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More elk/bison death news from the Yellowstone area courtesy Billings Gazette:

Toxic gases in park likely cause of bison deaths
By MIKE STARK
Of The Gazette Staff

Five bison dropped dead in Yellowstone National Park apparently after being overwhelmed by toxic gases in a geothermal area.

The bison carcasses were found March 10 along the Gibbon River near Norris Geyser Basin. It's likely the animals were killed by hydrogen sulfide or carbon dioxide trapped at ground level by unusually cold and windless weather, according to a report on the incident released Tuesday.

"The bison were in the wrong place at the wrong time," said Henry Heasler, Yellowstone's lead geologist. "It looks like they just dropped where they stood. Obviously it was a very quick death."

Although the bison deaths were the result of "a rare combination of events," similar incidents have happened before in Yellowstone.

Related deaths

In 1899, park records show, six bears, an elk, squirrels and other small animals died apparently because of toxic gases in Death Gulch in the upper Lamar Valley. Other reports are sprinkled through the park's history.

But this month's incident is unusual in that several bison died.

Bear biologists found the dead bison - two adults, two calves and a yearling - lying on their sides with their legs straight out. They were all within about 30 yards of each other. The body positions indicated that the bison died "very rapidly" and with little or no struggle, according to the report. Wildlife officials estimated that the bison had died about a week earlier.

Heasler said the bison were probably grazing and resting in a snow-free depression in the ground along the Gibbon River, about 300 yards downwind from several geothermal gas vents at Norris.

Cold front theory

As a cold front moved in during early March, the dense cold air kept the toxic gases close to the ground. The bison likely died as the calm conditions kept the bison surrounded by the gases, park officials said.

Blood, teeth and other samples were taken from the animals. Geologists also began running tests on the air in the area.

At some of the vents, researchers measured hydrogen sulfide gas at more than 200 parts per million, well beyond what's considered safe for people and animals.

"If it's above 150 parts per million, if a human breathes it, they take one breath and they drop," Heasler said.

The gas, formed from cooling magma underground and responsible for the "rotten egg" smell often associated with thermal features, is considered a chemical asphyxiant when it's at concentrated levels. Chemical asphyxiation can paralyze breathing and affect the brain stem and other organs.

Another gas associated with volcanic areas, carbon dioxide, is also a regular presence in Yellowstone. Park officials don't have equipment to monitor the gas but, in light of the latest incident, they are now looking to buy some.

Heasler said park officials are still trying to figure out exactly what happened to the bison.

It doesn't appear that the incident has anything to do with a brief upswing in temperatures at Norris Geyser Basin last year that caused park officials to close portions of the trails nearby. Monitoring equipment at Norris did not indicate an unusual burp of gas in early March, Heasler said.

The best guess is that the cold weather, with lows near zero, arrived with barely any of the wind that tends to sweep the gases out of low-lying areas in the park.

"We've got an interesting Norris mystery here," Heasler said. "Norris presents us with lots of these, as do other geyser basins."

Visitors to the park are not at risk, Heasler said.

Park officials have been walking geyser boardwalks with monitors and have only found hydrogen sulfide at less than one part per million. And at the spot where the bison died, which is off trail and away from visitor attractions, animals are going about their business without any apparent effects.

"This is not a threat to people," Heasler said.

Park geologists plan on becoming more aggressive on keeping track of the gases at Yellowstone, particularly around Norris, Heasler said.

"One question is, 'Is this starting to occur more now compared to earlier?' There's not enough statistics to say that," Heasler said. "Just because we're measuring and seeing things we didn't see before doesn't mean they didn't exist before."

Copyright © The Billings Gazette, a division of Lee Enterprises