Dating of paleolakes in the central Altiplano of Bolivia

The deposits of the extensive paleolakes of the Bolivian Altiplano correspond to an alternation of lacustrine episodes and dry periods, a phenomenon commonly explained by climatic changes. The lack of precise dating of the deposits has prevented detailed reconstruction of the evolution of paleolakes that would allow a determination of the relative influence of interconnections between neighboring basins and local climatic conditions. Among the five paleolakes of the northern Altiplano (Titicaca basin) and the three of the central Altiplano (Poopó, Coipasa and Uyuni basins) only the two most recent paleolakes have been dated. m long sediment core recovered from the Salar de Uyuni contained 11 lacustrine layers (L1–L11) separated by 12 salt crusts. Radiocarbon dating shows that the younger layers L1 and L2 are related to the Tauca lacustrine phase (12 000–16 000 cal yr bp) and layers L3 and L4 to the Minchin lacustrine phase (30 000–73 000 cal yr bp). Layer L5, located at 46 m below the surface, contains a volcanic ash with well-preserved biotites is here dated by the 40Ar/39Ar method. The measured age of 191 000±5000 y is the first date ever obtained from a pre-Minchin paleolake (Lake Escara). This result combined with 14C and U/Th data obtained on lakes Tauca and Minchin from various authors suggests that the duration of the lacustrine events increased regularly during the Pleistocene. In contrast, paleolake levels in the northern Altiplano decreased from the oldest known (Lake Mataro, 3950 m) to the present Titicaca level (3806 m). A progressive erosion of the threshold between the northern and central Altiplano may have allowed important volumes of water to overflow from the northern basin into the central Altiplano. The existence of large paleolakes in the central Altiplano was thus not only dependent on local climatic conditions but also on additional inputs from the north, where climatic conditions could have been quite different from those prevailing in the south.