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. 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 48,000 to 80,000 square kilometres (19,000 to 31,000 sq mi). Water levels varied, possibly reaching about 3,800 metres (12,500 ft) 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.

Lake Tauca formed on the Altiplano,^[2] a 1,000 by 200 kilometres (620 mi × 120 mi) wide high plateau^[3] in the central Andes, the world's longest mountain chain. It lies about 3,800 to 4,000 metres (12,500 to 13,100 ft) metres above sea level between^[2] Bolivia's Eastern and Western Cordilleras.^[3] The Altiplano is underlain by a Paleozoic basement, which is covered by Cretaceous^[4] to Pleistocene sediments, which form its surface.^[5] Uplift of the Altiplano began in the Miocene,^[2] and it became a closed basin during the late Pliocene-early Pleistocene.^[6] The Andean Central Volcanic Zone and the Altiplano–Puna volcanic complex are in the Cordillera Occidental.^[7] The Cordillera Real is glaciated, while the southern Altiplano peaks are ice-free owing to the dry climate.^[8]

As of 2018, temperatures at 3,800 metres (12,500 ft) altitude range between 6.3–8.5 °C (43.3–47.3 °F).^[9] Climate varies strongly from north to south: mean temperatures and precipitation drop from 15 °C (59 °F) and 700 millimetres (28 in) in the north to about 7 °C (45 °F) and 100 millimetres (3.9 in) in the southern Lípez area,^[3] reflecting the direction of prevailing winds and the topography. The climate is arid,^[10] evaporation rates throughout the Altiplano exceed 1,500 millimetres per year (59 in/year).^[11] A larger daily than seasonal temperature range,^[12] strong winds and high insolation are other aspects of the Altiplano climate.^[13] Much of the water balance in the present-day Altiplano-Atacama area is maintained by groundwater flow.^[14]

Most precipitation is recorded between October and April,^[15] when easterly winds associated with the South American Summer Monsoon transport humidity from the Amazon.^[12][16] They are the principal source of precipitation in the Altiplano,^[17] and responsible for its increase in a south-north direction.^[18] The climate of the Altiplano is usually dry when westerly winds prevail,^[12][16] but frontal systems can bring snowfall, mainly in winter but sometimes in summer.^[19] Its location at the edge of the monsoon belt makes the Altiplano climate sensitive to climate oscillations,^[20] such as the El Niño-Southern Oscillation.^[21]

Lake Titicaca is a large lake in the northern Altiplano and covers a surface area of 8,560 square kilometres (3,310 sq mi);^[18] it usually^[22] drains through the Rio Desaguadero to Lake Poopó.^[12] The Salar de Uyuni, at an altitude of 3,653 metres (11,985 ft) with an area of 10,000 square kilometres (3,900 sq mi), and the Salar de Coipasa, covering 2,500 square kilometres (970 sq mi) at an altitude of 3,656 metres (11,995 ft),^[23] are large salt pans.^[24] Lake Titicaca and the southern salt flats are two separate water basins;^[15] a sill at 3,703 metres (12,149 ft) altitude separates Poopó from Coipasa.^[12] Another sill, only a few metres high,^[25][26] at 3,672 metres (12,047 ft) altitude separates Coipasa from Uyuni.^[12] Channels, for example the Rio Laca Jahuira between Poopó and Coipasa,^[27] indicate the past connections between lakes;^[12] water levels exceeding 3,700 metres (12,100 ft) result in the formation of a waterbody spanning Poopó, Coipasa and Uyuni.^[18] Sometimes during wet years, like 1986, the salt flats fill with water^[28] and become partially connected.^[29][25][30]

At its maximum extent, Lake Tauca was larger than Lake Titicaca:^[31] it stretched for over 600 kilometres (370 mi)^[32] and flooded the basins now occupied by Lake Poopó, Salar de Uyuni and Salar de Coipasa.^[25] Lake Tauca was the Altiplano's largest palaeolake^[33] for at least the past 120,000 years^[34] and was comparable in size to present-day Lake Michigan.^[35] 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.^[24]

Water depths reached 110–120 metres (360–390 ft),^[12] (an altitude of about 3,770 metres (12,370 ft) above sea level^[36]) making Lake Tauca the largest lake on the Altiplano during the last 100,000^[37]-130,000 years.^[38] Disagreement about water levels at different sites may reflect varying isostatic rebound (uplift after the lake disappeared^[39]) of the former lakebed,^[40][41] or neotectonic processes.^[42] Minor water-level fluctuations occurred during the lake's existence.^[43]

