Brimstone Basin - a Cool Place at Yellowstone!

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Bleached and steaming ground that lacks vegetation is another visual clue that indicates ongoing hydrothermal activity at Yellowstone. The bleached areas are called acid-sulfate ground. They form when high heat flow and large emissions of acid gases, such as carbon dioxide (CO₂) and hydrogen sulfide (H₂S), alter the surface rock and create poor growing conditions for most types of plants. Yellowstone's acid-sulfate areas emit much greater quantities of gas than what is emitted from the geyser basins. For scientists who study magmatic gases, these areas are "the place to be" for learning more about the underlying magma system.

Brimstone Basin, along the Southeast arm of Yellowstone Lake, contains an area of acid-sulfate ground that covers about 1.1 km² (0.4 mi²). This area is different from most of Yellowstone's acid-sulfate regions because there are no visible thermal water features such as hot springs, fumaroles or mudpots. Partly because of a lack of thermal features, but also due to its remote location, Brimstone Basin is rarely visited or studied.

Brimstone Basin is one of the more unusual settings at Yellowstone. Along route to the acid-sulfate ground there are oddities such as two adjoining streams, one carrying white opaque water and one with clear water. The clear water looks like something you might want to drink, but the stream drains areas with large amounts of diffuse CO₂ and H₂S degassing and is highly acidic with pH values less than 3. The acid waters at Brimstone Basin have high concentrations of dissolved aluminum, which can drop out as a whitish precipitate if the pH increases. This can occur when the acid creek water mixes with regular groundwater.

Brimstone Basin is rich in sulfur, and even the casual observer will be struck by a preponderance of physical evidence that H₂S gas is emitting from the ground. At many places a strong acrid odor hangs in the air, and one can find mounds of rocks that are coated with sulfur-bearing minerals. Other unusual sites include sections of bleached white ground that are covered by brittle gray-to-black colored sulfur flows. Although no one was around to see it, we believe the sulfur flows formed when sulfur deposits were ignited during a forest fire.

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Prior to 2008, the acid-sulfate ground at Brimstone Basin was assumed to be the remains of an extinct hydrothermal system with no current connection to a heat source. This assumption is understandable given the lack of thermal water features and the fact that all gas escapes from the ground at the same temperature as surrounding air.

An extensive survey to measure the amount of gas discharge from Brimstone Basin concluded that the CO₂ and H₂S emissions are similar to what's been measured at known active thermal areas. By analyzing helium and carbon in the gas and oxygen and hydrogen in the water, scientists determined that the gases do arise from a warm system at depth beneath Brimstone Basin. So indeed, the area is a remnant from an older, differently active system, but it's not dead yet.