cloud would depend on its constituents. The cloud that suffocated 37 people at Lake Monoun, for example, was almost entirely carbon dioxide, an odorless, colorless gas that is present in the atmosphere in small quantities, Evans said.
Volcanic carbon dioxide killed 142 people during an eruption on the island of Java in 1979, and 16 students who were near a Colombian volcano in 1949.
The 300 injured survivors in Cameroon are reporting that the gas cloud had the characteristic rotten-egg odor of hydrogen sulfide. Some, but not all, scientists think it could have been the killing agent.
At low concentrations, in the range of 10 to 500 parts per million, hydrogen sulfide causes irritation of the eyes, the esophagus and the airways of the lung, according to chest specialist Alex Herbert of the University of Alberta.
At higher concentrations, according to toxicologist James Popp of the Chemical Industry Institute of Toxicology, hydrogen sulfide blocks the central respiratory center in the brain, causing the victim to stop breathing "very, very quickly."
Accident in 1950
A 20-minute release of hydrogen sulfide from a sulfur removal unit at a refinery in Poza Rica, Mexico, on Nov. 24, 1950, killed 22 people and hospitalized 320.
Two oil field workers were killed by hydrogen sulfide released during the blowout of a well Oct. 17, 1982, at Lodgepole, in Canada's Alberta province, Herbert said, and minor injuries from exposure to the gas are common in the industry.
Exposure to this gas is a worldwide occupational hazard because the drilling of oil wells can release pressure that keeps the gas dissolved in the oil.