Tauca and earlier lake phases^[2] occupied basins now filled by Lake Poopó^[44] and major salt flats such as the Salar de Uyuni, Salar de Coipasa, Salar de Empexa, Salar de Laguani,^[18] and Salar de Carcote with several tens of metres of water.^[45] Salar de Ascotán may^[46][47] or may not have been part of Lake Tauca,^[45] while Lake Titicaca was never contiguous with any Altiplano lake.^[48] Long embayments extended into the surrounding landscape (e.g at Salar de Ascotan, Salar de Empexa, Salar de Laguani^[18] and the Desaguadero valley to Nejapa^[49]), and several mountains formed peninsulas or islands in Lake Tauca.^[50] 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.^[47][51] The present-day cities of Challapata,^[52] Oruro and Uyuni are located in areas flooded by Lake Tauca.^[53] Muddy^[a] deposits formed in the deeper lake basins like Uyuni.^[55] The terrain above 3,800 metres (12,500 ft) was affected by glaciation,^[19] and glacial outwash reached the lakeshore.^[55]

The shores of Lake Tauca and earlier lake phases featured beach ridges,^[56] erosional benches, fan deltas and reefs. Lake sediments^[57] formed gravel and ooid beaches,^[26] mudflats,^[58] sandy deposits and tufa structures. Some caves along the shores were flooded.^[59] 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,^[60] and the debris avalanche from the Tata Sabaya volcano rolled over terraces left by Lake Tauca.^[61]

• Salar de Ascotan
• Salar de Carcote
• The course of the Lauca River
• Tata Sabaya with the landslide to the left
• 3D geography of Salar de Uyuni, remainder of Lake Tauca

A later phase in lake levels (known as the Ticaña phase) was lower, at 3,657 metres (11,998 ft);^[62] the drop from Tauca was abrupt. The late phase of Lake Tauca, Coipasa, had a water level of 3,660 metres (12,010 ft)^[63] or 3,700 metres (12,100 ft)^[64] and covered an area of about 32,000 square kilometres (12,000 sq mi).^[12] The connection between the Poopó and Coipasa basins narrowed and large islands formed in the southwestern Uyuni and between the Uyuni and Coipasa basins,^[36] which were still connected at Salinas de Garci Mendoza and Llica.^[65]

Alternative estimates for its surface area:

At a water level of 3,720 metres (12,200 ft), the total volume of the lake has been estimated to be 1,200 cubic kilometres (290 cu mi)^[71] to 3,810 cubic kilometres (910 cu mi) at a level of 3,760 metres (12,340 ft).^[56] Such volumes could have been reached within centuries.^[72] The quantity of water was sufficient to depress the underlying bedrock, which rebounded after the lake disappeared; this has resulted in altitude differences of 10 to 20 metres (33 to 66 ft).^[41] Water temperatures were cold—generally in the low single digits to around 10 °C (50 °F) depending on the method used—according to δ¹⁸O of carbonates,^[73][74] but similar to present-day Lake Titicaca, they might have been higher than that of the surrounding land.^[75] The area of Lake Tauca was large enough to influence the climate of the surrounding land, e.g by increasing air humidity^[b][76] and precipitation in and around the lake.^[77]

The lake was deep and saline,^[78] with salinity increasing from the Tauca to the Coipasa stages.^[79] Salt was predominantly NaCl^[c] and^[d] Na₂SO₄.^[43]

Estimated concentrations of selected dissolved salts and elements (for a 3,720 metres (12,200 ft) lake level)::

Tauca was fed by the Desaguadero^[e] from the north, Río Grande de Lipez on the south, the Río Lauca on the northwest^[18] and the glaciers of the two cordilleras on the east and west.^[89] The lake's total drainage basin has been estimated at 200,000 square kilometres (77,000 sq mi).^[90] Published estimates of Lake Titicaca's contribution vary widely. Strontium-isotope work suggests a large contribution^[f] via the Rio Desaguadero,^[91] while other studies argue that Titicaca's outflow could not^[g] have supplied most of Tauca's water.^[95] Earlier theories emphasised glacial meltwater, but increased precipitation and/or reduced evaporation are now^[h] considered necessary to form lakes of Tauca's size^[98] even if a contribution of meltwater cannot be discounted completely.^[84]

