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A soda lake or alkalin river is a lake on the strongly alkaline side of neutrality (in other words, a pH value above 7, typically between 9 - 12). They are characterized by high concentrations of carbonate salts, typically sodium carbonate (and related salt complexes), giving rise to their alkalinity. In addition, many soda lakes also contain high concentrations of sodium chloride and other dissolved salts, making them saline- or hypersaline lakes as well. High pH and salinity often coincide, because of how soda lakes develop (see "Geology, geochemistry and genesis"). The resulting hypersaline and highly alkalic soda lakes are considered some of the most extreme aquatic environments on Earth.〔Grant, W. D. (2006). ''Alkaline environments and biodiversity.'' in ''Extremophiles'', 2006, UNESCO / Eolss Publishers, Oxford, UK〕 In spite of their apparent inhospitability, soda lakes are often highly productive ecosystems, compared to their (pH-neutral) freshwater counterparts. Gross primary production (photosynthesis) rates above 10 g C m−2 day−1 (grams carbon per square meter per day), over 16 times the global average for lakes and streams (0.6 g C m−2 day−1), have been measured. This makes them the most productive aquatic environments on Earth. An important reason for the high productivity is the virtually unlimited availability of dissolved carbon dioxide. Soda lakes occur naturally throughout the world (see Table below), typically in arid and semi-arid areas and in connection to tectonic rifts like the East African Rift Valley. The pH of most freshwater lakes are on the alkaline side of neutrality and many exhibit similar water chemistries to soda lakes, only less extreme. ==Geology, geochemistry and genesis== In order for a lake to become alkalic, a special combination of geographic, geological and climatic conditions are required. First of all, a suitable topography is needed, that limits the outflow of water from the lake. When the outflow is completely prevented, this is called an endorheic basin. Craters or depressions formed by tectonic rifting often provide such topological depressions. The high alkalinity and salinity arise through evaporation of the lake water. This requires suitable climatic conditions, in order for the inflow to balance outflow through evaporation. The rate at which carbonate salts are dissolved into the lake water also depends on the surrounding geology and can in some cases lead to relatively high alkalinity even in lakes with significant outflow. Another critical geological condition for the formation of a soda lake is the relative absence of soluble magnesium or calcium. Otherwise, dissolved magnesium (Mg2+) or calcium (Ca2+) will quickly remove the carbonate ions, through the precipitation of minerals such as calcite, magnesite or dolomite, effectively neutralizing the pH of the lake water. This results in a neutral (or in slightly acidic) salt lake instead. A good example is the Dead Sea, which is very rich in Mg2+. In some soda lakes, inflow of Ca2+ through subterranean seeps, can lead to localized precipitation. In Mono Lake, California and Lake Van, Turkey, such precipitation has formed columns of tufa rising above the lake surface. Many soda lakes are strongly stratified, with well-oxygenated upper layer (epilimnion) and an anoxic lower layer (hypolimnion), without oxygen and often high concentrations of sulfide. Stratification can be permanent, or with seasonal mixing. The depth of the oxic/anoxic interface separating the two layers varies from a few centimeters to near the bottom sediments, depending on local conditions. In either case, it represents an important barrier, both physically and between strongly contrasting biochemical conditions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Soda lake」の詳細全文を読む スポンサード リンク
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