Cave Air Dynamics – Paha Sapa Grotto

Caves each have their own unique atmospheric chemistry and a distinct barometric response to external pressure changes. These characteristic footprints can vary from simplistic linear responses to highly complex multivariate systems with numerous feedback loops. The following sections discuss first the causes of airflow within caves, and then the nature of that air from a geochemical standpoint.

Jewel Cave Barometric Data - Andreas Pflitsch et al.

The last section will outline what is currently known about these atmospheric factors in Black Hills caves, who is doing the research, and a bit about the ongoing projects here in the Hills.

Cave Barometry & Airflow

In general, the larger the cave’s volume, the more substantial will be the flow of air in and out with a given change in barometric pressure. Other factors that determine flow rate are the size, number and elevations of any openings back to the surface.

: Barometric Inhale Cycle (High Pressure)

: Barometric Behavior Depends on Cavity Volume, and the Number & Size of Openings.

: Barometric Exhale Cycle (Low Pressure)

The response observed from barometric and other physical changes depends on a number of complexly interdependent factors that can either accentuate or retard flow under a given set of conditions. A cave’s size, geometry, wall roughness and passage tortuosity all effect airflow characteristics, as do the relative sizes and proximity of openings to one another.
Caves can breathe for a number of different reasons. Barometric shifts are one of the primary drivers for flow, but movement may also be induced from chimney effects and wind variations when there is an elevation difference between multiple openings.

A difference in air temperature and/or density between openings or even portions of an opening can lead to observable airflow.

These air density differences are usually small and tend to be seasonal and/or diurnal in nature.

Low to moderate levels of air flow can occur in response to daily and seasonal temperature fluctuations, depending on cave size and geometry.

Cold air can be trapped in low spots and lead to ice accumulations where a single entrance is present and the interior recesses are significantly below the opening.

illustrations-of-mechanisms-for-cave-airflow-for-thermally-driven-flows-airflow_1

(a) Chimney Flow
(b) Restricted Gravity Well / Sink
(c) Barometric Siphon and Chimney Flow
(d) Blow Hole
(e) Unrestricted Gravity Well / Sink
(f) Barometric bell

The adjacent diagram shows the thermal regime in a small pocket type cave. Air movement in response to seasonal thermal patterns will cause breathing in during the fall and winter months and out during the spring and fall seasons.

It is not uncommon for a breathing cave to have contributions to it’s flow patterns from more than one of the above mechanisms. Careful observations and flow measurements are usually required to separate the effects of each.
In a barometric cave where the number of openings are few, and in which the effects of chimney flow are minimal to absent, the inflow and outflow rates will be directly related to the system’s volume. This means that the cave’s ultimate volume or “size” can be estimated for the portions of the system above the water table.

Additionally, if enough is known about the average passage characteristics in the portions that are mapped, a tentative”length” can be approximated for the system. Cave barometry is an extremely powerful tool in the assessment of the significance and size of cave resources.

Cave Atmospheric Chemistry

The chemistry of spelean atmospheres are by no means static or simple in terms of gaseous makeup. Depending on the types of carbonates present, the minor and trace element contents of those carbonates and their proximity to other nearby lithologies, a cave’s geochemical signature can vary from relatively uniform to highly complex.

In general, larger caves with fewer entrances have more complex and unique atmospheres, but smaller caves can have distinct and surprisingly multifaceted signatures as well.

All of the gases present in the outside atmosphere are also found in the average cave system. Given this framework, there will typically be compositional shifts within the cave’s atmosphere that have been induced by the geochemistry and biochemistry of the wall rock and biota present. The cave’s barometry, geometry and temperature also play a key role in determining the flow and exchange rates of the interior air with the outside as well as the diffusion of cave specific gaseous components outwards.

The following chart shows all of the major atmospheric gases and a suite of others that have been recognized and studied in various cave systems worldwide. This list is by no means all inclusive, as many others are possible. The average atmospheric value for each gas is listed as an index for assessing possible anomalous concentrations in a cave’s environment. Some of the most likely sources for an anomalous gas concentration are also given.

The lighter-than-air gases are all highlighted in blue at the top of the list, and will diffuse rapidly if generated within the cave environment. Anomalous levels of these gases in a cave’s atmosphere would therefore indicate a relatively high inward flux rate, or a very slow rate of exchange with the surface. The light gases will typically have the most variable concentrations within cave air over time due to their lower densities and higher diffusion rates relative to the average air mixture.

