Chungará is located at the northeastern edge of the Lauca Basin. It lies in a tectonic-volcanic basin, with a maximum water depth of 40 m, a surface area of 21.5 km2, and a volume of about 385 million m3. The main inflows are the Chungará River (300 to 500 ls-1) and several springs on the western margin. There is no surface outlet. Hydrological balance suggested a large groundwater outflow to the Cotacotani lakes to the north. However, a chemical and isotopic survey of the Chungará and Cotacotani lakes shows a more complex picture, and a more independent hydrological evolution of each lake. The Chungará lake is polymictic, oligotrophic, contains 1.2 g l-1 TDS, and the water chemistry is alkaline (pH ranges between 7 and 10) and of (Na+)-(Mg 2+)- (HCO3-) – (SO4 2-) type. Lake water temperatures in spring (early November) range between 12 ºC (surface) and 6 ºC (bottom); electric conductivity showed no changes with depth (1450 mS). Oxic conditions at the bottom of the lake allow development of invertebrate benthic communities. The origin of the lake is related to the emplacement of the Parinacota volcano debris avalanche that blocked the Chungará River. The age of this episode is controversial, ranging from late Pleistocene (ca. 18 ka) to early Holocene (8 Ka). A seismic survey in the lake revealed two seismic units overlying the massive volcanic bedrock in the smaller, shallower eastern subbasin and also in the main, deeper, NW-SE trending basin. An older seismic unit, with less well-defined reflections, is found in the northwestern basin. Detailed sedimentological studies of short cores in the western bay (20 m water depth) and Livingstone cores in the eastern shelf (5 m water depth) have shown large depositional, paleohydrological and limnological changes in the lake during the late Holocene (ca. 4ka - modern). Sedimentary sequences identified in a 3.6 m long core from the eastern subbasin show alternation of lake sub environments (lacustrine shelf, macrophyte-dominated littoral, and peat bog) that reflect century- to millennial-scale limnological and hydrological changes in the lake during the last 4000 years. Diatom assemblages show fluctuations in the percentages of the dominant freshwater planktonic species in the lower units, and a sharp decrease in the upper unit (55 cm) where meso- to polisaline benthic diatom percentages slightly increase. Modern sediments in the deeper areas are composed of black, organic-rich, massive to faintly laminated mud. The sedimentological, chemical and stable isotope composition of a 210Pb-dated short core indicated large changes in the water balance of the lake, during the last 500 years. The results obtained from the short cores studies and the seismic survey showed the potential of Chungará Lake sedimentary sequence as a high-resolution archive of environmental and climate change in the tropical Andes during the Holocene. In November 2002 an international expedition, including Spanish, Chilean and US- institutions, retrieved fifteen Kullenberg cores along several transects in the lake. The longest ones (up to 8 meters) reached the basal seismic unit and recovered the whole post-Parinacota volcano collapse sequence. Basal sediments are coarse and contain gravels and sands. Preliminary core catcher studies showed a variety of sedimentary facies. Magnetic susceptibility analyses also show several sequences reflecting changes in the depositional sub environments in the lake. The on-going multidisciplinary study of the cores will help to solve some of the regional controversies related to Holocene climate change and the relationships between human communities and environmental changes in the past and it will provide useful data for the preservation and management of this Reserve of the Biosphere.
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