September 30, 2013 at 5:58 pm #402MikeKeymaster
By NADIA WHITE
Star-Tribune staff writer
A toxic mix of geothermal gases and a deep, still cold snap is being
blamed for the mysterious death of five bison at Yellowstone National
Park’s Norris Geyser Basin earlier this month.
The bison appear to have keeled over where they stood, their legs
still perpendicular to their bodies. Their discovery by bear
researchers on March 10 turned park scientists into sleuths.
The bison were found in a depression near the Gibbon River, downhill
and downstream of several gas vents. There was no sign of a chase or
agony of death.
“It looked like these bison had been standing up and then fell over
dead,” said Yellowstone geologist Hank Heasler, who examined the site
and wrote a report on the incident along with Cheryl Jaworowski and
park bear biologists. “They don’t have their front legs under them
like they were sleeping on the ground and then froze to death or
starved; they don’t look like they were pawing the ground a lot and
being chased by predators.”
That much, the biologists told him.
But what killed them?
Because the bison were found in a geyser basin, earth scientists
joined in the detective work. Causes such as a lightning strike were
considered, but a walk through the area with a hydrogen sulfide gas
(H2S) monitor seemed to tell the tale.
Heasler said he found hydrogen sulfide levels “as high as my meter
would measure,” more than 200 parts per million.
Hydrogen sulfide causes the “rotten egg” smell people often associate
with geothermal areas. It is classified as a chemical asphyxiant, but
is not usually dangerous for people because they easily smell it at
levels as low as 1 ppm and can leave the area when the smell gets
unpleasant, according to information provided by the park.
Animals may not sense the gas or make the connection that it is
dangerous. But even in an area as rich in geothermal gasses as
Yellowstone, such strange animal deaths are very unusual. Unusual,
but not unheard of, Heasler said.
In 1889, in an area of the upper Lamar Valley known as Death Gulch,
geologist Walter Weed found six dead bears, 1 elk, some squirrels,
pikas and other small animals and insects. Other scientists have
recognized the dangers of toxic gases in Yellowstone over the years,
according to Cheryl Matthews, a spokeswoman for the park.
“Usually in Yellowstone we do not have to worry about toxic
accumulation of gasses because its part of windy Wyoming,” Heasler
said. “So we need the right atmospheric conditions, the right
geologic conditions, for gasses to accumulate in toxic levels.”
The bison appeared to have been dead for about a week. The two
adults, two calves and one yearling appeared to have been grazing and
resting in a snow-free ground depression near the river.
What happened in the first week of March that might have killed them
all so swiftly?
There is no weather station for Norris Geyser Basin, but an
electronic temperature log at the museum there showed that the
temperature plunged to 1 degree in the early morning hours of March 2.
The scientists speculate that in the cold, still night of March 2, a
toxic blend of hydrogen sulfide and its companion gas, carbon
dioxide, spilled from the thermal vents some 300 yards and 30 feet
higher than the buffalo and settled where they grazed, asphyxiating
“We don’t know for certain what happened … There are no diagnostic
tests for confirming CO2 or hydrogen sulfide poisoning,” Heasler
said. But after more than a week of detective work, “the pieces fit
The explanation for the deaths, Heasler suspects, will continue to
The interior of Yellowstone remains closed to the public as crews
clear roads and ready visitor stations for seasonal tourists. But
Heasler walked the boardwalks around the Norris Geyser basin Tuesday
in a light breeze and found no measurable gasses on the boardwalk.
“Norris is anything but constant,” he said. “It’s a very, very
dynamic area, as are all the geyser basins in the park. That’s part
of the magic of Yellowstone, is that it’s not a static place. Nature
has its way here and it changes on a daily basis.
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. Heasler is investing in a pricey carbon
dioxide monitor so both gasses known to be in the area can be
measured, as well as oxygen levels.
Additional information can be found at
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