The heavier-than-air gases are all highlighted in green at the bottom of the chart, and in general will tend to be enriched in cave environments that have passages below the levels of any entrances. Heavy gases usually have the highest residence time and consistent gas compositions within a given cave, as the rates of exchange and diffusion are slower and they often have higher concentrations in areas of stagnated flow. The three heavier than air gases that have the highest probability of enrichment in spelean environments are:

Radon

Radon in Cave Atmospheres

Radon Chemistry and Radiochemistry

Radon (Rn) is the heaviest of the naturally occurring noble or inert gases. This radioactive gaseous element has no stable form in nature but is rather a mixture of its 36 naturally occurring albeit short-lived radioactive isotopes. The longest lasting of these isotopes (Rn 222) has a ½ life of just 3.8 days, so at any given time there is not much to be found in nature and in fact, there are only a few tens of grams of Radon present at any time in all of the Earth’s atmosphere.

This does not seem like much of an abundance to be concerned with, but this merely represents the steady-state flux of a gas that is constantly being regenerated. As such, it still represents an important and omnipresent atmospheric component. Radon reaches a steady-state concentration quickly and does not build up past a certain level due to this constant decay, so it is always present and generally found to be at a very constant and uniform concentration within the atmosphere as a whole.

Within caves, however, there is far more variability than is seen within the atmosphere, and that variability can give valuable clues as to the atmospheric dynamics and passage geometries found within the cave system. Radon can also provide insights into the nature of the solutions and bedrock geochemistry involved in groundwater evolution and the system’s speleogenesis. Radon is moderately soluble in groundwater at normal spelean environmental temperatures and pressures. As a consequence, its concentration in subsurface atmospheres varies with the degree of ventilation, groundwater solution equilibrium, background Rn gas levels in the surrounding formations and temperature.

Like carbon dioxide, radon (Rn) concentrations in cave-air typically exhibit seasonal variations, with summer maxima and winter minima where there is at least some groundwater transport through the vadose horizon. Less variability is typical in systems that are completely dry, although ventilation, Rn gas values in the host rock and temperature related variation may still be evident.

The relatively steady concentration of ambient radon gas in both above ground and subsurface atmospheres is due to constant additions from the radioactive decay of uranium (U) and thorium (Th) found in natural materials. The decay chains for these two actinide elements are a significant source of the normal background radiation in our environment, and the origin of most of the helium (He) present within our atmosphere.

One of the principal ways in which radioactive elements decay is by alpha particle emission. An alpha particle is nothing more nor less than a Helium (He) ion that quickly captures a couple of stray electrons to become helium (He) gas.

Although Radon is radioactive, it is still a nonreactive noble gas from a chemical standpoint, which means that it is inert and essentially non-toxic from purely chemical considerations. It does not dissolve readily in water, nor does it react chemically with organic tissues, or is it absorbed or used in any way by the body. Radon has absolutely no tendency to accumulate in the environment or within living organisms, either. Regardless of its seemingly innocuous chemical behavior, however, radon’s radiochemical activity must always be addressed when its effects on living organisms are being considered.

The adjacent nuclear reaction is the portion of the decay of Uranium 238 into Lead 206 that produces Radon 222 gas.
Rn 222 is the most abundant of the radon isotopes and the only isotope that the name “radon” formally applies to, although in practice they are all usually lumped together under this name.

Both uranium (U) and thorium (Th) readily substitute for the calcium (Ca) atoms found within mineral structures, and even though these two elements are found at very low levels in most rocks, enough radon is produced thereby to keep the background atmospheric concentration at about 6 x 10-16 moles/liter.

The equivalent to this chemical concentration is usually expressed as a radiochemical dosage when radon measurements are reported, however.

Here in the U.S., this value equates to 0.4 pCi /L (picocuries/liter); elsewhere in the world, it is expressed as 0.0148 Bq/m3 (Becquerels per cubic meter).

For those interested in the numbers, a picocurie is just 0.000,000,000,001 (one-trillionth) of a Curie, which is one of several internationally used units of radioactivity measurement.

