Importance of Caves and Karst

Why are Caves and Karst Important?

Caves and karst make landscapes diverse, fascinating, and rich in resources, including the largest springs and most productive groundwater on Earth. A total of 175 different minerals occur in limestone caves, a few of which have only been found in caves (Moore and Sullivan 1997). Caves and karst provide a unique subsurface habitat for rare animals. Caves preserve fragile archaeological and paleontological materials for millennia. Throughout history people have used caves for many purposes: from guano mining to tourism. The potential of caves as natural laboratories may be their most significant future use.

The many uses of caves and karst are a measure of their importance. Don't forget about their purely aesthetic value, however, and the sense of adventure and exploration they provide.

Select a topic to learn more about the importance of caves and karst:

 

Water Resources

Undoubtedly water is the most commonly used resource from karst. Although the lack of surface water is characteristic of karst, groundwater is bountiful in karstic regions. These areas contain some of the greatest-producing water wells in the world. In fact, most of the drinking water in the United States is stored naturally in cave-bearing rocks, such as limestone.

Historical narratives describe the vital role of groundwater in karst for human consumption; many of these accounts predate Biblical times in Europe and the Middle East. Communities were generally located along the margins of karst areas, downstream from large springs that provided water for domestic and agricultural uses. The development of well-drilling technology enabled more people to live in areas with karst. Prior to this people channeled water from springs toward towns and fields, pumped water by hand or with wind power, or simply collected and carried water in vessels. These methods are still used today in parts of the world where drilling wells is not practical or affordable.

 

Cave Minerals and Mineral Resources

Most mineral deposits in caves are made of calcium carbonate (CaCO3). The most common cave mineral is calcite; aragonite and gypsum are also common. Calcite is the constituent of most varieties of speleothems. Calcite lines cave walls in Jewel Cave, a national monument in South Dakota. Aragonite is the second most common cave mineral after calcite. It is a polymorph of calcite (which means both minerals have the same mineral composition, CaCO3, but have a different crystal structure). Aragonite is metastable relative to calcite, which means that its internal structure will change to that of calcite over time (Hill and Forti 1997). Speleothems made of aragonite are fairly uncommon. Aragonite nearly always takes on a needle-like form in caves (Hill and Forti 1997). When it does occur, aragonite crystals can be blue, green, white, or brown. Gypsum is the third most common cave mineral. It is usually colorless (as selenite) or white (as alabaster), but it can be tinted yellow, tan, brown, blue, pink, gray, reddish-brown, or black. Mammoth Cave is known for its gypsum, which was mined from the cave during prehistoric times (Hill and Forti 1997). We don't know for sure why these people wanted gypsum; it may have been mined for the medicinal value of the sulfates it contains, similar to Epsom salts. Cave rims (shells or projections that usually form around the lips of holes in cave floors) made of gypsum are found in Guadalupe Mountains, New Mexico.

The other 172 cave minerals are much less common than the primary three: calcite, aragonite, and gypsum. Nevertheless, an interesting anecdote is that more than 40 of the known cave minerals contain phosphorus. The phosphorus in these minerals is derived from bat droppings called guano. Bat guano was the most highly rated fertilizer in the 19th and early-20th centuries. More recently, guano has been replaced by cheaper and more easily obtained chemical fertilizers. Minor guano-mining operations continue today, however, and you can still buy this product in small quantities to fertilize your cherished flowers and potted plants.

The most common mineral resources extracted from karst areas are quarried rock. Limestone, dolomite, marble, gypsum, travertine, and salt are all mined in large quantities throughout the world.

 

Other Commercial Uses

Because of their constant temperatures, people have used caves to store food, such as potatoes and apples. Caves have also served as subsurface farms for mushrooms, rhubarb, and celery. People have also used caves to store cheese during the aging process. Some of the stronger varieties of cheese must be cured as long as two years to produce their characteristic sharp flavors and crumbly, dry textures. Furthermore, the blue penicillin mold in Roquefort cheese develops only in a cool, wet atmosphere such as a cave. Not surprisingly, then, penicillin was originally developed in the caverns below Roquefort, France (Gee 1994).

In the 1700s, French immigrants were the first to use American caves for storing and aging wine. Later, German immigrants found caves especially suited for aging lager beers, which require storage at low temperatures for several months. The caverns beneath Saint Louis, Missouri, were largely responsible for attracting German brew masters to the city in the mid-1800s (Gee 1994).

