SEDIMENTARY PROCESSES/Karst and Palaeokarst 679 with extensive gypsum karsts known from Russia and the Ukraine, but their greater solubility renders such landforms more dynamic and, for rock salt, ephemeral in all but the most arid climates Karst features may also develop, though rarely, on very weakly soluble rocks, such as basalt, granite, or quartzite Rock solubility and water are the primary factors in karst development Arid climates, whether hot or cold, support little karst Physical rock properties also are important Highly porous rocks seldom support well-defined karst features, which instead are favoured by low porosity and good secondary permeability, in the form of fractures, focusing the drainage into specific conduits through the karst rock The removal of rock in solution allows the development of drainage through the rock, rather than just across its surface as happens largely with rocks removed by mechanical erosion Consequently, karst landscapes generally lack well-developed surface drainage but have underground drainage conduits, or caves Hence a significant component of karst terrains typically lies beneath the surface, sometimes extending to depths of hundreds, or even thousands, of metres Intimately associated with the dissolutional aspects of karst are depositional ones The latter include clastic sediments within the caves and, particularly, minerals deposited by precipitation from karst waters both above and below ground Many subdivisions of karst have been proposed Relict karst is used to denote landforms inherited from earlier climatic or drainage regimes but still subject to modification by the current conditions Palaeokarst refers to karst features buried by younger rocks and so largely isolated from current karst modification; where uncovered by later denudation, this isolated karst is called exhumed karst Biokarst encompasses small-scale sculpting of limestone by animals and plants, although the distinction between dissolutional sculpting (true biokarst) and mechanical excavation (bioerosion) is seldom made Pseudokarst is, as its name implies, ‘false karst’ Such features superficially resemble karst but form by quite different processes, such as lava tubes, soil piping, and thermokarst, or cryokarst, formed by localized melting of permafrost Often karst geomorphology is regarded as a specialist discipline that is of limited general application to geology or geomorphology However, $12% of Earth’s terrestrial, ice-free surface is composed of limestone, with 7–10% supporting some form of karst landscape Furthermore, as much as 25% of the world’s population may depend to some extent on karst water supplies Consequently, the study of karst is crucial to understanding landscape and drainage development over a significant area of Earth’s surface Karst Processes The basic process of karst dissolution involves ion dissociation For rock salt (NaCl), gypsum (CaSO4Á2H2O), and quartz (SiO2), this requires only the presence of water but, on a global scale, the outcrop area of evaporites and the low solubility of quartz render them of only minor and localized significance for karst development Limestone (CaCO3) and dolomite (CaMg[CO3]2) are by far the dominant karst rocks but experience very low rates of dissociation in pure water The addition of free Hỵ ions greatly increases the rate of carbonate dissociation and hence even weak acids become effective solvents In most karst environments, carbonic acid, derived from atmospheric or soil CO2, is the main source of Hỵ ions, although other organic or inorganic acids may be significant locally The solubility of CO2 increases with decreasing temperature, in common with other gases The same is true also for limestone, in marked contrast to most solids for which solubility increases with temperature Nonetheless, the availability of liquid water and biogenic CO2 is far more significant for karst development than are low temperatures Water with an excess of Hỵ ions is commonly referred to as aggressive, and continues to take up HCO3 ions until an Hỵ / HCO3 equilibrium is reached at saturation, as in the following equation: CO23 ỵ 2Hỵ ! HCO3 ỵ Hỵ ! H2 CO3 Different karst waters may reach saturation at different concentrations, determined by the initial CO2 concentration, but this is not a simple straight-line relationship and mixing of different karst waters may increase aggressivity This phenomenon, called mixing corrosion, may be significant in certain karst environments Carbonate solubility also is increased by the foreign ion effect, the addition of ions such as Naỵ, Kỵ, and Cl Seawater is saturated and cannot directly dissolve limestone, but mixing with freshwater can considerably increase carbonate solubility and is of major significance in certain environments In the same way that an increase in CO2 concentration increases carbonate uptake, so degassing of CO2 from a saturated solution causes reprecipitation of calcite Dissolution can and does occur in static or laminar flow conditions, though constrained by diffusion rates through the boundary layer Permeable soil or sediment cover, even when vegetated, may offer only limited resistance to downward percolation of water to the limestone beneath Although this subsoil water movement may be slow, its dissolutional efficacy is enhanced by higher CO2 concentrations generated by