Being a closed lake, water left Lake Tauca through evaporation.^[99] Based on a 60,000-square-kilometre (23,000 sq mi) surface area, the evaporation rate has been estimated at over 70,000,000,000 cubic metres per year (2.5×10¹² ft³/a)—comparable to the discharges of the Nile or Rhine—^[100] of which a fraction——returned to the lake through precipitation.^[101] The lake waters fed aquifers, such as at Quebrada Puripica northeast of Laguna Miscanti^[102] and present-day aquifers in the Altiplano.^[103] If lake levels reached an altitude of 3,830 metres (12,570 ft),^[104] which is however unlikely,^[105] the lake may have drained into the Pilcomayo River and from there through the Río de la Plata into the Atlantic Ocean.^[106] Formerly an outlet into the Pacific Ocean may have existed at Salar de Ascotán, before being obstructed by lava flows,^[107][108] but this is questionable.^[109] A 1985 proposal that an Altiplano-wide lake catastrophically drained into the Rio Beni during the Holocene^[110] has received little support.^[111]

According to strontium isotope data, there may have been little water exchange between Tauca's Uyuni and Coipasa basins.^[112] During the Coipasa lake cycle, the Coipasa-Uyuni and Poopó basins had a limited connection;^[113] given the height of the sill between the two basins and evidence found at Poopó,^[100] water may have flowed from the Coipasa-Uyuni basin into the Poopó basin.^[114]

Lake Tauca left lake sediments up to 5 metres (16 ft) thick in the southern Altiplano,^[115] adding to the sediments left by older lakes.^[116] These rocks contain amphibole, clay minerals such as illite, kaolinite and smectite, feldspar, plagioclase, potassium feldspar, pyroxene and quartz, resembling Altiplano soils.^[117]

Aquatic plants grew in Lake Tauca when temperature, substrate conditions^[37] and depth allowed.^[118] Common taxa include characeae,^[119] Myriophyllum, Isoetes^[37] (indicating the formation of littoral communities)^[120] and Pediastrum.^[37] Algae^[121] and diatoms^[i] lived in the lake.^[37] Rocks^[58] and shoreline sediments contain fossils of brine shrimp,^[42] gastropods^[126] like Biomphalaria, Littoridina and Succineidae snails,^[119] molluscs^[59] (occasionally forming coquina sediments^[58]) such as Pisidium clams,^[119] and ostracods.^[126]

Algae^[121] and to a lesser degree stromatolite-forming microbes^[j][125] produced carbonate bioherms (reef-like knolls) in the lake^[121] and along its shorelines, forming complex reefs about 300 kilometres (190 mi) long.^[3] They constitute shapes ranging from crusts on exposed surfaces to large mushroom- or dome-shaped mounds^[127] that occasionally form structures resembling barrier reefs.^[128] Some dome-shaped bioherms reach a size of 4 metres (13 ft), forming reef-like structures on terraces. They developed around objects jutting from the surface, such as rocks. Tube- and tuft-shaped structures also appear on these domes.^[128] These grew in several phases,^[121] often beginning during earlier lake stages and spanning multiple lake level fluctuations.^[121] Similar structures have been described elsewhere (for example, in Germany's Ries crater). The algal taxa that build the bioherms of Lake Tauca are hard to identify, but Chara species have been recognized.^[125] The water above the tufa deposits was probably less than 20 metres (66 ft) deep.^[126]

During humid periods when lakes formed in the Altiplano, grasslands and woodlands dominated the vegetation of the region.^[129] Polylepis may have thrived in favourable salinity and climatic conditions^[37] and was more widespread than today^[130] but less widespread than during earlier lake stages. This may either reflect a climate during Lake Tauca that was not as suitable^[131] or impaired pollen preservation in the deeper Lake Tauca.^[120] Peak Polylepis and Acaena spread tends to occur during climates that are intermediate between full glacial and interglacial ones.^[132]

