Snow Traps Deadly Gas

Scientists have measured remarkably high levels of carbon dioxide (CO2) in the soil near dead trees on the flanks of California's Mammoth Mountain, a dormant volcano. The gas builds to hazardous levels beneath the winter snow, researchers report in the current issue of Geology. When the snow melts, it punishes the soil beneath with acidity rivaling that seen in the world's worst acid rain regions.

Mammoth Mountain, which last erupted about 700 years ago, shadows the resort town of Mammoth Lakes near Yosemite National Park. Early this decade, trees started dying in several patches near the summit of the volcano; those dead zones now cover about half a square kilometer. From the high levels of CO2 in the air surrounding the dead trees, geologists concluded in 1995 that a swarm of earthquakes 6 years earlier had created fissures through which about 500 metric tons of the gas a day was seeping to the surface from a reservoir of CO2 several kilometers below.

In the latest study, geochemists Kenneth McGee and Terrence Gerlach of the U.S. Geological Survey in Vancouver, Washington, installed two subsurface monitoring stations and made hourly measurements of carbon dioxide for a year, beginning in October 1995. They found that snow cover could dramatically alter CO2 levels. At one station, the soil CO2 concentration was more than 10 times above average for 2 months when deep snow retarded its diffusion into the air. That could prove lethal if a skier ventured into a low-lying pocket where CO2 might accumulate, McGee says, although the U.S. Forest Service already posts warnings near the tree-kill zones.

The researchers also found what McGee calls "astounding" acidity--about pH 4--from carbonic acid, which forms when CO2 dissolves in water and reacts with soil minerals. "Most soil has an extraordinary capacity to buffer acid," notes McGee. But Mammoth soil, made of rather young granite, lacks the proper composition to neutralize acid. This harms fungi, bacteria, and insects living in the soil.

The researchers now operate six CO2 monitoring stations at Mammoth, one of the few such networks in the world, says geochemist Susan Brantley of Pennsylvania State University in University Park. "It will be exciting to watch for correlations between gas emissions and seismic activity at the mountain," she says. Few such correlations have yet been seen, but a long-term series of measurements might reveal a pattern.