Lake Tauca

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. The lake covered large parts of the southern Altiplano between the Eastern Cordillera and the Western Cordillera, covering an estimated of the basins of present-day Lake Poopó and the Salars of Uyuni, Coipasa and adjacent basins. Water levels varied, possibly reaching in altitude. The lake was saline. The lake received water from Lake Titicaca, but whether this contributed most of Tauca's water or only a small amount is controversial; the quantity was sufficient to influence the local climate and depress the underlying terrain with its weight. Diatoms, plants and animals developed in the lake, sometimes forming reef knolls.
The duration of Lake Tauca's existence is uncertain. Research in 2011 indicated that the rise in lake levels began 18,500 BP, peaking 16,000 and 14,500 years ago. About 14,200 years ago, lake levels dropped before rising again until 11,500 years ago. Some researchers postulate that the last phase of Lake Tauca may have continued until 8,500 BP. The drying of the lake, which may have occurred because of the Bølling-Allerød climate oscillation, left the salt deposits of Salar de Uyuni.
Lake Tauca is one of several ancient lakes which formed in the Altiplano. Other known lakes are Lake Escara, Ouki, Salinas, Minchin, Inca Huasi and Sajsi, in addition to several water-level rises of Lake Titicaca. The identity of these lakes is controversial; Sajsi is often considered part of Lake Tauca, and the lake is frequently divided into an earlier and a later phase.
The formation of Lake Tauca depended on a reduction in air temperature over the Altiplano and an increase in precipitation, which may have been caused by shifts in the Intertropical Convergence Zone and increased easterly winds. It was originally supposed that glacial melting might have filled Lake Tauca, but the quantity of water would not have been sufficient to fill the whole lake. The lake was accompanied by glacial advance, noticeable at Cerro Azanaques and Tunupa. Elsewhere in South America, water levels and glaciers also expanded during the Lake Tauca phase.

Description

Overview

Lake Tauca existed on the Altiplano, a high plateau with an average altitude of, covering an area of or. The highland is in the Andes, the world's longest mountain chain which was formed during the Tertiary with a primary phase of uplift in the Miocene. Its central area, which contains the Altiplano, is formed by the eastern and western chains: the Eastern and Western Cordillera of Bolivia, which reach an altitude of. The Eastern Cordillera creates a rain shadow over the Altiplano. The climate of the Altiplano is usually dry when westerly winds prevail; during the austral summer, heating induces easterly winds which transport humidity from the Amazon. A north-south gradient exists, with mean temperatures and precipitation decreasing from and in the north, to and in the southern Lípez area. Although precipitation decreases from north to south, the evaporation rate throughout the Altiplano exceeds. Most precipitation is recorded between October and April. Occasionally during winter, frontal disturbances result in snowfall. 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. The terrain of the Altiplano consists primarily of sediments deposited by lakes and rivers during the Miocene and Pleistocene. A Paleozoic basement underlies Cretaceous and Tertiary sediments. The Andean Central Volcanic Zone and the Altiplano–Puna volcanic complex are in the Cordillera Occidental.
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 Bonneville in North America. Today, the Altiplano contains Lake Titicaca, with a surface area of, and several other lakes and salt pans. The latter include the Salar de Uyuni, at an altitude of with an area of, and the Salar de Coipasa, covering at an altitude of. Lake Titicaca and the southern salt flats are two separate water basins, connected by the Rio Desaguadero when Titicaca is high enough. The theory that the Altiplano was formerly covered by lakes was first proposed by J. Minchin in 1882. The formation of such lakes usually, but not always, coincided with lower temperatures. No evidence has been found for lake expansions in the Altiplano region below an altitude of.

Geography

Larger than Lake Titicaca, Tauca was over long and covered the area of the present-day Lake Poopo, Salar de Uyuni and Salar de Coipasa. Lake Tauca was the largest paleolake in the Altiplano in the last 120,000 years at least, and comparable to present-day Lake Michigan. Several different estimates for its surface area exist:

Surface	Surface	Details	Date of estimate
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

Estimates of lake levels	Date of estimate
	2002, 1995
	2013
	2001, 2006
	2013
Almost	2005