Reports of lake deposits on the Altiplano go back to 1861.^[133] In 1882, John B. Minchin reported encrustations around Lake Poopó and the salars south of Coipasa. He proposed that a lake with an area of about 120,000 square kilometres (46,000 sq mi) left these encrustations, and that nitrate deposits in the Atacama and Tarapaca formed from water draining from this lake. Some estimates of the size of this lake claimed that it reached from Lake Titicaca as far as 27° South. In 1906, Steinmann used the name ‘Lake Minchin' for the Uyuni basin and called the lake covering the Poopó–Coipasa basins ‘Lake Reck'.^[134] The name was applied in honour of John B. Minchin.^[135] Later it was found that Lake Titicaca was not part of Lake Minchin and the theory was put forward that meltwater from glaciers had formed the lake. A different lake stage encompassing Lake Titicaca was also defined: Lake Ballivian.^[136] The lake episodes "Escara" and "Tauca" were first defined in 1978,^[137] the latter with a shoreline at 3,720 metres (12,200 ft) elevation above sea level.^[89] Lakes Tauca and Minchin have been considered the same lake system and called Lake Pocoyu, after the present-day lakes in the area.^[138] The relationship between various deposits in the southern Altiplano and these around Lake Titicaca was unclear at the beginning of the research history.^[139] Researchers identify these lakes through terraces, sediments, bioherms^[140] and drill cores.^[141]

Before Lake Tauca, there were Ouki^[k] (120,000–98,000 years ago), Salinas (95,000–80,000 years ago), Inca Huasi (about 46,000 years ago) and Sajsi^[l] (24,000–20,500 years ago,^[146] during the Last Glacial Maximum^[34]).^[145] A deepening trend in the southern Altiplano paleolakes during the Pleistocene contrasts with a decreasing trend in the level of Lake Titicaca. This pattern probably occurred because the threshold between the two basins progressively eroded, allowing more water from Titicaca to flow into the southern Altiplano.^[147]

A number of sometimes-contradictory names and dates exist for these paleolakes:^[114] Inca Huasi and Minchin are sometimes considered the same lake phase,^[145] and earlier research defined a "pre-Minchin" lake.^[6] Other researchers have suggested that Lake Minchin^[148][42] and Ouki are combinations of several different lake stages,^[114] or separate the Coipasa phase from Lake Tauca.^[70]

Escara was identified in the central Altiplano,^[140] and it may be the oldest Altiplano lake cycle.^[149] Lake levels reached an altitude of 3,780 metres (12,400 ft);^[150] perhaps reaching the size of Lake Tauca and Ouki.^[60] This stage left 8 metres (26 ft) thick deposits at the town of Escara.^[151]

Tuffs dated to 1.87 million years^[60] and 191,000 years ago have been identified in the deposits of Lake Escara.^[152] The latter date is commonly cited as the age of Lake Escara^[153] but is considered questionable.^[154] The L5 sediment^[141] and S10 layers in Salar de Uyuni have been linked to Escara.^[155] Contemporaneously with Lake Escara,^[156] an extended Lake Titicaca^[81] (Lake Ballivian) may have covered parts of the northern Altiplano.^{[150] [156]}

A humid period 46,000–36,000 years ago led to the formation of a lake in the Altiplano, which like the humid period itself is named "Lake Minchin";^[157] the lake formed in the same area as Lake Tauca.^[158] Their relative size is uncertain^[159] but Minchin was probably the smaller lake.^[37][160] Earlier papers regarded Lake Minchin as larger than Lake Tauca,^[39] due to incorrect attribution of shorelines:^[161] A shoreline at 3,760 metres (12,340 ft) was attributed to Lake Minchin until dating established that it formed during the Tauca phase 13,790 BP.^[162]

The name ‘Lake Minchin' has been used inconsistently—sometimes for a specific palaeolake,^[163] sometimes for a time period, sometimes for the highest lake level, sometimes for sediment formations,^[21] or the entire late Pleistocene lake system in the Altiplano.^[135] An alternative theory postulates that Lake Minchin was formed by several lakes, including Ouki and Inca Huasi,^[148][145] and by unreliable radiocarbon dates.^[164] Sometimes the term "Minchin" is also applied to the whole hydrological system Titicaca-Rio Desaguadero-Lake Poopó-Salar de Coipasa-Salar de Uyuni,^[135] or to the highest ancient lake in the Altiplano (usually known as Lake Tauca).^[165] There are also contradictions between lake level records in different parts of the system.^[16] This confusion has led to calls to drop the usage of the name "Minchin".^[21]

