Speleothems, commonly known as cave formations, are secondary mineral deposits formed in a cave. Speleothems typically form in limestone or dolomite solutional caves. The term "speleothem," as first introduced by Moore, is derived from the Greek words spēlaion "cave" + théma "deposit". The definition of "speleothem," in most publications, specifically excludes secondary mineral deposits in mines, tunnels, and other man-made structures. Hill and Forti more concisely defined "secondary minerals" which create speleothems in caves:
A "secondary" mineral is one which is derived by a physicochemical reaction from a primary mineral in bedrock or detritus, and/or deposited because of a unique set of conditions in a cave; i.e., the cave environment has influenced the mineral's deposition.
Origin and composition
More than 300 variations of cave mineral deposits have been identified. The vast majority of speleothems are calcareous, composed of calcium carbonate in the form of calcite or aragonite, or calcium sulfate in the form of gypsum. Calcareous speleothems form via carbonate dissolution reactions. Rainwater in the soil zone reacts with soil CO2 to create weakly acidic water via the reaction: As the lower pH water travels through the calcium carbonate bedrock from the surface to the cave ceiling, it dissolves the bedrock via the reaction: When the solution reaches a cave, degassing due to lower cave pCO2 drives precipitation of CaCO3: Over time the accumulation of these precipitates form stalagmites, stalactites, and flowstones, which compose the major categories of speleothems. Calthemites which occur on concrete structures, are created by completely different chemistry to speleothems.
Types and categories
Speleothems take various forms, depending on whether the water drips, seeps, condenses, flows, or ponds. Many speleothems are named for their resemblance to man-made or natural objects. Types of speleothems include:
Dripstone is calcium carbonate in the form of stalactites or stalagmites
* Stalactites are pointed pendants hanging from the cave ceiling, from which they grow
** Soda straws are very thin but long stalactites having an elongated cylindrical shape rather than the usual more conical shape of stalactites
** Helictites are stalactites that have a central canal with twig-like or spiral projections that appear to defy gravity
*** Include forms known as ribbon helictites, saws, rods, butterflies, hands, curly-fries, and "clumps of worms"
** Chandeliers are complex clusters of ceiling decorations
** Ribbon stalactites, or simply "ribbons", are shaped accordingly
* Stalagmites are the "ground-up" counterparts of stalactites, often blunt mounds
** Broomstick stalagmites are very tall and spindly
** Totem pole stalagmites are also tall and shaped like their namesakes
** Fried egg stalagmites are small, typically wider than they are tall
* Columns result when stalactites and stalagmites meet or when stalactites reach the floor of the cave
Flowstone is sheet like and found on cave floors and walls
* Draperies or curtains are thin, wavy sheets of calcite hanging downward
** Bacon is a drapery with variously colored bands within the sheet
* Rimstone dams, or gours, occur at stream ripples and form barriers that may contain water
* Stone waterfall formations simulate frozen cascades
Cave crystals
* Dogtooth spar are large calcite crystals often found near seasonal pools
* Frostwork is needle-like growths of calcite or aragonite
* Anthodites are flower-like clusters of aragonite crystals
* Cryogenic calcite crystals are loose grains of calcite found on the floors of caves, and are formed by segregation of solutes during the freezing of water.
Speleogens are formations within caves that are created by the removal of bedrock, rather than as secondary deposits. These include:
* Cave popcorn, also known as "coralloids" or "cave coral", are small, knobby clusters of calcite
* Cave pearls are the result of water dripping from high above, causing small "seed" crystals to turn over so often that they form into near-perfect spheres of calcium carbonate
* Snottites are colonies of predominantly sulfur oxidizing bacteria and have the consistency of "snot", or mucus
* Calcite rafts are thin accumulations of calcite that appear on the surface of cave pools
* Hells Bells, a particular speleothem found in the El Zapote cenote of Yucatan in the form of submerged, bell-like shapes
Speleothems made of sulfates, carbonates, mirabilite or opal occur in some lava tubes. Although sometimes similar in appearance to speleothems in caves formed by dissolution, lava stalactites are formed by the cooling of residual lava within the lava tube. Speleothems formed from salt, sulfur and other minerals are also known. Speleothems made of pure calcium carbonate are a translucent white color, but often speleothems are colored by chemicals such as iron oxide, copper or manganese oxide, or may be brown because of mud and silt particulate inclusions.
