Brimstone Basin, Yellowstone: A Cold Thermal Area That Still Breathes Volcanic Gas

Brimstone Basin, on the southeast shore of Yellowstone Lake, looks like one of Yellowstone's thermal areas: little vegetation, and ground bleached white with patches of yellow and black where sulfur minerals collect and hydrogen sulfide, the "rotten egg" gas, escapes. But it has none of the usual signs of heat, no steaming ground and no hot or even warm springs. It seemed to be a prehistoric thermal area, now dead. In 2012 Deborah Bergfeld of the USGS and colleagues showed in the journal Chemical Geology that it is not.

Brimstone Basin and other areas of Yellowstone known to release carbon dioxide from the ground. Image from the USGS.

Acid-altered ground

Yellowstone has many patches of acid-altered ground like this, bleached white and gray. At most of them, geothermal gases, carbon dioxide (CO₂) with or without hydrogen sulfide (H₂S), seep through the ground in step with the heat coming out.

A sulfur flow (darker gray) on altered ground, with Yellowstone Lake behind. Image from the USGS.

A surprising amount of gas

A field campaign in 2008 measured the gas rising through the soil, the diffuse flux, at about 300 spots across the basin. It added up to about 277 metric tons of CO₂ and 0.6 metric tons of H₂S a day: cold gas coming out as fast as it does in much hotter "acid-sulfate" areas of the park.

Two kinds of altered ground (light blue and pale pink) and the gas measurement sites (red dots) from the 2012 study. Image from the USGS.

Clues to heat below

Along Alluvium Creek, a cold, acidic stream through the main areas of upflow, the team found spots where gas rises so strongly that the water seems to boil, and collected samples there.

Gas bubbling up through Alluvium Creek. A funnel traps it and sends it through tubing to a collection bottle. Image from the USGS.

The chemistry told several stories:

• Compared with other acid-sulfate areas, the gas is rich in CO₂ relative to H₂S and high in helium.
• The isotope ratios of carbon and helium show that some of the gas comes from magma.
• The methane and ethane, and the carbon isotopes in the methane, point to other gas coming from heated sedimentary rock.
• The rising CO₂ is abundant enough to strip heavy oxygen (¹⁸O) from the groundwater, pushing the local streams off the meteoric water line that surface water worldwide normally follows. That could only happen if the CO₂ started out at or near boiling, about 90 °C.

Together these point to a hot source for the gases and warm conditions still lying deep beneath a basin that looks cold at the surface.

Measuring gas flux over altered ground at Brimstone Basin. Image from the USGS.

Sources

• USGS Yellowstone Volcano Observatory, "Gases in Brimstone Basin Indicate Heated Pasts," 2013, on the study by Deborah Bergfeld and others in Chemical Geology; rewritten in hubnx's own words. Images from the USGS.