A concentration of 1.0 pCi/L means that in one liter of air at that at level there will be about 2.2 radioactive disintegrations per minute.

At a level of 4.0 pCi/l, therefore, approximately 9-10 radioactive disintegrations will occur each minute per liter of air (see radon reference level below).

This amounts to about 1 per second within the average human lung volume of 6 liters.

The adjacent nuclear reaction shows the portion of the decay of Thorium 232 into Lead 208 that produces Radon 220 gas.
Rn 220 is often referred to as “thoron” but due to its relatively low abundance (5-15%), it is typically lumped together with Rn 222 as “radon”.

A quick comparison will show that there are approximately 27 Bq/m3 for each 1.0 pCi / L, so conversion from one unit to the other is quite straightforward.

Radon Reference Level

A reference value of 4.0 pCi/L – (108 Bq/m3) has been established by the Environmental Protection Agency (EPA) as the level at which radon levels are considered to exceed the “normal” or “acceptable” background range for living spaces; this concentration is about 10 times what background atmospheric levels are known to be. The reference value, however, is not based on any observational criteria or known set of physiological effects at that level. It is rather a concentration at which “concern” should be considered prudent, most likely because it represents an environment in which radon levels are a full order of magnitude (10X) higher than average background levels. Allowable workplace levels have been similarly set by the EPA at the higher value of ~15 pCi/L – (400 Bq/m3) which is about 4 times the level established for living spaces.

The health effects due to radon inhalation are typically estimated using the so-called “Linear No threshold” (LNT) relationship. This assumes that the radiation effects produced from low level irradiation can be predicted quantitatively by linear extrapolation of the effects produced with high level irradiation. This assumption implies, therefore, that any amount of radiation, however small, is potentially harmful.

The LNT hypothesis provided a decidedly conservative approach to radon risk assessment for the purpose of standard setting. However, its application has likely lead to some rather overly conservative radiation protection standards being put in place, particularly with respect to the dangers of low level irradiation. The LNT approach in this application has been repeatedly criticized due to the lack of sufficient definitive evidence required to support its use to assess the health effects of low-level radiation. The risks of lung cancer attributed to low level radon exposure are simple extrapolations from extremely high level (lethal) dosages, and the assumption of simple linearity is neither justified nor prudent based on the available evidence. https://pdfs.semanticscholar.org/404b/3d6f916752ff78864f16158e29db26454c20.pdf

The adjacent nuclear reaction shows the portion of the decay of Thorium 232 into Lead 208 that produces Radon 220 gas.
Rn 220 is often referred to as “thoron” but due to its relatively low abundance (5-15%), it is typically lumped together with Rn 222 as “radon”.

There is quite a bit of variation in concentration seen at ground level in some areas which is primarily due to differences in local rock types and their geochemical affinities. With the measurements I have performed, the atmospheric average is more closely approached at a few meters above the ground.

The following diagrams show the complex radiochemical mechanisms for uranium (U) and thorium (Th) decay, which transition through no fewer than 10 daughter forms each before they decay into one of several stable isotopes of lead (Pb). These decay chains each produce a single radon (Rn) daughter in the central part of the string, which makes the various radon isotopes the parents for all the reactions down-chain from that point.

The radon isotope phase is the only point within each chain where the product is a mobile gas, and the principal stage at which environmental dispersion takes place. All of the various daughter products are much less mobile post-transition metals that range from moderately to extremely toxic, both chemically and radiochemically. This is why radon exposure levels should always be considered when any long term extended exposures are a possibility.

Radon

Thoron

Actinon

Click on any of the above decay chain diagrams for more details

Radon gas is almost 8 times as dense as air and diffuses slowly, so it tends to become enriched in low spots and within closed spaces. Some would argue that this is not the case or even possible, but from a purely empirical standpoint, the low and closed spaces measured within caves follow this trend more often than not.

Radon exposure levels can become very significant when in a highly enriched closed space like a uranium mine, thorium mine, or within a well or basement dug into rocks enriched in the suite of lithophile elements that U and Th are typically associated with. Radiological exposures to radon are usually considered as averaged over time and can approach prohibitive levels with prolonged exposures at high concentration levels.

Radon in North American Caves

In general, radon concentrations in caves are higher than within open air settings, which is understandable considering that caves typically have restricted air exchange potential with the external atmosphere.