 

Tourism and Recreation

Tourism in cave and karst areas is big business. In the United State alone, more than 150 caves are open to the public, and several million visitors pass through each year (Moore and Sullivan 1997). Many of these caves are skillfully lighted and host well-maintained trails. For example, sections of Mammoth Cave in Kentucky, the world's longest cave, have been developed and are popular tourist destinations. Carlsbad Cavern in New Mexico, which like Mammoth Cave is a national park and commonly visited, contains some of the world's largest rooms and passages. Developed caves, also known as show caves, offer ready opportunities to become familiar with many of the features presented in this knowledge center.

"Wild" caves remain in their natural state; they are located throughout the country on public and private land. For most people a visit to a wild cave is a once-in-a-lifetime adventure, but for thousands of cavers worldwide it is a regular pastime. Recreational activities in scenic, karstic areas also include car touring, boating, hiking, fishing, camping, swimming, backpacking, nature watching, and photography.

 

Archaeology

Our present knowledge of the early development of human beings and their cultures is intimately associated with the exploration and study of caves. People have long used caves as dwelling places, burial sites, storehouses, and ceremonial places. Many skeletal remains of primitive relatives of Homo sapiens sapiens have been found in caves in Africa and Asia. Many artifacts are extremely well preserved in the constant environments of caves. Scientists date flowstone in caves using uranium-series techniques, which is handy for determining the ages of artifacts in relation to the flowstone.

Although the outer parts of caves and shallow sandstone caves were used for shelter, it is highly improbable that prehumans or humans ever dwelt for long periods deep in caves. The dark zone in most caves is too damp and cool for prolonged human habitation. In fact, during extensive archaeological excavations of numerous caves, archaeologists have failed to find major accumulations of occupational debris beyond the twilight zone (Moore and Sullivan 1997). Although it is unlikely that they lived there, this is not to say that prehumans or early humans did not travel deep into caves. Archaeologists have found charred remains of cane torches, carrying bags, and articles of clothing woven from plant fibers miles from the entrance of Mammoth Cave, for example. Scrapes and chips on walls coated with sulfate tell us that these early cavers were in search of crystals and mineral salts (Gee 1994). Also, cave drawings, many of which are deep in caves, demonstrate that humans lived at the same time as the species depicted and give insights into the artists eating and hunting habits.

 

Paleontology

Caves house paleontological resources in two ways. First, fossils are preserved within cave-forming rock. Caves may provide the best or only exposure of subsurface bedrock, and fossils preserved within the cave-forming rock may become exposed through cave-forming processes. Second, fossils accumulate within cave and karst features. Openings in the ground surface - such as caves, sinkholes, and tubes - attract and trap animals. Rich deposits of fossil bone can accumulate as remains associated with organisms inhabiting the cave or can be transported there. Predators frequently transport prey into caves and generate accumulation of bones. In addition packrats are responsible for the redistribution of fossil material while building and dismantling their nests in shelters, such as caves. These accumulations of bone and plant material, called middens, are important sources of paleontological data for researchers.

Cave environments are conducive to the preservation of paleontological resources. The relatively constant temperature and humidity of caves aid long-term preservation of organic material. Moreover, the reduction or absence of sunlight limits the adverse effects of solar radiation on fossils. Also caves provide protection from the forces of weathering and erosion that can destroy paleontological resources. In a few rare cases, footprints and claw marks have been preserved in cave dust and sediments exposed on the cave floor for thousands of years (Santucci et al. 2001). Moreover, ancient remains may be transported into inaccessible cavities that provide refuge from trampling, scavenging, scattering, and other disturbances.

Within caves in arid regions, where humidity is usually very low, preservation can occur through desiccation or mummification. This scenario allows the preservation of animal remains, such as soft tissue, hair, and dung, that would decay rapidly under other conditions. In arid regions, caves may contain mineral-rich waters that aid in preservation of fossil material.

Fossils found in association with archaeological resources can provide archaeologists with valuable information. Likewise, examination of prehistoric art is helpful to paleontologists who are studying Pleistocene vertebrates. It helps them to date the extinction of many animal species. Furthermore, the realistic pictures made by cave artists give information about the form and color of extinct animals that could not be obtained by merely examining skeletons (Moore and Sullivan 1997).