The layer S4 in Salar de Uyuni drill cores has been linked to Lake Minchin.^[155] Moisture increased in the Brazilian and Bolivian Amazon^[166] and lakes formed at Laguna Pozuelos,^[167] in landslide-dammed valleys in northwestern Argentina^[168] and in Bolivia's Lipez region.^[169] Bioherms and stromatolites grew in these lakes.^[170] Increased sedimentation took place in river valleys,^[171][172] forming for example the "Minchin Terrace" in the Pisco River valley.^[173] The Lake Minchin stage coincided with global climate cooling^[174] and regional glacial advance extending to the southern Altiplano/Puna,^[175][176] such as the formation of the N-III moraines at Choquelimpie,^[177] the Canabaya glaciation in the Cordillera Apolobamba^[178] and the Choqueyapu I advance in the Eastern Cordillera.^[179]

The existence of Lake Tauca was preceded by a dry period, with minor lake events recorded in Salar de Uyuni in the Late Pleistocene at 28,200–30,800 and 31,800–33,400 BP.^[158] The era may have been drier than the present;^[180][181] ice had disappeared from Nevado Sajama^[93] and the Amazon rainforest had retreated. A dry period is also noted in Africa and other parts of South America around 18,000 BP.^[182] The drying of earlier lake stages had left carbonate^[58] and salt deposits; including an about 20 metres (66 ft) thick salt layer in the Salar de Uyuni.^[183]

Water levels began to rise around 18,000 years ago and reached their peak 16,500 years ago.^[12] This highstand, which is named the "Tauca" stage, continued until 15,500 years ago.^[12] Water levels decreased between 14,500 and 13,500 years ago,^[12] leading to the "Ticaña" stage.^[184][140] Water levels dropped by 100-metre (330 ft)^[62] and Lake Tauca disconnected from Lake Titicaca^[185] as water levels in the Huiñaimarca basin of Lake Titicaca decreased.^[129] The drying of Lake Tauca during the Ticaña lowstand^[186] has been linked to the Bølling–Allerød^[130] and Antarctic Cold Reversal climate oscillations.^[187] Transitions between lake cycles occurred in timespans of no more than a millennium.^[12]

The Coipasa phase commenced about 13,300 years ago and reached its peak between 12,900 and 11,800 years ago.^[188] Lake Tauca was smaller during the Coipasa phase than during the Tauca proper:^[189] Lake levels rose to about 3,720 metres (12,200 ft).^[36] The Tauca-Coipasa double highstand is also observed at Salar de Atacama.^[190] After the Coipasa phase, water levels decreased between 11,800 and 10,200 years ago.^[188] Further drying during the Holocene was gradual.^[191] After 10,000 BP, another drought lasted from 8,500 BP to 3,600 BP,^[192] and peaked from 7,200 to 6,700 BP^[193] although a minor wet period may have occurred between 10,600 and 3,200 years ago.^[194] The world's largest salt pan-including 10 metres (33 ft) of material at Salar de Uyuni-was left behind when Lake Tauca dried up.^[67][127]

Published radiometric ages span a wide range (72,600–7,200 BP),^[195] reflecting differences in dating methods and the deposits^[m] being dated.^[196] Scatter in ages obtained by radiometric dating can yield misleadingly long durations for lake stands,^[197] but the structure of the shorelines imply that they last longer than a century.^[96] Water level fluctuations took place within less than a thousand years.^[12]

The first research, by Servant and Fontes in 1978, indicated a lake age between 12,500 and 11,000 BP according to C-14 dating.^[198] These were bracketed by dates between 12,360 ± 120 and 10,640 ± 280 BP for the highest deposits at Salar de Coipasa and Salar de Uyuni, and 10,020 ± 160 and 10,380 ± 180 BP for deposits which formed shortly before the lake dried.^[199][40] The reliability of the dates was questioned in 1990,^[110] and a later estimate was set at 13,000 to 10,000 BP.^[200] In 1990, Rondeau proposed ages of 14,100 to 11,000 BP based on radiocarbon dating and 7,000 to 14,800 BP based on uranium-thorium dating.^[137]

In 1993 it was suggested that Lake Tauca had an earlier phase, with water levels reaching 3,740 metres (12,270 ft), and a later phase reaching 3,720 metres (12,200 ft).^[200] Research published in 1995 suggested that the lake remained shallow for over a millennium before rising to, and stabilising at, its maximum level. Water levels between 13,900 and 11,500 BP reached 3,720 metres (12,200 ft); 3,740 metres (12,270 ft) was reached between 12,475 and 11,540 BP, and 3,760 to 3,770 metres (12,340 to 12,370 ft) between 12,200 and 11,500 BP.^[201]

