Lake Tauca is a former lake in the Altiplano of Bolivia. It is also known as Lake Pocoyu for its constituent lakes: Lake Poopó, Salar de Coipasa and Salar de Uyuni. At high water levels, the lake spread across the southern Altiplano between the Eastern and Western Cordilleras, flooding the basins that now hold Lake Poopó and the Uyuni and Coipasa salars. Published area estimates range from about . Water levels varied, possibly reaching about above sea level. It was a saline lake. The lake received water from Lake Titicaca, although estimates of Titicaca's contribution vary widely. In any case, Tauca was large enough to influence local climate and to depress the underlying terrain under its own weight. Diatoms, plants and animals lived in the lake, sometimes forming reef-like carbonate knolls.
The duration of Lake Tauca's existence is uncertain. Research published in 2011 suggested that lake levels began rising around 18,500 BP and reached their main highstand around 16,000–14,500 years ago. Around 14,200 years ago, lake levels fell, then rose again and persisted until about 11,500 years ago. Some researchers suggest that the final phase may have continued until about 8,500 BP. When the lake dried—possibly during the Bølling–Allerød oscillation—it left the salt deposits of Salar de Uyuni.
Lake Tauca is one of several ancient lakes which formed in the Altiplano. Lake Tauca was one of several named Altiplano palaeolakes (including Escara, Ouki, Salinas, Minchin, Inca Huasi and Sajsi), alongside repeated highstands of Lake Titicaca. How these named lake phases relate to one another is debated. Sajsi is often treated as part of the Tauca system, and Tauca itself is frequently divided into an earlier Ticaña phase and a later Coipasa phase.
Lake Tauca formed when the Altiplano became colder and wetter, likely because shifts in the Intertropical Convergence Zone (ITCZ) strengthened easterly moisture transport. Early explanations emphasised glacial meltwater, but melt alone would not have provided enough water to fill the lake. Glaciers advanced alongside the lake, leaving clear evidence at Cerro Azanaques and Tunupa. Elsewhere in South America, water levels and glaciers also expanded during the Lake Tauca phase.
Description
Overview Lake Tauca formed on the Altiplano, a wide high plateau in the central Andes, the world's longest mountain chain. It lies about metres above sea level between Bolivia's Eastern and Western Cordilleras. The Altiplano is underlain by a Paleozoic basement, which is covered by Cretaceous to Pleistocene sediments, which form its surface. Uplift of the Altiplano began in the Miocene, and it became a closed basin during the late Pliocene-early Pleistocene. The Andean Central Volcanic Zone and the Altiplano–Puna volcanic complex are in the Cordillera Occidental. The Cordillera Real is glaciated, while the southern Altiplano peaks are ice-free owing to the dry climate.
, temperatures at altitude range between . Climate varies strongly from north to south: mean temperatures and precipitation drop from and in the north to about and in the southern Lípez area, reflecting the direction of prevailing winds and the topography. The climate is arid, evaporation rates throughout the Altiplano exceed . A larger daily than seasonal temperature range, strong winds and high insolation are other aspects of the Altiplano climate. Much of the water balance in the present-day Altiplano-Atacama area is maintained by groundwater flow.
Most precipitation is recorded between October and April, when easterly winds associated with the South American Summer Monsoon transport humidity from the Amazon. They are the principal source of precipitation in the Altiplano, and responsible for its increase in a south-north direction. The climate of the Altiplano is usually dry when westerly winds prevail, but frontal systems can bring snowfall, mainly in winter but sometimes in summer. Its location at the edge of the monsoon belt makes the Altiplano climate sensitive to climate oscillations, such as the El Niño-Southern Oscillation.
Lake Titicaca is a large lake in the northern Altiplano and covers a surface area of ; it usually drains through the Rio Desaguadero to Lake Poopó. The Salar de Uyuni, at an altitude of with an area of , and the Salar de Coipasa, covering at an altitude of , are large salt pans. Lake Titicaca and the southern salt flats are two separate water basins; a sill at altitude separates Poopó from Coipasa. Another sill, only a few metres high, at altitude separates Coipasa from Uyuni. Channels, for example the Rio Laca Jahuira between Poopó and Coipasa, indicate the past connections between lakes; water levels exceeding result in the formation of a waterbody spanning Poopó, Coipasa and Uyuni. Sometimes during wet years, like 1986, the salt flats fill with water and become partially connected.
Geography
At its maximum extent, Lake Tauca was larger than Lake Titicaca: it stretched for over and flooded the basins now occupied by Lake Poopó, Salar de Uyuni and Salar de Coipasa. Lake Tauca was the Altiplano's largest palaeolake for at least the past 120,000 years and was comparable in size to present-day Lake Michigan. Lake Tauca was one of many lakes which formed around the world during glacial epochs; others include the Baltic Ice Lake in Europe and Lake Missoula in North America, although most of them formed behind ice dams.
Water depths reached , (an altitude of about above sea level) making Lake Tauca the largest lake on the Altiplano during the last 100,000-130,000 years. Disagreement about water levels at different sites may reflect varying isostatic rebound (uplift after the lake disappeared) of the former lakebed, or neotectonic processes. Minor water-level fluctuations occurred during the lake's existence.