 

Habitat for Cave Animals

An obvious characteristic of caves is darkness. Beyond the twilight zone, which extends only a short distance in from the entrance, caves are completely absent of light. Cave animals called troglobites have adapted to total darkness and permanently live in the dark zone of caves. Their adaptations include extra long feelers for finding their way around in the dark.

The environment at the surface in which we live is very changeable. By contrast cave environments are quite stable; the climate within caves is almost completely uniform. The atmosphere in a deep cave rarely changes, except for slight variations in barometric pressure and carbon dioxide content (Moore and Sullivan 1997). The temperature remains virtually constant throughout the year, a feature especially beneficial to animals that cannot regulate their body temperatures.

In addition, many caves have high relative humidity that is nearly constant. In humid caves, water continually drips from ceilings and walls. Cave walls are sometimes so moist that aquatic organisms can readily crawl over them. Small crustaceans and flatworms, which normally live only in pools or streams outside of caves, are found on the ceilings of humid caves!

Humid caves are typical, but there are also very dry caves. Life in arid caves is rare. Nevertheless, such caves are of great interest to paleontologists and archaeologists because their environments are conducive to the preservation of artifacts and fossils - evidence of earlier habitation by humans or extinct species of animals.

 

Environmental History

Caves contain dissolution features, sediments, and speleothems, all of which preserve a record of the geologic and climatic history of an area. Because subsurface voids and deposits are protected from surface weathering and disturbance, karst faithfully preserves a record of environmental change. Temperature, precipitation, the nature of soil and vegetation cover, glaciation, fluvial erosion and deposition, and patterns of groundwater flow can usually be read from cave patterns and deposits (Berger 1995).

In the upper levels of caves where most speleothems form, cave temperature represents the average surface temperature over the preceding few years. Therefore, as they form, speleothems record long-term surface climate change and can be determined using isotopic dating methods. For example, as climate cools, the abundance of the isotope oxygen-18 (18O) in the calcite at inland cave sites is reduced (the calcite is said to become lighter). This indicates that the dominant control on 18O/16O ratios is the lowering of air temperature. At coastal sites, the opposite trend may apply because of reduced evaporation of 18O at the sea surface (Ford 1997). Samples of dripstones can be dated by measuring radioactive decay, and their temperatures at the time of formation can be estimated using isotopic analysis. Annual climate from thousands of years ago can be determined in the case of certain fast-growing speleothems (Berger 1995).

Using radiometric methods (e.g., thorium 230) scientists have now dated thousands of speleothems in various climatic zones. They have performed closely spaced, sequential sampling along the vertical axes of stalagmites, which shows periods of growth alternating with periods of no growth, the latter usually representing droughts. As the body of data has grown, studying paleoclimates in this manner has become more and more feasible. Scientists have been able to separate local effects from regional ones and map changes in precipitation patterns during the last several glacial stages of the Pleistocene Epoch (Moore and Sullivan 1997).

Studies of speleothems also give us information about past temperature. Analysis and interpretation of data from speleothems indicate that the average surface temperature in midlatitude caves reached a peak of 3°C above present about 8,000 years ago. From 15,000 to 80,000 years ago, it was as much as 10°C colder than now. It was warmer than present from 80,000 to 120, 000 years ago, colder from 120,000 to 170,000 years ago, and colder for an undetermined period before that (Moore and Sullivan 1997). Further studies should give us a detailed picture of climate change during the past 300,000 years (Moore and Sullivan 1997).

Fossils recovered from stratified deposits in caves also yield information about past climate. For example, the changes from assemblages of boreal and temperate species in stratified cave deposits tell us about the succession of glacial advances and retreats (Santucci et al. 2001).

Oxygen-18/Oxygen-16 Ratios

One of the most useful of all climate indicators is the oxygen-18/oxygen-16 ratio measured in organic sediments and in ice (Schmidt 1986). It is also significant in climate reconstructions from calcite precipitates (Ford 1997). Nearly all oxygen in our atmosphere has an atomic weight of 16 (that is, there are eight protons and eight neutrons in its nucleus). A small percentage (0.2%), however, has two extra neutrons, giving it an atomic weight of 18. Both kinds of oxygen behave the same chemically, combining with hydrogen to form water, for example. The only difference is that oxygen-18 is 12.5% heavier that oxygen-16. This mass difference influences the physical behavior of water. For instance, water molecules containing the heavier oxygen-18 do not evaporate as readily as those containing lighter oxygen-16, and this effect is more pronounced at lower temperatures. This fact may be used in reconstructing past climates in several ways.

