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INTRO Most speleothems form by similar processes, surface water passes downward through soil above the limestone, absorbs carbon dioxide (CO2), and becomes acidic. As a weak acid, the water is able to dissolve a small amount of the limestone rock as it passes through cracks and pores whilst travelling through the cave. As this water drips into the air-filled cave, dissolved carbon dioxide is given off. Because the water has lost carbon dioxide, it cannot hold as much dissolved calcium. The excess calcium is then precipitated on the cave walls and ceilings to make up many of the different kinds of formations. Most calcium is precipitated in the cave as the mineral calcite (CaCO3).
Many factors impact the shape and colour of speleothem formations including the rate and direction of water seepage, the acidity of 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 calcite, the primary mineral in limestone. It is a slightly soluble mineral whose solubility increases with the introduction of carbon dioxide. Most other solution caves that are not composed of limestone or dolostone are composed of gypsum (CaSO4).
Stalactites - Stalactites, in general are not used for past environmental studies because the growth layers being spread over the entire specimen are thin. Stalactites form when the central canal of straw stalactites becomes blocked, so that drips overflow and run down the outside, forming a typical carrot shape. Crystals orient themselves radially around the central section as the deposition of successive sheaths occurs, thus increasing the length and girth of the stalactite at the same time. Progressively fewer rings are found from the top to the bottom which accounts for the conical shape. Cross-section shape varies due to uneven deposition on the sides of the speleothem.
Stalagmites - In general, slow drips favour stalactite formation and fast drips favour stalagmite formation, stalagmites are much broader than stalactites because the drips splash out over them. Stalagmites often thicken into more complex shapes. The uneven deposition commences when a small hollow, called a 'splash cup', develops on the top of a stalagmite. When the splash cup is large enough to contain a proportion of the drops, some of the solution flows over the sides of the depression rather than splashing. Carbon dioxide is easily lost from the thin film of water and so calcite precipitates, causing widening of the top of the stalagmite. The solution dribbles down the sides, thickening it into a ribbed, inverted cone shape. The whole process may start again and, if repeated several times, the overall effect is of a tiered cake. Stalagmites are considered to be excellent speleothems for past environmental studies as their chronological layers of calcite crystals are often substantial. Many of the Australian studies using speleothems have been carried out on uniform growth stalagmites, because their evolution is simpler than the tiered-cake specimens. With time, the stalactites and their accompanying stalagmites will meet, forming columns. Once a column is formed, its circumference is increased by water flowing down it and depositing more calcite. The original slender column thickens, becoming a massive pillar.
Stromatolitic-stalagmite - In illuminated cave entrances the growth of CaCO3 structures is not controlled just by abiotic factors. With the presence of light, cyanobacteria, algae and higher photosynthetic plant forms can inhabit the surfaces of any speleothems. The photosynthetic organisms remove carbon dioxide from the percolating waters and precipitate calcium carbonate. Where complex photosynthetic bacterial colonies are active and calcite-saturated waters are present, the speleothems are said to be phototropic because they grow faster on the sides toward the light and so lean toward it. Characteristic specimens have a smooth upper surface with large regular transverse corrugations. When wet they have a blue-green colour or are white; when dry they have a grey-black appearance. Cyanobacteria are the cause of the deep green or blue-green colour on the surface of the damp stalagmites. Stromatolitic-stalagmites are capable of preserving long accurate records as they are robust like their dark-zone counterparts. The cyanobacteria have a gelatinous sheath that is sticky and acts as the perfect trap for pollen. Pollen gives information about the surface environment around the cave entrance. Since specific plants are known to occur only within narrow climate ranges, the presence of their pollen implies specific climates for that region when the calcite layer was being deposited. Stromatolitic-stalagmites can also contain amino acids, lipids and n-alkanes, all suitable fingerprints for microbes. Amino acids expand the range of past environmental studies back a million years when calibrated against the radiocarbon ages.
drapery or curtain - Also known as shawls, these speleothems are formed by saturated calcite solutions draining down inclined surfaces and building up hanging sheets of calcite from their leading edge. Their growth rates are unknown as only few have been studied. Shawls can have vertical extents of metres, indicating long periods of growth. Their advantage over stalagmites and flow-stones for past environment studies is that the potential record is clearly delineated and accessible, not buried within concealing outer layers of calcite.
subaqueous calcite - Moonmilk in wet caves is a soft, white, watery colloid that resembles milk. When dry, it is white and fine-grained like milk powder. In Australia the semi-liquid form is rare, but the dried material is common. The minerology of moonmilk is complex and it has been shown to contain a variety of calcium and magnesium carbonate minerals in their hydrous and anhydrous forms. Optical and electron microscope studies reveal microcrystals, often as fine needles, among which are bacterial spheres and filaments. The random orientation and size of the microcrystals implies that the mineralisation is biologically induced. Often moonmilk is found in shallow caves and is associated with tree roots, a place where the supply of water and nutrients would encourage bacterial growth. It is possible that even moonmilk deposits found deep inside caves and those that do not contain remains of bacteria may be a result of the bacterial decay of speleothems.
calcite raft - Calcite rafts which litter the floor of some dry areas of the cave are evidence that these passages were once flooded too. Calcite rafts are clusters of crystals that float on the surface of deep, still pools. Scanning electron microscopy shows that the growing surface of the raft is flat whereas its underside is pinnacled with spar crystals. A bio-film on the surface of the pool increases the probability of raft development as compared to pool crystal formation. Calcite rafts float and grow as long as they are supported by surface tension. They sink from their own weight when the pool surface is disturbed or drips from the roof fall onto them. Sunken rafts litter the pool floors and crystals can be added to both sides of the raft. If rafts continually fall in one place a cone is formed that resembles a stalagmite. As the water level rises and falls, some rafts cling to cave walls, indicating the past water levels. If they coalesce over the pool surface to form a crust, the resultant crystal floor can become so thick that it is strong enough to walk on.
Flowstone - Flowstone results when surfaces in the crystal gallery are totally covered with a thin film of water. From this shallow water the crystallising calcite is uniformly added to the crystal mass below. When there is a flow over a series of ledges, the calcite is deposited in massive sheets as 'frozen' rapids and waterfalls. Flowstones are used for past environmental studies as they too are constructed of layers of calcite crystals that have been deposited in chronological order. In some tourist caves, paths have been carved through flowstone, exposing layers of calcite with a variety of textures, colours and thicknesses. As a result of the way they were laid down, flowstones often contain substantial quantities of detrital materials that may be a mine of past environment indicators. On the other hand, they may also contain substances that interfere with uranium series dating, making the reliability of the absolute dates obtained from flowstone uncertain. Despite this, flowstone has an important role in archaeological and sediment studies because the material buried beneath a layer of flowstone must be older.
Climate proxies - Samples can be taken from speleothems to be used like ice cores as a proxy record of past climate changes. 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. Stalagmites are particularly useful for palaeoclimate applications because of their relatively simple geometry and because they contain several different climate records, such as oxygen and carbon isotopes and trace cations. These can provide clues to past precipitation, temperature, and vegetation changes over the last approximately 500,000 years.
Another dating method using electron spin resonance (ESR) ââ‚¬" also known as electron paramagnetic resonance (EPR) ââ‚¬" 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 (organic matter), 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.