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Caldera Chronicles: Capturing Yellowstone’s elusive gases
CALDERA CHRONICLES

Caldera Chronicles: Capturing Yellowstone’s elusive gases

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When you visit Yellowstone’s geyser basins, the sights, sounds and smells that surround you are evidence of gases from beneath the surface escaping into the atmosphere.

These gases are composed mostly of water vapor (steam), which is particularly easy to see during the morning as white clouds that emerge from hot springs, steaming ground, geysers, bubbling pools, fumaroles and frying pans. Other gases escaping with water vapor, such as carbon dioxide (CO2), hydrogen sulfide (H2S), methane, nitrogen, carbon monoxide and helium, are colorless, but their presence is given away by bubbling through water and wet ground and, in the case of H2S, a distinctive rotten-egg smell.

Gases not only provide some of the geyser basins’ more interesting touristic features, they can also yield important information about the state of Yellowstone’s volcanic activity. For example, measuring the rate at which gases are emitted from the volcanic system, their chemical compositions and how these factors change over time can provide early warnings of when magma enters Earth’s deep crust and its subsequent movement upward toward the surface.

Importantly, the information that can be gleaned from a gas sample is only as good as the quality of the sample itself. Scientists must therefore find ways to collect high-quality gas samples with minimal contamination from the atmosphere and sampling equipment. Because thermal areas pose a range of dynamic and hazardous conditions, such as hot and corrosive liquids and gases and hot and unstable ground, scientists must carefully assess if a feature can be safely approached and, if so, what type of equipment will be needed to provide a quality sample for laboratory analysis.

At the heart of the sampling equipment is a glass bottle with a stopcock from which air has been evacuated prior to field work. The pre-evacuated sample bottle may also contain an alkaline (high pH) solution, into which acidic gases, such as CO2 and H2S, dissolve and steam condensation is facilitated. This results in enrichment of the non-reactive gases, like helium, in the bottle headspace above the alkaline solution.

How gas is captured in the sample bottle without contamination from the atmosphere depends on the characteristics of the thermal feature being sampled. A corrosion-resistant device is used to divert the gases as they are emitted from the thermal feature, for example, a titanium tube inserted into a fumarole or an inverted plastic funnel placed on the surface of a pool or partially buried in the ground. The trapped gases are allowed to flow through tubing connecting this device to the closed sample bottle until all preexisting air has been purged from the setup. At some sites, long lengths of tubing and a painter’s pole may be used to allow sampling at a safe distance from the thermal feature.

Once the gas flow is captured, the setup is purged of air and the sample bottle stopcock is opened, allowing gas to flow in and steam to condense. To encourage steam condensation and discourage boiling of the alkaline solution during the sampling process, which can take up to 30 minutes, the sample bottle is often cooled using a cold wet cloth. Once sampling is complete, bottles are carefully packed and shipped to the laboratory, where the next step of their journey on the way to providing critical information in monitoring the Yellowstone volcanic system will be carried out.

Over the past several decades, gases have been sampled from all of the major thermal areas of Yellowstone, allowing for a better understanding of the types and quantities of gases emitted from different areas of the park. These data are crucial for not only volcano monitoring but also better understanding Earth’s inner workings.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Jennifer Lewicki, research geologist with the U.S. Geological Survey in Menlo Park, CA.

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