One technique involves measuring how the oxygen-18/oxygen-16 ratio varies in ice cores drilled from the Antarctic or Greenland ice sheets. Like sediments, these ice sheets are layered and stratified, with the youngest layers nearer the top of the ice sheet. In many cases annual layers can be counted like tree rings. Because the ice is formed from snow which in turn is derived from evaporation from the oceans, a higher oxygen-18/oxygen-16 ratio in the ice indicates warmer conditions, allowing a greater fraction of oxygen-18 to be evaporated. Thus, ice cores may provide important climatic records extending back 120,000 years or more (Schmidt 1986).

At the same time, growth of continental glaciers and ice sheets preferentially removes oxygen-16 from the oceans, leaving behind water that is slightly enriched in oxygen-18. This in turn finds its way into the skeletal and shell structures of sea creatures that contribute their hard parts to oceanic sediments. Calcium carbonate (CaCO3) is the principal constituent of these fossil remains, and the oxygen in these molecules is drawn from the seawater in which they lived, recording the oxygen-18/oxygen-16 ratio that prevailed during their lifetimes. In effect, the creatures of the sea unwittingly recorded climatic conditions around them for our later analysis.

The use of oxygen-18/oxygen-16 ratios in calcium carbonate (limey) sediments is complicated by another effect: when creatures incorporate oxygen from seawater into their skeletal parts, the resulting oxygen-18/oxygen-16 ratio depends not only on that of the water but also on the water temperature, with a greater proportion of oxygen-18 being deposited at lower temperatures. Thus, the oxygen-18/oxygen-16 ratio in limey fossils reflects two different climatic variables: the temperature of the ocean water and the amount of freshwater ice tied up in continental glaciers. Separating these two effects can be difficult, especially since they are related: colder temperatures tend to result in more polar ice. Nevertheless, because of the great span of geologic ages represented in limestones from which oxygen-18/oxygen-16 ratios may be derived, this method has proved to be a key element in deciphering the climate puzzle for the last 500 million years (Schmidt 1986).

In addition, the calcite of speleothems also records past changes in temperature. In calcite the oxygen-18/oxygen-16 ratio varies with the temperature of formation, because oxygen-18 concentrates in calcite relative to water, and the concentration becomes less with increasing cave temperature (Moore and Sullivan 1997).

Oxygen-18 values in cave calcite, in addition to being related to the temperature of calcite formation, also are related to the oxygen-18 content of the original depositing water, which varies with other aspects of the climate besides temperature. To obtain the most accurate temperature values, speleologists analyze small quantities of the water that are almost always trapped and preserved in microscopic cavities in stalagmites. This trapped water has lost its original oxygen isotope record by exchange with the oxygen of the enclosing calcite, but hydrogen isotope analysis can be used from a well-known relationship between hydrogen and oxygen isotope ratios to estimate its original oxygen-18 content. When compared with the oxygen-18 content of samples enclosing calcite, these values give a precise measure of the ancient temperature of the cave and of the surface above it (Moore and Sullivan 1997).

 

Sinks and Sources of CO2

The formation of karst is part of the global carbon cycle in which carbon is exchanged between the atmosphere, surface and groundwater, and carbonate minerals. Reconciling all the sources and sinks in the carbon cycle is necessary to explain the process of global warming, for example. Scientists must be able to account for all the carbon in the global system, including carbon in karstic systems. Furthermore, they must be able to separate natural movements, or fluxes, of carbon from fluxes caused by human activities.

Karstic activity ties up carbon derived from rock and from dissolved carbon dioxide (CO2) as aqueous bicarbonate ions (HCO3-). Deposition of dissolved carbonate minerals is accompanied - and usually triggered - by release of some of the carbon as CO2. In many locations with karst, CO2 emission is associated with the deposition of calcareous sinter (e.g., tufa and travertine) at the outlet of cold or warm springs (Berger 1995).

