Into the depths: Studying CO2 in karst caves

Around ten percent of the Earth’s continental surface is covered by karst rock, especially limestone and dolomite, which is naturally permeable and easily soluble. These rocks are interlaced with many cracks, fissures, and crevices caused by mechanical stress such as the uplift of entire mountains. When it rains, the water seeps into the ground, where it mixes with CO₂ produced by soil bacteria during the decomposition of plant matter. This CO₂ converts the rainwater into carbonic acid, which attacks the limestone and slowly dissolves it, especially along the existing cracks and fissures. Water moves primarily along these cavities, which, over time, become increasingly larger and connect to form entire cave systems.

The cave air has an average temperature of 8–9°C both in summer and winter. In summer, the air outside is usually warmer than in the cave; in winter, it is usually colder. Gas becomes heavier the colder it is and the more CO₂ it contains. If the soil dries out, the gas presses into the cave. Depending on the temperature, it can then escape back into the atmosphere through the soil layer. 

Physical processes in the cave

If the CO₂ is dissolved in water, it increases its density. The water thus becomes heavier and accumulates at the interface between the vadose and phreatic zones (i.e., at the boundary layer to the groundwater). There, it continues to move downwards. This leads to fingering, the formation of finger-like structures of CO₂-enriched water in the groundwater. Because of these fingers, the CO₂ can increasingly enter the groundwater. “However, there is a lack of reliable data on these processes under karst conditions,” says Keim. Finding this out is the focus of his doctoral project.

The idea came from geologist Harald Scherzer from the Laichingen Cave and Heritage Association. He suspected fluctuating CO₂ levels in the cave, wanted to find out more, and contacted Keim’s doctoral supervisor, Professor Holger Class. Because the results are interesting not only for cave research but also for climate modelers or CO₂ storage in the soil, a research project was created from this. In order to measure the CO₂ content, a measuring station over 6 m high was transported to the Laichingen Deep Cave and set up.

It consisted of a water column, which was essentially a six-meter-long pipe filled with tap water. They imitated a cave lake to measure gas exchange with the water. Sensors were attached inside the pipe at depths of 0.15 and 5.6 m in order to measure the CO₂ content of the water. Sensors were installed above the pipe to measure the CO₂ content of the cave air. The data provides information on how the CO₂ concentration in the water changes at different depths depending on the CO₂ concentration in the air. From this, it is then possible to deduce the conditions and time periods under which the fingers occur. 

Keim can analyze the recorded data in his office at the University of Stuttgart because it is transmitted online. In addition, the temperature in the cave was measured and freely accessible data on the daily amount of precipitation as well as the minimum, maximum, and average daily temperature from the German Weather Service was used and linked to the measurement data. The measurements in the Laichingen Deep Cave began in April 2021 and are still ongoing. “We started by describing what we had observed,” says Keim. “For example, we have seen that every spring, the CO₂ concentration tends to be rather low and increases over the summer. In the summer of 2021, we had a particularly high CO₂ concentration, which was not the case in the following two years and was then quite high again in 2024. So we naturally asked ourselves: why is that?”

CO₂ values in the Laichingen Deep Cave

A lower CO₂ concentration was generally measured in the winter months through to early summer (i.e., in dry and cold periods) than in summer and fall. Peak CO₂ concentrations were also reached in the cave air during periods of high precipitation. Because the temperature outside was lower than inside the cave in winter, the air outside was heavier and forced its way inside.

If the upper soil layer dried out, gas exchange between the cave and atmospheric air took place, and the CO₂ concentration decreased. In summer, the CO₂ concentration tended to be higher. The scientists interpreted the particularly sharp increase in concentrations in late summer and early fall 2021 as the result of a relatively wet summer followed by an extremely dry period. In periods of high precipitation and when the CO₂ was stored in the karst system for a while, the concentration leveled off at 15,000 ppm. This equilibrium value repeatedly occurred in the cave air.   

Mobility of CO₂ gas in the karst system

The question was also whether the increased visitor traffic in summer (the Laichingen Deep Cave is a show cave from April to October) also increases the CO₂ concentration. Daily fluctuations confirmed this pattern, but the variations in the measurement curves were minor and therefore negligible over the course of the year. Scientists found the events around March 5, 2023 particularly striking. On that day, the Baden-Wuerttemberg Cave Rescue Service conducted a major exercise, during which active ventilation was installed down to the measuring station in a neighboring shaft. This greatly reduced the CO₂ concentration. This data could still be relevant for further university studies.

The scientists also made some surprising observations when measuring the gas exchange with the water. Class and his colleagues documented the results in 2023. They found that the same fluctuations in concentration occurred at shallow water depths as in the cave air, albeit not as strongly. When CO₂ is dissolved in water, it increases its density and sinks to the bottom. However, at a depth of 1 m, the smallest disturbances (e.g., minimal temperature differences or small vibrations) are still sufficient for the CO₂ to return to the gas phase (i.e., back into the cave air).

However, at greater water depths, the CO₂ concentration was high. A peak value was reached at a depth of 5.85 m. Once CO₂ had sunk, it was trapped there (i.e., it did not rise again). Large water depths are therefore a highly efficient and permanent trap for CO₂ provided there is no considerable mixing. “To our knowledge, this is the first time that the mobility of CO₂ in still water has been modeled in a cave,” says Keim. The data sets are freely available to other scientists and modelers for their own research.

In collaboration with researchers from the University of Bordeaux, scientists from the University of Stuttgart also aim to investigate how CO₂ moves through the cave. In doing so, they can build on the preliminary work of the French scientists. “Ultimately, we aim to better understand which processes lead to which effects in caves and how climate change affects them,” explains Keim.

Manuela Mild | SimTech Science Communication