Chemistry
Many factors impact the shape and color of speleothem formations including the rate and direction of water seepage, the amount of acid in the water, the temperature and humidity content of a cave, air currents, the above ground climate, the amount of annual rainfall and the density of the plant cover. Most cave chemistry revolves around calcium carbonate, the primary mineral in limestone and dolomite. It is a slightly soluble mineral whose solubility increases with the introduction of carbon dioxide. It is paradoxical in that its solubility decreases as the temperature increases, unlike the vast majority of dissolved solids. This decrease is due to interactions with the carbon dioxide, whose solubility is diminished by elevated temperatures; as the carbon dioxide is released, the calcium carbonate is precipitated. Most other solution caves that are not composed of limestone or dolomite are composed of gypsum, the solubility of which is positively correlated with temperature.
Speleothems are studied as climate proxies because their location within cave environments and patterns of growth allow them to be used as archives for several climate variables. The principal proxies measured are oxygen and carbon isotopes and trace cations. These indicators, alone and in conjunction with other climate proxy records, can provide clues to past precipitation, temperature, and vegetation changes over the last ~ 500,000 years. A particular strength of speleothems in this regard is their unique ability to be accurately dated over much of the late Quaternary period using the uranium-thorium dating technique. Stable isotopes of oxygen and carbon are recorded well in speleothems, giving high-resolution data that can show annual variation in temperature and precipitation. By sampling along a dated transect of a speleothem, these isotope values and speleothem growth rates provide a paleoclimate records similar to those from ice cores. Variations in precipitation alter the width of new ring formation, where close ring formation shows little rainfall, and wider spacing for heavier rainfall. The geometrical way in which stalagmites grow is also used in paleoclimate applications, which varies based on the height the water is falling from, and the rate of flow. Weaker flows and short travel distances form more narrow stalagmites, while heavier flow and a greater fall distance tends to form more broad ones. Additionally, drip rate counting and trace element analysis on the water drops themselves have been shown to record shorter-term variations in the climate at high resolution, such as drought conditions attributed to the El Niño-Southern Oscillation climate events. A recent technique is the use of CT-scanning on intact specimens to analyze density, where more dense speleothem development indicates higher moisture availability.
Absolute dating
Another dating method using electron spin resonance – also known as electron paramagnetic resonance – is based on the measurement of electron-hole centers accumulated with time in the crystal lattice of CaCO3 exposed to natural radiations. In principle, in the more favorable cases, and assuming some simplifying hypotheses, the age of a speleothem could be derived from the total radiation dose cumulated by the sample and the annual dose rate to which it was exposed. Unfortunately, not all the samples are suited for ESR dating: indeed, the presence of cationic impurities such as Mn2+, Fe2+, or Fe3+, humic acids, can mask the signal of interest, or interfere with it. Moreover, the radiation centers must be stable on geologic time, i.e., to have a very large lifetime, to make dating possible. Many other artifacts, such as, e.g., surface defects induced by the grinding of the sample can also preclude a correct dating. Only a few percents of the samples tested are in fact suitable for dating. This makes the technique often disappointing for the experimentalists. One of the main challenge of the technique is the correct identification of the radiation-induced centers and their great variety related to the nature and the variable concentration of the impurities present in the crystal lattice of the sample. ESR dating can be tricky and must be applied with discernment. It can never be used alone: "One date only is No date", or in other words, "multiple lines of evidence and multiple lines of reasoning are necessary in absolute dating". However, "good samples" might be found if all the selection criteria are met.
Calthemites: secondary deposits not formed in caves
Secondary deposits derived from concrete, lime, mortar or calcareous material as found on man-made structures outside the cave environment or in artificial caves, can mimic the shapes and forms of speleothems, but are classed as calthemites. The occurrence of calthemites is often associated with concrete degradation, but could also be linked to leaching of lime, mortar or other calcareous material. Despite similar appearances, "calthemites" are not considered to be "speleothems", and vice versa, as per their definitions.