Research in 1999 indicated an earlier start of the Tauca lake cycle, which was subdivided into three phases and several sub-phases. Around 15,438 ± 80 BP (the Tauca Ia phase), water levels in Salar de Uyuni were 4 metres (13 ft) higher than the current salt crust. Lake levels then rose to 27 metres (89 ft) above the salt flat, accompanied by freshwater input (Tauca Ib). Around 13,530 ± 50 BP (Tauca II), the lake reached an altitude of 3,693 metres (12,116 ft)^[124] but no higher than 3,700 metres (12,100 ft).^[202] Between 13,000 and 12,000 BP, the lake reached its greatest depth—110 metres (360 ft)—of the Tauca III period. Dates of 15,070 BP and 15,330 BP were obtained for the highest shoreline, at 3,760 metres (12,340 ft).^[202] After 12,000 BP, water levels decreased abruptly by 100 metres (330 ft).^[203] An even-earlier start was proposed by 2001 research, based on sediments in the Uyuni basin, which determined that Lake Tauca began developing 26,100 BP.^[158] A 2001 review indicated that most radiometric dates for Lake Tauca cluster between 16,000 and 12,000 BP, with lake levels peaking around 16,000 BP.^[70] A 2005 book estimated the duration of the Lake Tauca phase at between 15,000 and 10,500 BP.^[204]

Research in 2006 postulated that the Lake Tauca transgression began 17,850 BP and peaked at altitudes of 3,765 to 3,790 metres (12,352 to 12,434 ft) between 16,400 and 14,100 years ago.^[205] Spillovers into neighbouring basins may have stabilized the lake levels at that point,^[194] and the level subsequently dropped over a 300-year period.^[205] The following Coipasa phase ended around 11,040 +120/-440 BP, but its chronology is uncertain.^[194]

A 2011 lake history study set the beginning of the lake-level rise at 18,500 years ago. Levels rose slowly to 3,670 metres (12,040 ft) 17,500 years ago, before accelerating to 3,760 metres (12,340 ft) by 16,000 years ago. Contradictions between lake depths determined by shorelines and diatom-fossil analysis led to two lake-level-rise chronologies: one reaching 3,700 metres (12,100 ft) 17,000 years ago and the other reaching 3,690 metres (12,110 ft) between 17,500 and 15,000 years ago. The lake level would have peaked from 16,000 to 14,500 years ago at 3,765 to 3,775 metres (12,352 to 12,385 ft) altitude. Shortly before 14,200 BP, the lake level would have begun its drop to 3,660 metres (12,010 ft) by 13,800 BP.^[142] The Coipasa phase began before 13,300 BP and reached its peak at 3,700 metres (12,100 ft) 12,500 years ago. The Coipasa lake's regression was nearly complete around 11,500 years ago.^[74] A 2013 reconstruction envisaged a lake level rise between 18,000 - 16,500 years BP, followed by a highstand between 16,500 - 15,500 and a decrease in lake levels between 14,500 - 13,500 years BP.^[206]

The formation and disappearance of Lake Tauca was a major hydrological event^[38] that was accompanied by several millennia of wetter climate.^[144] Its formation (and the later Coipasa phase) is associated with the Central Andean Pluvial Event (CAPE) as precipitation increased between 18° and 25° degrees south.^[207] The CAPE chronology is often subdivided into two phases. The first is wetter and begins in the late glacial (17,500-15,900 years ago); the second is drier and continued into the early Holocene (until 9,700-8,500 years ago). Published start and end dates vary by study^[208][209] and in some places it may have lasted past 8,500 years ago.^[210] The dry period separating them^[211] coincides with the Ticaña lowstand. The second phase of the Central Andean Pluvial Event has been subdivided further into a wetter earlier and a drier later subphase.^[212] Moisture sources and seasonality were different during the two phases: The Tauca cycle had a simultaneous increase of summer and winter precipitation and a northerly moisture source; during the Coipasa lake cycle only summer precipitation increased and had a more southerly origin (from the Gran Chaco).^[209][64] The Central Andean Pluvial Event may not have been synchronous between the southern Altiplano and the southern^[213] and northern Atacama.^[214] There were no major century-scale precipitation changes during the Tauca period.^[215]

