Yellowstone Officials Suspect Toxic Gas in Bison Deaths
Yellowstone National Park officials announced today that-in a very rare combination of events-a concentration of toxic gases (hydrogen sulfide and carbon dioxide) along with unusually cold, dense air appear to be the most probable cause of death for five bison found at Norris Geyser Basin by Bear Management staff. Park staff noticed the animals while doing routine research in the area. The bison, estimated to have been dead for approximately a week, were found lying on their sides, with their feet perpendicular to their bodies; the unusual position of the carcasses indicates the bison died very rapidly, as a group.
According to the park's geologist Dr. Henry Heasler, the five bison (2 adults, 2 calves, one yearling) were likely grazing and resting in a snow-free ground depression along the Gibbon River near multiple geothermal gas vents in the Norris area. Cold, still air from a cold-front passing through the area around March 1 probably caused the geyser basin's steam and toxic gases to remain close to the ground, overwhelming the animals. Hydrogen sulfide and carbon dioxide can accumulate in topographically low areas on cold, calm nights because they are denser than air.
In the investigation following the deaths, Dr. Heasler and staff measured hydrogen sulfide gas (H2S) in some vents exceeding 200 parts per million (ppm), far above safe limits for humans or animals. The gas is classified as a chemical asphyxiant and is better known as "rotten egg" gas because of the smell. Since it is heavier than air, on an usually cold, still night, it could concentrate and overwhelm animals breathing it. Humans, who can easily detect the smell of the gas at the minute level of 1 ppm, are able to escape an area well before it reaches a toxic level. Generally, the fairly constant winds in the Yellowstone area dilutes and disperses gases so that it would be almost unheard of for a park visitor to be overcome by toxic fumes as the bison were.
Although rare, incidents such as this are not unheard of in the park's history. In 1889, the death of several animals (6 bears, 1 elk, some squirrels, pikas and other small animals and insects) were found by geologist Walter Weed in an area known as Death Gulch in the upper Lamar River valley. A second geologist, T.A. Jagger, visited the area in 1899 and noted 7 dead bears. A cursory survey of Yellowstone Research Library data indicates that many other people have recognized the dangers of toxic gases within Yellowstone (Traphagen, 1904; Ash, 1939; Frisbee, 1960; Fournier, 1959; Love and Good, 1970).
In an ongoing effort to learn more about the gases in the Norris Geyser Basin area, park staff plan to continue taking random air and vent samples of gases.