Radon is known to reach highly elevated concentrations in some North American caves (rarely up to 500 pCi/L and above), but in general American caves have somewhat lower Rn levels than have been reported in European caves and elsewhere.

Limestone has a higher background level of uranium than other sediments due to its high calcium (Ca) content, and therefor most limestone caves have radon levels that exceed atmospheric levels.

The primary determinant for finding elevated background levels, however, is the nature of the underlying and adjacent bedrock, as well as the geochemistry of the local groundwater. This is true within both cave-blessed and cave-deprived areas. To keep things in perspective, however, it best to be aware that many basements and the lower levels in well insulated homes will contain radon levels that are comparable with those found in many cave systems.

The only way to know for sure whether a particular cave has a high radon level is through testing. This is usually done through measuring long term averages at one or more locations within a cave or by spot checks in various locations at different times of the year.

The National Park Service (NPS) has done numerous long term radon monitoring studies in several of the nation’s largest cave systems which has looked at cumulative radon dosages over time for both its employees and visitors.

The EPA has done a similar analysis that also included recreational cavers. These studies concluded that neither employees nor frequent visitors or recreational cavers experienced significant increases in lifetime exposure levels, considering the short duration of most of their visits.

References

http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/09/378/9378952.pdf

https://www.nps.gov/subjects/caves/upload/inside_earth_6_2.pdf

https://caves.org/pub/journal/PDF/v69/cave-69-01-207.pdf

Radon in NPS Caves

Looking at the NPS sites evaluated in the chart above, there is some tendency for eastern and southwestern caves to have higher Rn levels than those in the west, but the data set is certainly not all inclusive. Some caves have quite a bit of variability in radon distribution, but others like Wind Cave have relatively homogeneous values.

Radon in North American Caves – Synopsis

Those of us that work in and or are frequent visitors to caves, therefore, are generally only rarely at a higher risk from these short term ventures into the Stygian depths. The possibility for encountering high Rn levels within a cave that has not been previously evaluated can usually be anticipated, however, based on local geology and nearby rock associations.

Radon tracking is not an imperative in most situations, however, due to the short term exposures that are generally experienced when visiting most caves. It is, however, advisable to consider when it is planned to visit a particular cave over an extended period of time.

Radon as a Cave Atmosphere and Geochemical Tracers

Aside form its obvious value for addressing health risks, radon testing and distribution profiling can also be an excellent tool for better understanding the dynamics of a complex cave system’s atmosphere, due to the element’s very high density and relatively slow diffusion characteristics. The complex patterns of flow, air exchange and stagnation can be in many cases better understood with information on the heavy gas components within a given system. To this end, radon is a a relatively easy, economical gas to measure and quantify. This is typically done through the use of short and long short term track etch monitors, as well by the use of hand-held continuously recording meters. Top of the line monitoring equipment can be quite pricey and is usually quite bulky, but a well protected hand-held device in a pelican case that is allowed to breath during spot checks works very well, gets the basic information desired and is a heck of lot more affordable.

Alpha Track Etch Type Radon Monitor

Handheld Radon Monitor

Thermal waters often have distinctive solute, gas and radioisotopic characteristics that may impart a distinctive fingerprint to the system’s groundwater and atmosphere. Radon analyses can in some cases help to determine the presence of hypogene components in the phreatic portions of a system where thermal waters may have had a hand in the dissolution process. This is particularly true if isotopic analysis are available, as groundwater radiochemistry can be be very distinctive depending on its source.

Hypogene Cave Features

Monitoring/profiling of radon levels has also provided insight into how elevated Rn levels within a cave may affect the distribution of its cavernicole (cave dwelling) inhabitants. Bats, in at least one study of four caves in Mexico, seem to have figured out that the highest radon areas in these caves are not the best areas for roosting and colonies. It would be very interesting to see whether this adaptability pattern extends to cave systems across North America and elsewhere.