Many explanations link Lake Tauca to changes in atmospheric circulation during Heinrich event 1,^[38] when cooling in the North Atlantic^[216] caused by a decrease of the Atlantic Meridional Overturning Circulation^[217] and its cross-equatorial transport of heat triggered a southward movement of the ITCZ and a strengthening of the South American Summer Monsoon.^[218][219] Condensational heating in the monsoon strengthened the Bolivian High^[n] and shifted it southward, increasing the transport of easterly moisture into the Altiplano.^[221] Earlier highstands of Altiplano lakes may also correlate to earlier Heinrich events,^[35] although not all Heinrich events led to lake expansions.^[222] Analogously^[o], the Younger Dryas has been linked to the Coipasa stage^[217] and CAPE II.^[224]

Other potential mechanisms:

• Increased cloud cover probably increased the effective precipitation by reducing evaporation rates.^[225]
• The lake may have contributed to increased precipitation by influencing land breezes^[226] and through evaporation, which increased precipitation in the lake centre four-fold.^[90]
• Persistent La Niña climatic conditions may have contributed to the lake's filling^[227][228] and the first CAPE,^[211] as well as facilitating glacier expansions.^[229]
• An intensification and southward shift of the South Atlantic Convergence Zone^[p] may have contributed to the precipitation increase^[231] but not all records agree.^[232]
• Vegetation changes and lake development would have decreased the albedo of the Altiplano, resulting in warming and moisture advection of moisture towards the Altiplano,^[233] but such positive feedback mechanisms were considered questionable in a 1998 study^[234] and might have been offset by larger glacier area.^[235]
• The overflow path from Lake Titicaca into the southern Altiplano was in existence for the last 50,000 years; this might explain why there is little evidence of large lakes in the southern Altiplano before that time.^[236]
• The early Holocene humid period might be linked to the northern hemisphere pluvials forced by the Milankovich cycle or warmer sea surface temperatures in the Southeastern Pacific, which weakened the South Pacific Anticyclone.^[215]

The ideal conditions for the development of paleolakes in the Altiplano do not appear to exist during maximum glaciation or warm interglacial periods.^[237] Insolation rates do not appear to be linked to lake-level highstands in the Altiplano;^[238] the lake expansion occurred when summer insolation was low^[192] although a 2019 study connected an insolation maximum between 26,000 and 15,000 years ago to the Tauca stage.^[216] The pattern of moistening does not support a role of the southern hemisphere westerlies in the Tauca humid period.^[239] A global climatic warming and a northward shift of the monsoon occurred around 14,500 years ago,^[240] increased occurrence of El Niño^[241] and the northward shift of the ITCZ accompanied the Ticaña lowstand.^[186]

There are few reconstructions of how the climate looked before and after the Lake Tauca highstand,^[12] one recent (2020) estimate is of a temperature 3 °C (5.4 °F) lower than today and a mean regional precipitation of 900 millimetres per year (35 in/year)^[9] as precipitation doubled in and around the Arid Diagonal.^[242] Temperature changes were correlated to Antarctic events and precipitation changes to climate oscillations in Greenland.^[220]

The humid period that formed Lake Tauca is associated with a slowdown^[q] of glacier retreat,^[12] or glacier^[r] expansion^[s] or slowdown in the Andes between 24° south latitude^[252] and Hualca Hualca in Peru^[253] during the Late Glacial Maximum.^[254] The equilibrium line altitude of glaciers in the dry Andes decreased by 700 to 1,000 metres (2,300 to 3,300 ft).^[255] The correlation is most clear in southern Bolivia (Tunupa^[33]),^[256] while farther south (Tocorpuri, Llano de Chajnantor, El Tatio) evidence is less^[t] clear.^[260][242] The Choqueyapu II glacier advance in the Eastern Cordillera may be related to the Tauca period.^[179] In the dry climate of the Altiplano, precipitation can play an important role in glacier growth alongside temperature^[u]:^[262] Moisture from Lake Tauca might have enhanced glacier growth^[263][264] at Sajama,^[265] Tunupa^[v] and other mountains^[w] close to the lake.^[262] Lake Tauca may have degraded^[267] or eroded traces of older glaciations.^[268]

During the Tauca phase, Lake Titicaca rose to 3,815 metres (12,516 ft)^[140]-3,825 metres (12,549 ft) altitude.^[269] Water level then dropped during the Ticaña phase and probably rose again during the Coipasa.^[270] The Lake Tauca highstands left terraces at the southern and eastern shores of Lake Titicaca^{[x][271][272]} and, together with older stages of higher water levels, formed the pampas around Titicaca.^[273] This was not the first time Lake Titicaca rose; Pliocene to Pleistocene lake-level rises are known as Mataro, Cabana, Ballivian and Minchin,^[56][274] whose exact chronology is unknown.^[275]

