Why do we monitor carbon dioxide emissions in Yellowstone, and how?

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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The carbon dioxide (CO₂) emitted from the Yellowstone magmatic-hydrothermal system has a number of characteristics that make it an important gas to monitor, including its great abundance, partial magmatic origin, and that it can provide information on the depth of the magma beneath the surface. In the early 1900s it was recognized by Eugene Thomas Allen and Arthur Lewis Day, geochemists from the Carnegie Institution of Washington, that Yellowstone gases are rich in CO₂. In fact, except for steam, CO₂ is typically the most abundant (>90%) gas emitted from the Yellowstone magmatic-hydrothermal system, and estimates of the rate at which CO₂ is emitted across Yellowstone suggest that it is among the highest of any volcanic system in the world. (It is important to note that this amount is very small compared to anthropogenic, or human-caused, CO₂ emissions.) A significant portion of this CO₂ originates from degassing of basaltic magma beneath Yellowstone.

Measuring the amount of CO₂ that is emitted from a volcanic system and how it changes over time is particularly useful because it provides hints about possible changes in the depth of magma beneath the surface. Much like a bottle of soda, which contains CO₂ dissolved in the liquid, CO₂ and other gases (like sulfur, chlorine, fluorine and helium) are dissolved in magma. When the cap is tightly on the soda bottle, the pressure inside the bottle is high and the CO₂ remains dissolved in the soda. When the cap is taken off, the pressure on the liquid decreases and the CO₂ bubbles out of the liquid and is released to the air.

Similarly, when magma is at great depth beneath the surface, the weight of the earth above it acts like a "cap" maintaining high pressure that causes gases to dissolve in the magma. However, when the magma begins to migrate upwards toward the surface, pressures on the magma decrease and gases are able to bubble and escape. Because different gases have different solubilities in magma, they are released at different pressures (depths). Carbon dioxide happens to be one of the least soluble gases in magma, so can escape from the magma at great pressure/depth. Other sulfur, chlorine and fluorine gases are more soluble and escape from the magma at shallower depths. Therefore, measuring the rate at which CO₂ is emitted from the volcanic system, its abundance relative to other gases, and how these factors change over time can provide early warnings of when magma enters the earth's deep crust and its subsequent movement upwards toward the surface.

The amounts of CO₂ and other gases emitted from Yellowstone thermal features, such as springs and fumaroles, have traditionally been measured by collecting gas samples and analyzing their chemical composition in the laboratory. The rate at which CO₂ is emitted from the ground has also been "mapped" across thermal areas using a portable backpack-mounted CO₂ fluxmeter. Both of these techniques have provided valuable information on how the chemical compositions of gases and emissions of CO₂ vary in space across Yellowstone. However, to better understand how gas compositions and CO₂ emissions change over time, we need to make measurements more frequently and year-round with automated instruments.

With the goal of monitoring potential magmatic (and hydrothermal) activity, YVO has installed and tested two types of instrumentation in Yellowstone thermal areas over the past several years. The first type of instrument, an eddy covariance station, makes high-frequency measurements of atmospheric CO₂ concentrations and wind speeds in three directions, calculates CO₂ emissions based on these measurements every half hour, and telemeters data to the USGS. The second instrument type, a Multi-GAS station, makes regular measurements of CO₂, H₂O and sulfur gas concentrations in the air downwind of gas vents.

To-date, eddy covariance and Multi-GAS instruments have measured substantial "background" changes in CO₂, H₂O, and H₂S concentrations and CO₂ emissions on a daily basis, largely caused by variations in the weather, while SO₂ has not been detected. Gas concentrations and emissions have been relatively stable over the longer term, however, and have not suggested changes in the deeper hydrothermal and magmatic systems. After background variations are well characterized, the goal of the YVO gas monitoring program is to detect anomalies that could provide indications of potential magma movement beneath Yellowstone, should it occur.