Laguna Negra is a lake in the Catamarca Province of Argentina. It lies on the Puna high plateau next to two other lakes and salt flats. The lake is less than 2 metres (6 ft 7 in) deep and forms a rough rectangle with a surface of 8.6 square kilometres (3.3 sq mi). Laguna Negra loses its water through evaporation, and is replenished through surface runoff and groundwater which ultimately originate to a large part from snowmelt. The waters of the lake are salty. On the southeastern shore of the lake, microbialites grow in the shallow water. These are structures formed by carbonate and microorganisms including diatoms and various bacteria. These structures have a layered internal structure and colours ranging from green to orange-pink to black. They have been compared to Precambrian stromatoliths, which were among the oldest forms of life on Earth.
Regional and local Laguna Negra lies at an elevation of 4,131 metres (13,553 ft) in the Tinogasta Department, Catamarca Province of Argentina, and near the San Francisco Pass between Chile and Argentina. The path to Monte Pissis passes close to the lake. It covers a surface of 8.6 square kilometres (3.3 sq mi) and has the rough shape of a rectangle. The average depth of the lake does not exceed 2 metres (6 ft 7 in). The lake periodically floods its beaches. To the northwest, a salt flat separates the lake from its neighbours to the north and covers over half of the lake basin. A prominent alluvial fan borders Laguna Negra to the southeast and its northward growth has generated a shallow water area at the southeastern margin of the lake (Stromatolite Belt). The waters of the lake are hypersaline, their principal salt is calcium chloride, although they have been described as mesosaline too. Arsenic is present at high concentrations. The high salinity prevents the water from freezing except at the margins of Laguna Negra, where salinity is lower due to inflow. Calcite and aragonite reach saturation concentration at points where groundwater enters the lake, leading to carbonate precipitation mainly along the southeastern margin of the lake. Conditions in the lake environments are alkaline and the consumption of carbon dioxide by degassing and photosynthesis facilitates carbonate precipitation. The lake is fed mainly by groundwater and by runoff that enters mainly from the southwestern side. The water mainly originates through annual snowmelt. Laguna Negra has no surface outflow and is a closed lake; the lake waters evaporate in the strong wind and at high temperatures, leading to the precipitation of salts such as gypsum, halite and polyhalite in a sequential manner from carbonates to salts.
Regional context Laguna Negra is the southernmost of three lakes aligned in north–south direction; the other two are Laguna de la Salina/Laguna Tres Quebradas to the north and Laguna Verde in the middle. They form the Laguna Verde Saline Complex, also known as Laguna Verde Complex or Salar de la Laguna Verde. The Salar de Tres Quebradas salt pan separates Laguna Verde from Laguna Tres Quebradas. Together these two lakes have a water surface of 26.2 square kilometres (10.1 sq mi). Lava flows from surrounding volcanoes have closed off the southern outlet of the valley occupied by the lakes. Smaller lakes exist nearby such as Laguna Azul northeast of Laguna Negra.
The lakes lie in the southernmost Puna, a high plateau at an elevation of 3,700 metres (12,100 ft) where a dry climate and Cenozoic uplift generated the Laguna Verde Saline Complex when block faulting generated separated drainage basins separated by north–south trending mountain ranges. The terrain consists mainly of volcanic rocks such as basalt and andesite; some summits exceed 6,000 metres (20,000 ft) elevation above sea level such as the 6,795 metres (22,293 ft) high Cerro Pissis. Evaporites, sand and silt cover the terrain around the lake.
Climate, vegetation and fauna The climate at Laguna Negra is cold, arid with strong winds. Temperature is highly variable, ranging between 30 – −10 °C (86–14 °F) in summer and between 8 – −30 °C (46 – −22 °F) in winter. Annual precipitation is less than 250 millimetres per year (9.8 in/year) and falls mainly as snow. Evidence from other lakes in the region suggests that the environment was wetter than today between about 15,000–14,000 and 13,500–11,300 years before becoming dry during the middle Holocene. After about 4,000 years BP precipitation has increased again. Presently, the climate is dominated by the South Pacific High anticyclone which draws dry air into the region. During summer, an Atlantic anticyclone conversely transports moister and warmer air to the area, resulting in the formation of convective clouds and precipitation. Climatic conditions together with the thin atmosphere at high elevations and high UV radiation limit the complexity of life at Laguna Negra, in particular of nonmicrobial life. In this sector of the Andes, peatlands are the key ecosystems, with the dominant plant species being Distichia muscoides, Oxychloe andina and Plantago rigida as well as graminoids and grasses. Mites are their most important fauna. Salt marsh grasses of the genus Spartina grow at the southern end of the lake, and copepods have been observed in ponds; both are linked to areas with lower salinity. The lakes are an important bird conservation site, and were added to the "South Sub-site of the Ramsar Sites, Lagunas Altoandinas y Puneñas de Catamarca" Ramsar Site in 2009.
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