Increased river discharge occurred in the Altiplano-Puna,^[276] Atacama region,^[277] the Parana basin^[278] and rivers draining into the Pacific Ocean^[y].^[279] Titicaca's outflow, the Rio Desaguadero, may have been eight times that of today.^[91] River terraces formed in Andean rivers during the Tauca highstand.^[280][145]

Lakes in the Puna^[281] and Atacama also formed or expanded^[161][282] during the Tauca and the later Coipasa stages.^[283][z] Examples include Laguna Miscanti,^[284] Lejía Lake,^[270] Laguna de los Pozuelos^[167] and Salar de Atacama.^[190] Salt flats experienced more frequent flooding.^[z][209] In Salar Aguas Calientes high-water levels lasted until 8,430 ± 75 BP.^[270] Tauca-era lake level rises occurred from Laguna Mar Chiquita^[219] in the south to Lake Junin in the north^[285] and left recognizable shorelines at many lakes.^{[z][286][169]} Higher lake levels have been found at the same time in other parts of the Altiplano and areas of the Atacama above 3,500 metres (11,500 ft) above sea level,^[287] but not at lower elevations^[aa].^[226]

During the CAPE, major environmental changes occurred in the Atacama,^[207] as vegetation zones moved downward^[279] and new ecosystems formed.^[289] Trees^[ab] and shrubs^[ac] grew along rivers and groundwater discharge areas, providing habitat to animals^[ad], which either no longer occur in the Atacama or went extinct, like megafauna^[ae].^[289] Increased wildfire activity formed the "Pica glass" in the Atacama.^[292] Other effects included increased groundwater recharge,^[z][293] which caused the formation of wetlands,^[294][z] and increased water supply from the mountains to the lowlands.^[295] The well dated record of Lake Tauca has been used to correlate climatic events elsewhere in the region.^[296][296]

Paleoindian settlement in South America commenced during the Lake Tauca and Ticaña stages,^[297] the Viscachani culture around Lake Titicaca was contemporaneous with Lake Tauca.^[298] Human dispersal increases towards the end of the Ticaña phase, while the Coipasa phase coincides with more established settlement^[299] (including in proximity of Lake Tauca^[224]) and the establishment of regional commercial networks.^[300] The more favourable environment during the CAPE,^[279] as oases^[207] and paleolakes formed, provided the environment for first settlers.^[301] The end of the paleolake phase coinciding with Lake Tauca was accompanied by the end of the first phase of human settlement;^[302] now humans left the desert^[303] and population in the Salar de Atacama area decreased.^[212] Some Inca towers on the Altiplano were built using rocks left by Lake Tauca.^[304]

Some fossil water reserves in the dry Andes formed during the Tauca phase,^[305] the groundwater in the northern Chilean Central Valley,^[306] around Peinado in the Puna^[307] and part of the groundwater under Pampa del Tamarugal for example date back to the Lake Tauca wet phase.^[308] Lake Tauca may have supplied water to the Rio de la Plata region, sustaining life there during dry periods.^[104]

The Lake Tauca and preceding cycles left evaporite deposits,^[309] with sediment layers left by the lake in the Salar de Uyuni reaching a thickness of 6 metres (20 ft).^[141] The salts are continually washed out and re-deposited by ephemeral rainfall, causing the salt surfaces of the Salars to become very flat and smooth.^[81] The submergence of a large part of the Altiplano under Lake Tauca reduced the production of dust there and its supply to Patagonia,^[310] but also replenished sediments, increasing dust supply once Lake Tauca dried up.^[311][312] The high aerosol content of the air in the Uyuni region has been attributed to fine sediments left by Lake Tauca.^[32] Diatomaceous deposits containing clay or calc were left behind by the lake,^[89] and ulexite deposits were formed by sediments in its deltas.^[313]

The taxonomic similarity between fish species of the genus Orestias in Lauca National Park and Salar de Carcote has been attributed to these watersheds' being part of Lake Tauca;^[46] in general the evolution of these fish was heavily influenced by the various lake cycles including these that preceded the Tauca cycle.^[314] The drying of the ancient lakes would have fragmented amphibious habitats, generating separate populations,^[315] a similar process affected plants and animals during the lakes' existence.^[316]

• Aucanquilcha, Irruputuncu, Uturunku