The Atacama Desert (Northern Chile) is among the driest places on Earth, with annual rainfall below 25 mm per year and very rare flash flood events. Despite the scarcity of rainfall in this area, there are several solution caves developed in the Oligocene-Miocene evaporites of the Cordillera de la Sal, an over 100 km long, NE-SW elongated belt located at an elevation around 2,446 m asl in the Pre-Andean depression, at about 150 km East from the Pacific coast of South America. Over 50 caves and more than 15 km of salt cave passages have been explored in the last 20 years in the Cordillera de la Sal, close to San Pedro de Atacama village. These karst systems are characterized by the presence of a great variety of secondary deposits that include stalactites, flowstones, precipitates that form crusts in the streambeds and at the groundwater seeps, coatings along the cave walls, earthy masses from the cave floors and efflorescence salts on ceiling rock outcrops. Salt precipitation in this environment is controlled by the temperature dependence solubility of the species in saline water, that control the different secondary cave deposits formation. In fact, most secondary deposits found in the salt caves of Atacama are derived from the evaporation of brines (i.e. halite, gypsum, blödite, leonite), or from dehydration of gypsum induced by the hot, arid climate of the area (bassanite, anhydrite). In the framework of the project "Reading" the salt caves of Atacama", funded by the National Geographic Research and Exploration Grant, this study aims to explore the microclimatic conditions that drive cave minerals deposition within Atacama caves. For this purpose, in November 2015 and March 2018 during two multidisciplinary scientific expeditions a cave micrometeorological monitoring was performed in three different caves equipped with temperature and relative humidity data loggers that continuously recorded air masses pattern in two different seasons (summer and winter). The results have shown that Atacama cave microclimate conditions are characterized by air temperatures ranging between 15-18 °C, depending on altitude, depth below the surface, and size/number of their openings of the karst systems. Unlike most of the caves in the world, where the underground atmosphere is characterized by saturation in water vapour, cave relative humidity in this area is always very low (between 5-15%) enforced by the constant airflow through the entire cave length. This micrometeorological features control the secondary minerals precipitation on cave conduits. In fact, during the long periods of extreme dryness, evaporation of the fluids that reach the underground passages is favoured. This fluids derive from rare flash flood events during which rainwater seepages through the fractures within the host rock. The salts dissolution allows the formation of brines. The evaporation of these resulting salt-rich fluids at the cave atmosphere interface causes secondary deposits to precipitate, mostly halides and sulphates.
Microclimatic control of secondary cave minerals deposition in Atacama Desert (Chile)
Laura Sanna
2019
Abstract
The Atacama Desert (Northern Chile) is among the driest places on Earth, with annual rainfall below 25 mm per year and very rare flash flood events. Despite the scarcity of rainfall in this area, there are several solution caves developed in the Oligocene-Miocene evaporites of the Cordillera de la Sal, an over 100 km long, NE-SW elongated belt located at an elevation around 2,446 m asl in the Pre-Andean depression, at about 150 km East from the Pacific coast of South America. Over 50 caves and more than 15 km of salt cave passages have been explored in the last 20 years in the Cordillera de la Sal, close to San Pedro de Atacama village. These karst systems are characterized by the presence of a great variety of secondary deposits that include stalactites, flowstones, precipitates that form crusts in the streambeds and at the groundwater seeps, coatings along the cave walls, earthy masses from the cave floors and efflorescence salts on ceiling rock outcrops. Salt precipitation in this environment is controlled by the temperature dependence solubility of the species in saline water, that control the different secondary cave deposits formation. In fact, most secondary deposits found in the salt caves of Atacama are derived from the evaporation of brines (i.e. halite, gypsum, blödite, leonite), or from dehydration of gypsum induced by the hot, arid climate of the area (bassanite, anhydrite). In the framework of the project "Reading" the salt caves of Atacama", funded by the National Geographic Research and Exploration Grant, this study aims to explore the microclimatic conditions that drive cave minerals deposition within Atacama caves. For this purpose, in November 2015 and March 2018 during two multidisciplinary scientific expeditions a cave micrometeorological monitoring was performed in three different caves equipped with temperature and relative humidity data loggers that continuously recorded air masses pattern in two different seasons (summer and winter). The results have shown that Atacama cave microclimate conditions are characterized by air temperatures ranging between 15-18 °C, depending on altitude, depth below the surface, and size/number of their openings of the karst systems. Unlike most of the caves in the world, where the underground atmosphere is characterized by saturation in water vapour, cave relative humidity in this area is always very low (between 5-15%) enforced by the constant airflow through the entire cave length. This micrometeorological features control the secondary minerals precipitation on cave conduits. In fact, during the long periods of extreme dryness, evaporation of the fluids that reach the underground passages is favoured. This fluids derive from rare flash flood events during which rainwater seepages through the fractures within the host rock. The salts dissolution allows the formation of brines. The evaporation of these resulting salt-rich fluids at the cave atmosphere interface causes secondary deposits to precipitate, mostly halides and sulphates.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