Bat adaptability to High Radon in several Mexican Caves

Bats have relatively long lives (~20 years) and extremely high metabolisms, so all things being equal, one would expect high rates of radon-related mortality in these creatures due to the amount of time they spend in high radon environments. To the best of my available knowledge, this theory has not been thoroughly tested as of yet…hmm…sounds like a great PHD or post-doc project to me. So many questions…so little time!!
Carbon Dioxide

Carbon Dioxide in Cave Atmospheres

Carbon dioxide is by far the most important and fundamental of all the gases found in cave atmospheres. It is responsible for both the original formation of the cave’s open space and for the precipitation of the mineral formations that will in time refill them. Due to its genetic ties to the cave forming process, relatively high abundance in cave air and its rapid response to equilibrium shifts, carbon dioxide is an excellent tracer for the study of cave atmospheric dynamics.

Cave-air CO2 concentrations are almost always considerably higher than the current 390 parts per million (ppm) atmospheric average. Cave-air CO2 levels are often observed to rise with increased distance from surface openings, as well.

It is not uncommon for ambient CO2 levels to rise to several thousand ppm and more in some caves and several percent CO2, though rare, can occasionally be encountered in narrow enclosed spaces.

At concentrations up to 2%, very few effects other than a decrease in working capacity are experienced. When CO2 levels rise above 3%, however, highly labored breathing and skin tone changes are the warning signs of approaching blackouts and possible asphyxia.

Fortunately, extremely high CO2 levels are only very rarely encountered in caves, but it is good to be mindful of the possibility thereof, none-the-less. CO2 monitoring, if for no other reason than the recognition and avoidance of these rare situations, should be a consideration when entering newly discovered cave systems for the first time.

Cave-air CO2 levels, at any concentration level, are directly related to the speleogenetic process and are strongly affected by both internal and external atmospheric dynamics. Cave atmospheric CO2 levels typically vary with temperature, the degree of surface air advection into the system, local groundwater chemistry and the CO2 gas content of the surrounding bedrock.
Due to the combined influences of these factors, the carbon dioxide levels within a cave will typically follow a definite pattern of seasonal and diurnal fluctuation, with significantly higher inner atmospheric levels during the warmer times of the day and year and lower levels during the cooler periods.

Seasonal Cave Air Variation

Dissolution mechanisms predominate during periods of high CO2 gas levels and precipitation typically takes place during periods of lower concentration, so these processes tend to follow the seasonal and diurnal patterns of CO2 fluctuation (see the section on reaction progress below).

Gases in general have much higher solubilities in cold water than in warm water, and the advected warmer air during the summer months also helps to seasonally increase the CO2 and all the other trace gas contents of cave-air.

The degree to which the gases present in the connected pore spaces of the surrounding formations controls gas levels within a cave are generally not well understood but are none-the-less likely to be significant.

Formational CO2 and other gases typically migrate along faults, bedding planes and other discontinuities within bedrock, just as groundwater or any other fluid will, and these transported gases may (often?) constitute a major component in cave-air chemistry.

Carbon Dioxide

Reaction Progress and CO2 Levels

The following two reaction series show the mechanisms responsible for cave dissolution and cave mineral precipitation, so they may be considered in light of how changing CO2 levels can affect the reaction progress in each cycle.

In the world of chemistry, Le Chatelier’s principle dictates that if you increase one of the reactants on the left side of a reversible reaction at equilibrium , it will drive the balance towards the products on the right. On the other hand, if you increase the amount of one of the products instead, it will shift the reaction back towards the left.

Carbon Dioxide

As a consequence, having a high and/or increasing CO2 level within a cave will tend to drive the reaction toward the right (product side) which favors the dissolution cycle, as outlined in the two linked reactions shown below.

The Dissolution Cycle

These same conditions will also simultaneously inhibit the precipitation cycle. This is because CO2 is one of the products on the right hand side of that reaction, so an increase in the CO2 level shifts the system back in the opposite direction towards the dissolved bicarbonate ion. This is illustrated in the precipitation reaction below.

As a consequence, having a high and/or increasing CO2 level within a cave will tend to drive the reaction toward the right (product side) which favors the dissolution cycle, as outlined in the two linked reactions shown below.

The Precipitation Cycle

Le Chatelier’s principle similarly dictates that decreasing the amount of one of the products on the right hand side of a reversible reaction at equilibrium will have the same effect as increasing one of the reactants and shifts the reaction towards the products.

Having a lower and/or decreasing CO2 level in a cave, therefore, will tend to favor the precipitation cycle and the growth of calcite and speleothems.