The Global Carbon Cycle

Carbon, an element that is essential to all forms of life, occurs in four reservoirs: (1) as carbon dioxide (CO2) in the atmosphere, (2) in organic compounds in the biosphere, (3) as dissolved carbon dioxide in the hydrosphere, and (4) in the calcium carbonate of rocks (e.g., limestone, dolomite, and aragonite) and in decaying and buried organic matter (e.g., peat, coal, and petroleum) in the lithosphere. Each reservoir is involved in the carbon cycle.

The key to the carbon cycle is the biosphere, where plants continuously extract CO2 from the atmosphere and then break the CO2 down by photosynthesis to form organic compounds. Animals consume plants and use these organic compounds in their metabolism. When plants and animals die, the organic compounds decay by combining with oxygen from the atmosphere to form CO2 again. The passage of material through the biosphere is so rapid that the entire content of CO2 in the atmosphere cycles every 4.5 years (Skinner and Porter 1995).

Not all dead plant and animal matter in the biosphere decays immediately back to CO2. A small fraction is transported and redeposited as sediment; some is then buried and incorporated in sedimentary rock where it locally forms deposits of coal and petroleum. The buried organic matter joins the atmosphere naturally only after uplift and erosion have exposed the rock in which it is trapped.

Carbon dioxide from the atmosphere also is distributed in the waters of the hydrosphere. There it is used by aquatic plants in the same way that land plants use CO2 from the atmosphere. In addition, aquatic animals extract calcium carbonate (CaCO3). When the animals die, the shells accumulate on the seafloor, mixing with any CaCO3 that may have been precipitated as chemical sediment. When compacted and cemented, the CaCO3 forms limestone. In caves CaCO3 forms calcite, the most common cave mineral, which is the primary constituent of most speleothems. In this way too some carbon joins the rock cycle. Eventually, the rock cycle will bring the limestone back to the surface where weathering and erosion will break it down; the calcium returns in solution to the ocean, and the carbon escapes as CO2 to the atmosphere.

 

Natural Laboratories and Research

The study of caves is an important means for understanding our world. It can broaden our grasp of the interaction of certain biologic and geologic processes that have been shaping our planet and its inhabitants for hundreds of millions of years. Vast subsurface areas provide unique, productive field sites for study because they allow direct observation and mapping of underground features. Furthermore, the origin, morphology, and distribution patterns of caves and karst are the dominant factors in controlling the nature of overlying land surface (e.g., distribution of sinkholes) and the directions of groundwater movement (Berger 1995). Karst systems provide answers to how water supplies can become contaminated or how disease can spread via underground connections between watercourses.

Concepts developed in the science of speleology have provided useful guidelines for environmental protection. For example, monitoring changes in an uncomplicated microenvironment such as a cave may have great utility in sounding a warning against potentially harmful broader changes at the surface that might be masked by other changes people have caused.

For geologists, caves hold the keys to the solution of many puzzles: the formation of certain mineral deposits, the nature of soil cover, and the rate of water movement through rocks. Because the great variety of subsurface voids and deposits are protected from surface weathering and disturbance, karst preserves a record of environmental change more faithfully than most other geologic settings (Berger 1995). Therefore, cave mineral deposits (e.g., dripstones) and their isotopic properties have established a new line of evidence about past climates in continental areas.

For biologists, caves pose many interesting questions: how do cave animals adapt to total darkness? How do cave animals subsist on such a limited food supply? How does the food web in caves work without sunlight? How do organisms living underground differ from their surface relatives? What does the study of these related species teach us about evolution?

Caves also provide intriguing opportunities for medical research. Some early uses of cave materials as medicines have proven to be scientifically valid. In the 16th and 17th centuries, for example, European physicians used dried moonmilk from caves for dressing wounds. The moonmilk acted as a dehydrating agent and stopped bleeding; it was also thought to have curative qualities. Now we know that moonmilk contains actinomycetes, mold-like bacteria that have antibiotic properties. Scientists have also discovered that sulfur bacteria found in caves produce vitamins of the B group, an important area of research that could supply useful nutritional and metabolic products. Only continued research and time will tell what miracle drugs will be developed from cave resources, but great potential seems to exist.

 

Links

Caves and Karst Index

Importance

Underground

Threats to Caves and Karst

Exploration

Caves and Karst in National Parks

Challenge Your Understanding

 

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