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. Since the caverns which house karst anchialine systems form through the dissolution of bedrock via water percolation, current karst anchialine systems developed around the last glacial maximum, approximately 20,000 years ago when the sea level was ~120 meters lower than today. Evidence of this can be seen in speleothems (stalactites and stalagmites), a terrestrial cave formation observed at 24 meters water depth in anchialine pools in Bermuda and 122 meters water depth in a blue hole in Belize. The marine transgression after the last glacial maximum caused saline groundwater to intrude into karst caverns resulting in anchialine systems. In some anchialine systems, lenses of freshwater overlay the saltwater environment. This is caused by the accumulation of freshwater from meteoric or phreatic sources above the intruded saltwater or the vertical displacement of freshwater from intruding saltwater. Horizontal white "bathtub ring" stains are observed in submerged sections of Green Bay Cave, Bermuda, indicating paleo-transition zones between freshwater and saltwater at a lower sea level.

Anchialine systems are also commonly found in coastal mafic volcanic environments such as the Canary Islands, Galapagos Islands, Samoa, and Hawaii. Lava tubes are the primary mechanism that creates anchialine systems in these volcanic environments. Lava tubes occur during eruptions of fluid-flowing basaltic pahoehoe lava. As lava flows downhill, the atmosphere and cooler surfaces come in contact with the exterior of the flow, causing it to solidify and create a conduit through which the interior liquid lava continues flowing. If the solid conduit empties of liquid lava, the result is a lava tube. Lava tubes flow towards lower elevations and typically stop upon reaching the ocean; however, lava tubes can extend along the seafloor or form from submarine eruptions creating anchialine habitats. Saltwater intruded into many coastal lava tubes during the marine transgression after the last glacial maximum creating many volcanic anchialine pools observed today. Volcanic anchialine systems typically can develop more rapidly than karst systems; on the order of thousands to tens of thousands of years due to their rapid formation at or near the Earth's surface, making them vulnerable to erosional processes.

Tectonic faulting in coastal areas is a less common formation process for anchialine systems. In volcanic and seismically activity areas, faults in coastal environments can be intruded by saline groundwater resulting in anchialine systems. Submerged coastal tectonic faults caused by volcanic activity are observed in Iceland and in the Galapagos Islands, where they are known as "grietas," which translates to "cracks." Faulted anchialine systems can also form from tectonic uplift processes in coastal regions. The Ras Muhammad Crack area in Israel is an anchialine pool created by an earthquake in 1968 from the uplift of a fossil reef. The earthquake resulted in a fault opening approximately 150 meters from the coastline, which filled with saline groundwater creating an anchialine pool with water depths of up to 14 meters. Deep anchialine pools created by faulting from the uplift of a reef limestone block are also seen on the island of Niue in the Central Pacific.

Hydrological processes can describe how the water moves between the pool and the surrounding environment. Collectively, these processes change the salinity and the vertical density profile, which sets the conditions for the ecological communities to develop. Although each anchialine system is unique, a box model simplifies the hydrology processes included in each system.

To predict mean salinity of an anchialine pool, the pool can be treated as a well-mixed box. Various sources (sinks) add (remove) water and alter the salinity. Below lists several important saline sources and sinks of the pool.

The ratio between the evaporation and water exchange with the surrounding, ${\displaystyle PS}$, implies if the box reach an equilibrium state or not.