Ouki

Ouki is an ancient lake in the Bolivian Altiplano. Its existence was postulated in 2006 by a group of scientists which had subdivided the Lake Minchin lake cycle in several subcycles. The Lake Minchin cycle had been previously identified in 1904 as a now disappeared lake in the central Altiplano. Sediments attributed to Lake Minchin may be part of Ouki instead. The dating is uncertain, with radiocarbon and uranium-thorium dating yielding different dates spanning the time between 28,200 and 125,990 ± 9,580 years ago.

Whether Ouki existed is a subject of controversy. In 2011, scientists claimed that the lake did not exist outside of the Lake Poopo basin. The formation of Ouki is associated with a major glaciation and was probably caused by increased precipitation, which has also been observed elsewhere.

Ouki reached a water level of approximately 3,735 metres (12,254 ft), and it may have covered the Salar de Uyuni, the Salar de Coipasa and the Lake Poopo area, although the Uyuni and Coipasa basins may have been filled by a separate lake Salinas instead. Estimates of the surface area depend on the assumed lake levels and whether the lake covered only the Lake Poopo basin or also the Uyuni/Coipasa basins. It ranges 10,400–46,500 square kilometres (4,000–18,000 sq mi).

The Ouki lake is one of the so-called deep lake cycles, along with Lake Tauca, from which it is separated by a period where water levels dropped below 3,700 metres (12,100 ft). It has left characteristic tufa deposits which have shapes of inverted cones. The depth of the lake did not exceed c. 80 metres (260 ft). The subsequent Salinas lake cycle may simply be a shrinking stage of the Ouki lake. Likewise, the existence Lake Minchin may be in part based on misattributing deposits left by Ouki. The Ouki lake cycle may be subdivided into individual phases in the future.

The Ouki lake was populated by species such as Pisidium bivalves, ostracodes and the Biomphalaria andecola snail. Waters had a high concentration of Sr. While one model inferred from strontium isotope data assumes that most of the Ouki water was contributed by the Poopó basin, another assumes a 69% contribution by waters from Lake Titicaca. Waters from the Poopó basin would have spilled into the Uyuni/Coipasa basin. The drying of the lake left deposits containing amphibole, illite, plagioclase feldspar, potassium feldspar, kaolinite, pyroxene, quartz and smectite.

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