Anchialine system

An anchialine system (ˈæŋkiəlaɪn, from Greek ankhialos, "near the sea") is a landlocked body of water with a subterranean connection to the ocean. Depending on its formation, these systems can exist in one of two primary forms: pools or caves. The primary differentiating characteristics between pools and caves is the availability of light; cave systems are generally aphotic while pools are euphotic. The difference in light availability has a large influence on the biology of a given system. Anchialine systems are a feature of coastal aquifers which are density stratified, with water near the surface being fresh or brackish, and saline water intruding from the coast at depth. Depending on the site, it is sometimes possible to access the deeper saline water directly in the anchialine pool, or sometimes it may be accessible by cave diving. Anchialine systems are extremely common worldwide especially along neotropical coastlines where the geology and aquifer systems are relatively young, and there is minimal soil development. Such conditions occur notably where the bedrock is limestone or recently formed volcanic lava. Many anchialine systems are found on the coastlines of the island of Hawaii, the Yucatán Peninsula, South Australia, the Canary Islands, Christmas Island, and other karst and volcanic systems. Anchialine systems may occur in karst landscapes, regions with bedrock composed of soluble sedimentary rock, such as limestone, dolomite, marble, gypsum, or halite. Subterranean voids form in karst landscapes through the dissolution of bedrock by rainwater, which becomes mildly acidic by equilibrating with carbon dioxide from the atmosphere and soil as it percolates, resulting in carbonic acid, a weak acid. The acidic water reacts with the soluble sedimentary rock causing the rock to dissolve and create voids. Over time, these voids widen and deepen, resulting in caves, sinkholes, subterranean pools, and springs. The processes to form these karst morphological features occur on long geological timescales; caverns can be several hundred thousand to millions of years old.