IANIGLA   20881
Unidad Ejecutora - UE
capítulos de libros
Birkenland Trykkeri A/S
Lugar: Birkenland, Norway; Año: 2007; p. 181 - 197
After the regionalization of the most important cryogenic regions in South America and with the help of the MAAT maps, including Andean geomorphology, superficial temperature measurings, considering cryotic characteristics, cryogenic processes or restrictive factors, as well as indirect methods (geophysics) the Andean permafrost so far detected was classified in 6 main groups; the order is according to their static position or movement, supposed ice content, their hygric potential and degradation caused by the global warming (Trombotto, 2000). Figuring the surface of the most important cryogenic regions the area with permafrost was calculated to be 100,000 km2 approximately. Approximately continuous permafrost (Garleff & Stingl, 1986) is applied to the type of permafrost which is very much restricted by topography as well as thermal conditions and is limited to the MAAT isotherms of –2 °C and – 4 °C (at the Central Andes, 33° S and approximately 4500 m ASL) with precipitations between 500 – 900 m/y and limited to the isotherms of –1 °C and –2 °C with a precipitation of 300 mm/y in the case of the Argentine Puna region. According to direct observations in the Southern Andes, the occurrence of permafrost near Lake Vintter (approximately 44° S) had been registered at an elevation of 2060 m in recent years and at 51° 30´ S in Santa Cruz at an elevation of 980 –1100 m in the years 1977 and 1978. The term island permafrost was introduced for the phenomenon of isolated patches at 4000 m ASL in the Central Andes of Mendoza. It appears because of the topography but also due to special conditions, where the exposition and shadow have a key role. The types of this case are: zonal and azonal. Towards the Equator, in the region of the Chimborazo (1° 21’, 6275 m ASL, Ecuador), a typical tropical environment but with dry conditions, Heine (1994) reported island permafrost below 5000 m. The lower limit of Andean permafrost is restricted by a type of creeping permafrost which may be observed in some parts through rock glaciers. At present the lower permafrost limit at the Cordón del Plata (Central Andes, Mendoza) is found at an elevation of 3700 – 3800 m. Rock glaciers may be polarized in two mayor varieties with periglacial landforms and relatively similar processes: a) talus rock glaciers and b) debris rock glaciers. Barsch and King (1989) corroborated that one of its terminal frozen bodies in the Morenas Coloradas rock glacier (Mendoza) has a permafrost thickness of over 50 m. In the Central Andes (31º-35º S) creeping permafrost appears in groups of rock glaciers building a typical pattern of rock glaciers. At the volcano Lullaillaco (24º43’ 6739 m ASL) the permafrost table is at 50 cm depth but rock glaciers don’t seem to be present in the South American Dry Diagonal (arid zone between 24º-27º S). The variety of dry permafrost (cold permafrost conditioned by low temperatures and arid or semiarid conditions) with temperatures below zero but without visible ice is also a frequent phenomenon, and should not be neglected when considering cryotic characteristics of the Central Andes and the region of the Desert Andes. Relict ice, or mesas, permafrost below a sediment cover of varied thickness and with CaCO3 minerals was identified by Hurlbert & Chang (1984) in the saline lakes of Bolivia and Chile, at 4300 m and approximately 22° S. The authors assigned this type of ice to the Little Ice Age. These forms would tend to disappear gradually with the regional warming. Degraded permafrost has been emphasized for certain parts of rock glaciers mainly (Francou et al., 1999) and for covered glaciers, affected by global or regional warming with a clear physiography of thermokarst or collapse structures (facies of thermokarst of Corte, 1980) and supraglacial lagoons. Zones with old thermokarst in the Central Andes reveal past warming holocene episodes, but now they appear to be reactivated. Between 1991 and 1993 the 0 °C isotherm at the Cordón del Plata (Andes of Mendoza, Argentina) was found at 3865 m. Because rock glaciers are directly correlated with the discharge of the Andean rivers (Trombotto et al., 1999), climatic variations will rebound in the future regional water supply. While in the last years the glaciers are turning into small insignificant bodies in the high mountains, the periglacial level with creeping permafrost and linked with rock glaciers is expanding altitudinally (Trombotto, 2003).Thermic particularities and regional dry conditions seem to reinforce the effects of periglacial processes and the persistence of ground ice in the Central Andes. Obtained low values of thermal diffusivity may very well express why the ice remains for such a long time below the the Andean cryolithozone (Trombotto & Hernández, 2006). A remarkable change in the active layer and suprapermafrost of this rock glacier of the Cordón del Plata however is registered at the monitoring sites at the nose or lower parts of the rock glaciers. The observed changes imply direct consequences for the cryogenic environment and the Andean creeping permafrost. At the Morenas Coloradas rock glacier for example (Balcón I, 3560 m a.s.l.) the permafrost table is found at great depth. Andean passes and mountain roads are being affected by climatic changes through catastrophic hazards like mud flows. Additional investigations are required for the distribution of Andean permafrost. It is necessary to map it, to study its topography, its ice content and its behaviour regarding global climatic change as well as its importance to domestic water supply.