Tauca and earlier lake phases occupied basins now filled by Lake Poopó and major salt flats such as the Salar de Uyuni, Salar de Coipasa, Salar de Empexa, Salar de Laguani, and Salar de Carcote with several tens of metres of water. Salar de Ascotán may or may not have been part of Lake Tauca, while Lake Titicaca was never contiguous with any Altiplano lake. Long embayments extended into the surrounding landscape (e.g at Salar de Ascotan, Salar de Empexa, Salar de Laguani and the Desaguadero valley to Nejapa), and several mountains formed peninsulas or islands in Lake Tauca. The ridge that separates the Salar de Uyuni and Salar de Coipasa was a peninsula in the lake; San Agustín, San Cristóbal and Colcha formed islands. The present-day cities of Challapata, Oruro and Uyuni are located in areas flooded by Lake Tauca. Muddy deposits formed in the deeper lake basins like Uyuni. The terrain above was affected by glaciation, and glacial outwash reached the lakeshore.
The shores of Lake Tauca and earlier lake phases featured beach ridges, erosional benches, fan deltas and reefs. Lake sediments formed gravel and ooid beaches, mudflats, sandy deposits and tufa structures. Some caves along the shores were flooded. Volcanic eruptions affected Lake Tauca; the Chita tuff was emplaced in the lake approximately 15,650 years BP, when the lake may have been regressing, and the debris avalanche from the Tata Sabaya volcano rolled over terraces left by Lake Tauca.
A later phase in lake levels (known as the Ticaña phase) was lower, at ; the drop from Tauca was abrupt. The late phase of Lake Tauca, Coipasa, had a water level of or and covered an area of about . The connection between the Poopó and Coipasa basins narrowed and large islands formed in the southwestern Uyuni and between the Uyuni and Coipasa basins, which were still connected at Salinas de Garci Mendoza and Llica.
Alternative estimates
Alternative estimates for its surface area: Surface Surface Details Date ofestimate 43 17 1981 80 31 Possibly triggered by a large spillover from Lake Titicaca, 13,000 years ago 1995 33–60 13–23 2006 50 19 2009 52 20 At a water level 2011 48 19 Around 12,000 BP, and extending towards the Lípez area 2012 55 21 2013 56.7 21.9 2013 Alternative estimates of lake levels Date ofestimate 2002, 1995 2013 2001, 2006 2013 Almost 2005
Hydrology
At a water level of , the total volume of the lake has been estimated to be to at a level of . Such volumes could have been reached within centuries. The quantity of water was sufficient to depress the underlying bedrock, which rebounded after the lake disappeared; this has resulted in altitude differences of . Water temperatures were cold—generally in the low single digits to around depending on the method used—according to δ18O of carbonates, but similar to present-day Lake Titicaca, they might have been higher than that of the surrounding land. The area of Lake Tauca was large enough to influence the climate of the surrounding land, e.g by increasing air humidity and precipitation in and around the lake.
The lake was deep and saline, with salinity increasing from the Tauca to the Coipasa stages. Salt was predominantly NaCl and Na2SO4.
Salt concentration Comment Source The earlier estimate may be incorrect; many salinity values came from marginal deposits, which tend to be less saline. Later research
Estimated concentrations of selected dissolved salts and elements (for a lake level):: Mineral Concentration Source Sodium chloride Chlorine Sodium Sulfate Magnesium Potassium Calcium Boron Lithium or and Bromine
Tauca was fed by the Desaguadero from the north, Río Grande de Lipez on the south, the Río Lauca on the northwest and the glaciers of the two cordilleras on the east and west. The lake's total drainage basin has been estimated at . Published estimates of Lake Titicaca's contribution vary widely. Strontium-isotope work suggests a large contribution via the Rio Desaguadero, while other studies argue that Titicaca's outflow could not have supplied most of Tauca's water. Earlier theories emphasised glacial meltwater, but increased precipitation and/or reduced evaporation are now considered necessary to form lakes of Tauca's size even if a contribution of meltwater cannot be discounted completely.
Being a closed lake, water left Lake Tauca through evaporation. Based on a surface area, the evaporation rate has been estimated at over —comparable to the discharges of the Nile or Rhine— of which a fraction——returned to the lake through precipitation. The lake waters fed aquifers, such as at Quebrada Puripica northeast of Laguna Miscanti and present-day aquifers in the Altiplano. If lake levels reached an altitude of , which is however unlikely, the lake may have drained into the Pilcomayo River and from there through the Río de la Plata into the Atlantic Ocean. Formerly an outlet into the Pacific Ocean may have existed at Salar de Ascotán, before being obstructed by lava flows, but this is questionable. A 1985 proposal that an Altiplano-wide lake catastrophically drained into the Rio Beni during the Holocene has received little support.
According to strontium isotope data, there may have been little water exchange between Tauca's Uyuni and Coipasa basins. During the Coipasa lake cycle, the Coipasa-Uyuni and Poopó basins had a limited connection; given the height of the sill between the two basins and evidence found at Poopó, water may have flowed from the Coipasa-Uyuni basin into the Poopó basin.
Lake Tauca left lake sediments up to thick in the southern Altiplano, adding to the sediments left by older lakes. These rocks contain amphibole, clay minerals such as illite, kaolinite and smectite, feldspar, plagioclase, potassium feldspar, pyroxene and quartz, resembling Altiplano soils.
Biology