Although the speleogenetic processes of dissolution and precipitation take place over a wide range of environmental conditions, there generally appears to be an upper limit of about 4000 ppm CO2 in cave air for calcite precipitation to take place. Cave systems with higher year around average concentrations than this threshold value tend to have speleothems that no longer grow, or to be devoid of speleothems entirely.

Bethlehem Cave - Photo by Mike Hanson
Argon
Argon in Cave Atmospheres

Argon (Ar) makes up 1.3% of our atmosphere by weight and 0.934 % by volume. It can also be found everywhere dispersed through the pore space of rocks within the earth’s interior. Although held tightly within defects and holes within crystal lattices, it is released during heating events, and being a gas, will eventually make it way to the atmosphere. A major portion of the terrestrial pool of argon has been produced since the Earth’s formation from the radioactive decay of the tiny amounts of the isotope potassium-40 (40-K = 0.012% of all potassium) found within potassium bearing minerals. Measuring the production of argon-40 (40-Ar) from the decay of 40-K is one of the more commonly used methods for determining the Earth’s age (potassium-argon dating).

Argon (Ar) is the most abundant of the inert Noble gases within our atmosphere. It is a colorless, odorless, non-reactive gas that is the 3rd to 4th most abundant gas (Ar vs. H2O) in the air that we breath and the atmospheres of caves.

Although atmospheric argon is ultimately derived from the decay of radioactive 40-K, it is not measurably radioactive itself and consists of just 3 stable isotopes.

The adjacent nuclear reaction is the portion of the decay of Uranium 238 into Lead 206 that produces Radon 222 gas.
Rn 222 is the most abundant of the radon isotopes and the only isotope that the name “radon” formally applies to, although in practice they are all usually lumped together under this name.

Now for a bit of educated speculation on my part. To the best of my knowledge, the following theory on argon’s ability to accumulate in cave air is untested. Be that as it may…

Diffusion Coefficient Graph

Diffusion Coefficients

Argon (Ar) is the most abundant of the heavier-than-air gasses found in cave atmospheres, and like CO2 and radon (Rn), it should have the potential to accumulate in areas with restricted air flow.

Argon’s density is among the highest and it’s rate of diffusion the lowest of all the principal gases in cave atmospheres, so it seems logical that it may have the tendency to accumulate.

Argon has a very low rate of diffusion, which is the primary factor for predicting gas accumulation. Gaseous rates of diffusion are compared by their respective diffusion coefficients. Assuming an average cave temperature of 10C (50F), argon’s diffusion coefficient is 0.173 cm2/s, which is only 18% higher than that of CO2 (0.147 cm2/s). Overall, it is the third lowest after radon (0.104 cm2/s) amongst common cave-air gases.
Argon is also one of the densest of the atmospheric gases. It has a density of 1.78 kg/m3 , which is just 10% less than that of carbon dioxide (1.98 kg/m3). Carbon dioxide readily accumulates and often displays pronounced seasonal variations in cave air concentration and distribution.

Under the right conditions, therefore, argon (Ar) should accumulate in much the same way as does it’s far less abundant noble gas cousin, radon (Rn) and/or carbon dioxide (CO2).

Under which conditions that might be, however, has not been widely studied or recognized.; it should be, in my opinion.

Why? Because argon is the most abundant of the non-reactive noble gases present in cave atmospheres. Many of the noble gases (argon included) have been used as geochemical tracers of both aqueous and gaseous solutions in non-spelean environments, and these types of solutions are key participants in the formation and development of karst and caves.

Argon's Spectral Lines

A few speculations as to the conditions under which argon (Ar) enrichment might occur would perhaps be germane at this juncture.

The potential for accumulation is likely to be highest in low spots with restricted ventilation, as this is observed with the other heavy gasses, and there is no reason to suggest that this is not the case with Argon, as well.

High gaseous concentrations of 40-Ar might be reasonable to expected in caves when there is potassium (K) rich bedrock either stratigraphically below the cave or nearby.

Gaseous enrichment is also possible where cave networks intersect an argon saturated water table or where hypogene groundwaters such as thermal or other solute/gas enriched solutions are known or suspected to be involved in the speleogenetic process.

Hourglass Lake in Jewel Cave - Madison Aquifer Water Table

The Use of Argon Isotopes in Groundwater Dating & Source Area Characterization

Argon’s chemical and isotopic properties make it an ideal tracer of Earth processes. There is a considerable amount of information that argon isotopic compositions and concentrations can provide if the time and effort are put forth to acquire and assess them.

