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Sajsi is the name of an ancient lake in the Andes

The existence of ancient lakes in the Altiplano was proposed as early as 1882. At first, a Lake Minchin was identified with ages of 30,000 years or older; later additional lake cycles were recognized.[1] The minor ones include Inca Huasi, Salinas and Coipasa. The major ones include Lake Tauca and Ouki.[2] Because of this, it was proposed that Lake Minchin was actually a combination of several different ancient lakes.[3]

This lake occupied the area of the current Salar de Uyuni;[4] it also covered the Salar de Coipasa but it is not clear whether and how it extended in the Lake Poopo.[5] Its waters reached altitudes of 3,670 metres (12,040 ft) and its depth did not exceed 17 metres (56 ft).[6] A sample dated 23,700 ± 2,600 years ago by uranium-thorium dating indicates that lake levels at that point were about 15 metres (49 ft) deep in the Uyuni basin.[7] Water levels subsequently decreased to less than 5 metres (16 ft) above the present day levels before Lake Tauca formed,[8] however both the timing and the history of water levels are uncertain.[9] The surface area of the lake may have been 21,000 square kilometres (8,100 sq mi) during the highstand.[10] By 19,900 ± 900 - 18,700 ± 200 years ago, the Lake Tauca was beginning to form.[7] The so-called "L2" unit in drill cores in Salar de Uyuni may correspond to both the Sajsi and the later Lake Tauca cycle.[11] Strontium isotope data indicate that about 41% of the water in Sajsi came from Lake Poopo and 4% from Lake Titicaca.[12]

Radiocarbon dates have been obtained for Sajsi-age ooids and tufa, uncalibrated they range from 17,080 ± 720 to 20,830 ± 140 years ago.[13] Later dates indicated that the lake existed between 25,000 and 19,000 years ago and reached its maximum depth 23,000 years ago.[7] The existence of this lake coincides with the Last Glacial Maximum. Earlier, lakes had formed in the Laguna Blanca, the Salar de Atacama,[4] as well as the Pozuelos Basin in northwest Argentina.[14] Given evidence from the Bolivian Eastern Cordillera[15] and the small size of the glacial Sajsi and Inca Huasi paleolakes, it is likely that the Last Glacial Maximum was accompanied by a dry climate on the Altiplano[8] and indeed climate modelling shows that only a small precipitation increase - or none at all - would be needed to create the Sajsi lake.[16] Glacier expansion is recorded at that time in Northwest Argentina.[17] The second Heinrich event seems to coincide with the Sajsi lake period.[18]

A maximum in local insolation about 21,000 years ago coincides with the existence of the Sajsi lake but was probably not responsible for the lake's existence.[19] Farther south, precipitation in the drainage area of the Rio Salado had increased by 10 millimetres per year (0.39 in/year) during the Sajsi time,[20] lakes formed within the Western Cordillera[21] and the Bolivian Chaco likewise shows evidence of increased precipitation.[22] The Sajsi lake was apparently followed by Lake Tauca, but evidence is lacking.[23] Another theory postulates that Sajsi was simply a sub-phase of Lake Tauca,[24] an interpretation applied in particular to data taken from drill cores.[25]

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