Water depths reached. Water levels were about higher than Salar de Uyuni, or. According to research published in 2000, the lake level varied from. Some disagreement about water levels at various sites may reflect differing isostatic rebound of the land covered by the lake. The original 1978 research on the Tauca phase postulated its shoreline at. Of the previous lake cycles in the area, only the Ouki cycle appears to have exceeded that altitude.
A later phase in lake levels 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. Transitions between lake cycles occurred in about one thousand years.
Lake Tauca was the largest lake on the Altiplano during the last 100,000-130,000 years. Although the preceding paleolake was probably shallower, there is disagreement about the methods used to ascertain water depth. Some consider Minchin the larger lake; a 1985 paper estimated its size at, compared with Tauca's. Confusion may have resulted from the incorrect attribution of Tauca's shorelines to Lake Minchin; a shoreline at formerly attributed to Lake Minchin was dated to the Tauca phase at 13,790 BP. The theory that Tauca is the largest lake follows a deepening trend in the southern Altiplano paleolakes which contrasts with a decreasing trend in the level of Lake Titicaca during the Pleistocene. This pattern probably occurred because the threshold between the two basins progressively eroded, allowing water from Titicaca to flow into the southern Altiplano. The lakes left erosional benches, fan deltas and lake-sediment deposits, and eroded into moraines. 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 lake and its predecessors formed in the area currently occupied by salt flats such as the Salar de Uyuni, Salar de Coipasa, Lake Poopó, Salar de Empexa, Salar de Laguani, and Salar de Carcote—several tens of meters beneath the Tauca water level. The present-day cities of Oruro and Uyuni are located in areas flooded by Lake Tauca. Salar de Ascotán may or may not have been part of Lake Tauca. The submergence of a large part of the Altiplano under Lake Tauca reduced the production of dust there and its supply to Patagonia, but "restocked" the sediments and thus increased dust supply once Lake Tauca dried up. The terrain above was affected by glaciation. In the Coipasa basin, a major debris avalanche from the Tata Sabaya volcano rolled over terraces left by Lake Tauca.

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 in 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. Based on oxygen-18 data in lake carbonates, water temperatures ranged from or. Tauca may have been subject to geothermal heating.
The lake was deep and saline, with salinity increasing from the Tauca to the Coipasa stages. The salt content seems to have consisted of NaCl and Na₂SO₄. Estimated salt concentrations:

Salt concentration	Comment	Source

	The latter, earlier, estimate may be incorrect; many salinity values were obtained from deposits at the lake margins, which tend to be less saline
	Later research

Estimated salt concentrations :

Mineral	Concentration	Source
Sodium chloride
Chlorine
Sodium
Sulfate
Magnesium
Potassium
Calcium
Boron
Lithium	or	and
Bromine

Some of this salt penetrated aquifers beneath the lake, which still exist. A significant excess NaCl concentration has been inferred for Lake Tauca, possibly stemming from salt domes whose contents moved from lake to lake. Precipitation of calcium carbonate resulted in lake waters becoming progressively enriched in more soluble salts.
Glacial meltwater may have contributed substantially to Lake Tauca's development. Strontium isotope data indicates that water draining from Lake Titicaca through the Rio Desaguadero may have contributed between 70% and 83% of Lake Tauca's water, an increase of between 8 and 30 times the current outflow of Lake Titicaca via the Desaguadero. A drop in the level of Lake Titicaca about 11,500 BP may have resulted in its outflow drying up, favouring the disappearance of Lake Tauca. According to other research, the increased outflow of Lake Titicaca would have had to be unrealistically large to supply Lake Tauca with water if Titicaca was its principal source. Other estimates assume that one-third of Tauca's water came from Lake Titicaca, no more than 15% for any lake cycle, or the much-lower four per cent. During the Coipasa cycle, Lake Poopó may have contributed about 13% of the water. About 53% of Lake Tauca's water came from the Eastern Cordillera. About 60,000 years ago, the Desaguadero probably began transporting water from Lake Titicaca to the Uyuni area and the southern paleolakes. Tauca was fed by the 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. If lake levels reached an altitude of, 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 may have formed at Salar de Ascotán, into the Pacific Ocean, before it was obstructed by lava flows. A theory proposed by Campbell in 1985 that a former Altiplano-wide lake catastrophically drained into the Rio Beni during the Holocene has not received much support.
Although earlier theories postulated that large lakes formed from glacial meltwater, increased precipitation or decreased evaporation are today considered necessary for lake formation; a complete glacial melting would have had to occur in less than about a century to produce the required volume. The water volume would be insufficient to explain Lake Tauca's high water levels; however, some smaller lakes in the southern Altiplano probably expanded from glacial meltwater alone. The lake may have contributed to increased precipitation by influencing land breezes. 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. Minor water-level fluctuations occurred during the lake's existence.
Based on a surface area, the evaporation rate has been estimated at over —comparable to the discharges of the Nile or Rhine. Less than half of this evaporation returned to the lake as precipitation; in the central sector of the lake at Tunupa, this would have increased precipitation by 80%, delaying the retreat of glaciers in the area. Groundwater from Lake Tauca may have drained into the Quebrada Puripica, northeast of Laguna Miscanti. Given the height of the sill between the two basins and evidence found at Poopó, water may have drained from the Coipasa-Uyuni basin into Lake Poopó during the Coipasa cycle.
Glacial debris and ice were probably present at the lake, with fan deltas at Tunupa overlapping the Lake Tauca shore. At Tunupa and Cerro Azanaques, glaciers reached their maximum size shortly before the lake level peaked and probably contributed to water levels when their retreat began. Conversely, Lake Tauca may have eroded traces of older glaciations away.
Lake Tauca left up to thick sediments in the southern Altiplano, and tufa deposits formed in the lake. The continental environment Pleistocene sediments were formed from lacustrine carbonate deposits. These rocks contain amphibole, clay minerals such as illite, kaolinite and smectite, feldspar, plagioclase, potassium feldspar, pyroxene and quartz. The composition of these rocks resembles that of the Altiplano soils. The sedimentation rate in the Uyuni basin was about.