Noble gases like argon (Ar), helium (He), neon, (Ne), krypton (Kr) and xenon (Xe) are chemically inert and many of the complications that are common in interpreting isotopic data in other methodologies are absent when working with such tracers.

The Noble Gases

The patterns of noble gas isotopic variation are directly related to the source area and formations from which the gas originated and the path that it has taken prior to it’s final destination. This is true of both the anhydrous gases contained in the pore spaces of formations and those that are dissolved in groundwater. Although virtually all of the geochemical research done on argon isotopic variation has been done on groundwater, it should also be applicable to the argon that finds its way into cave air from formation gas sources.

Atmospheric Noble Gas Abundances

The patterns of Helium (He) and Argon (Ar) isotopic variation have been most widely studied, but Neon (Ne) Xenon (Xe) and Krypton (Kr) have more recently been looked at, as well. Argon’s abundance, however, makes it far easier to evaluated than most of the other noble gases.

Argon gas has about the same solubility in water as oxygen and thus can be readily dissolved in aqueous solutions. When surface precipitation enters the groundwater regime, it is generally saturated with argon and takes on its atmospheric isotopic signature.

The natural variation in Argon’s three stable isotopes has been used to determine the paleo-recharge temperature of groundwater. The dependence of argon’s solubility on temperature makes it possible to determine the recharge temperature of recent and paleo-groundwater by simply evaluating the total dissolved argon gas content and comparing it against it’s solubility limits as a function of temperature.

The assumption in this method is that the water does not pick up any additional argon along the flow path. Often enough, however, there are potassium mineral bearing lithologies within the aquifer. When there is dilution from additional argon, it is most likely to be due to radiogenic 40-Ar produced from reactions with these potassium bearing minerals within the rock matrix along the flow path (see below). If the flow path for groundwater is dominantly through carbonates, however, argon dilution is less likely to be as prevalent, and in karst aquifers, flow paths restricted to carbonates is generally the case.
Picture

The ratios of radiogenic 37-Ar, 39-Ar, and 40-Ar concentrations in groundwater can also provide information about the relative ages of the different groundwater sources involved in cave formation. These ratios should also be reflected in cave atmospheric values when exchange with outside air is at a minimum and lithologic and groundwater contributions to cave air are the greatest (during the cool season)

The half life of 37-Ar is only 35 days and that for 39-Ar is 269 years, so the later is used in most age estimations.

Age dating using the 39-Ar abundance is possible out to about 2000 years or so (~7 half lives).

Both 37-Ar and 39-Ar are primarily produced in the atmosphere from cosmic rays by the reactions shown in the adjacent diagram….but….

They can also be produced, under the right conditions, from in-situ reactions within the rock matrix through which the groundwater flows.

The presence of uranium or thorium in the rock matrix is the primary cause of this type of interference.

In general, only minimal interference from the in-situ reactions listed above occurs for the 39-Ar reaction, whereas the 37-Ar reaction is often masked by interference and this is most particularly the case in Ca rich matrices (like limestone!). This interference, however, can yield some fairly useful information (see next section).

The n(40-Ar)/n(36-Ar) Ratio in Groundwater Dating

The n(40Ar)/n(36Ar) ratio has also been used to aid in groundwater dating. This ratio has a constant value in the atmosphere as a whole of 295.5, but in groundwater, higher ratios are often the case.

Most aquifers contain some potassium-bearing minerals, and the 40-K within these rocks decays and over time may cause this ratio to increase. The isotopic ratio can be used to age date very old groundwater, as long as the relative production rate of 40-Ar and releas within the aquifer can be estimated. This rate can be deduced from measuring the levels of 37-Ar within the groundwater (see next section)

The groundwater n(40Ar)/n(36Ar) ratio of a given source can be substantially compromised by the addition of radiogenic 40-Ar from nearby potassium rich lithologies.

This source of contamination comes from rocks that are not within the aquifer itself but which are producing argon that has migrated into the system from nearby areas.

The usage of 37-Ar levels as an aid in interpreting Argon Isotope patterns

Because of the short half life of 37-Ar, groundwater has virtually no cosmogenically produced 37-Ar present. However, subsurface production of this isotope is common from rock matrix reactions involving the trace uranium and thorium found in many minerals.

By measuring the amount of 37-Ar present, the production rate within an aquifer or formation can be assessed, the neutron flux level can be thereby deduced, and the efficiency of mineral to water transfer of Argon estimated. All of these values are essential for defining how the other radon isotopes may be used and interpreted in hydrologic and spelean applications.

The usage of 39-Ar in Groundwater Dating

39-Ar has been mainly used in dating groundwater in conjunction with the usage of other isotopic methods. With a half life of 269 years, age determinations using 39-Ar overlap with the high end of the tritium range (3-H), and the low end of the carbon-14 (14-C) range. The 39-Ar method is most useful for dating young (~40 to ~2000 years B.P.) groundwater, and this is generally the age range most applicable to speleogenetic processes, as well.

Synopsis

Even though argon is one of the four most abundant components of cave air, it’s variability in abundance and distribution within spelean environments is essentially an unknown. To the best of my ability to ascertain, it has not been studied in cave environments to date. Argon is a heavier than air gas that should have the the potential to locally accumulate, very much like CO2 and it’s radioactive noble gas cousin radon (Rn). This gas should therefore be subject to variability in concentration and distribution due to oscillations in cave air dynamics.

Like radon, argon bears a radiogenic isotopic signature and has be used successfully as a tracer of geochemical processes within lithologic and hydrologic systems. By inference , argon should also be useful as a tracer of these processes as they are uniquely expressed in cave environments.

A systematic look at argon abundances and isotopic patterns within cave air and waters would seem overdue, but as a geochemist who is also a caver, that’s just might be my personal bias shining through. So be it.

“the eternal recurrence of the same”
Also sprach Zarathustra – Friedrich Nietzsche

Click on one of the above links for more information on the nature and distribution of these heavy gas components of cave atmospheres.

Current Research Relating to Black Hills Cave Barometry and Atmospheric Geochemistry

The question of the basic air flow mechanisms in barometric caves and determining the full size of both the Wind and Jewel Cave systems are the main aspects of an ongoing research project from the Cave- and Subway-Climatology working group at the Ruhr-University of Bochum in Germany. This project is being spear-headed by Andreas Pflitsch of the Ruhr-University and his graduate students along with National Park Service personnel at both caves. Numerous Pahasapa grotto members have also provided help and support for the project, and barometric monitoring has recently been expanded to other caves within the southern hills region.
https://ojs.zrc-sazu.si/carsologica/article/view/75

Another ongoing research project involves an assessment of the seasonal concentrations and distribution of radon, carbon dioxide ,water vapor and temperature within Black Hills Caves. This study is currently in its infancy and only a few caves have been evaluated so far, but the goal is to expand the testing and monitoring to a representative population of Hills caves so as to establish baseline atmospheric concentrations for these gases and atmospheric parameters.

One aspect of this study is the determination of the seasonal patterns of CO2 concentration in both barometric and non-barometric caves in the area and to see whether CO2 concentrations may be effecting and/or limiting the speleogenetic process therein. Black hills caves are generally wet but sparsely decorated on the average, which is unusual, considering their generally accepted age. One possibility is that this paucity may be due to high ambient CO2 concentrations above 4000 ppm, which has been suggested as an upper limit beyond which speleothem precipitation may not be able to occur. Defining this atmospheric parameter may help to answer this question.

Another planned aspect of this study is to determine the seasonal and average concentrations of radon within our caves. This is needed both to establish an exposure baseline for occasional entrants such as cavers, and to see whether bats intrinsically avoid high radon areas within our area, as has been observed/suggested in some Mexican caves. https://www.researchgate.net/publication/257611454_Indoor_radon_concentration_levels_in_Mexican_caves_using_nuclear_track_methodology_and_the_relationship_with_living_habits_of_the_bats

Jewel Cave Barometric Data - Andreas Pflitsch et al.

Cave Barometry & Airflow

Cave Atmospheric Chemistry

Seasonal Cave Air Variation

Carbon Dioxide

Carbon Dioxide

Argon's Spectral Lines

The Noble Gases

Atmospheric Noble Gas Abundances

Current Research Relating to Black Hills Cave Barometry and Atmospheric Geochemistry