Résumé
During the last deglaciation, on the Bolivian Altiplano ( 16 S), a wide paleolake covering at least 51,000 km2, named lake Tauca reached its maximum highstand between 16.5 and 15 ka. Overlooking this site, an ice-core from the Sajama ice-cap (covering the last 25,000 years) evidenced an oxygen 18 isotopic excursion of +7 ppm matching with the end of the Tauca phase, and more pronounced by about +5 ppm compared with the neighboring Andean ice-cores isotopic records. Here we i) provide a new and original experimental use of lacustrine diatoms (n=21) at medium resolution ( approximately 300 years) together with ostracods (n=4) for isotopic delta 18O paleolake water reconstruction (delta 18Olake), ii) detail a simple hydro-isotopical model with constraints given by literature to explain the strong features in the delta 18Olake signal and iii) explore whether the Tauca paleolake could contribute as a moisture source to precipitation at Sajama site when it disappears at 14.2 ka. Based on a new chronostratigraphy, the sedimentary sections cover lake Tauca phase ( approximately 18.7-14.1 ka) and Lake Coipasa phase ( approximately 12.6-11.7 ka). On centuries time scale, strong features consistently appear in delta 18Olake: an abrupt decrease during lake filling phases immediately followed by an increase during lake level stable phases. The highest variation occurred at approximately 15.9 ka with a delta 18Olake fall of about approximately 14 ppm concomitant with lake Tauca highstand, and followed by a delta 18Olake increase of a similar amplitude four centuries later. We also show that this unexpected re-enrichment of delta 18Olake can be partly explained with a simple hydro-isotopic model, based on Craig and Gordon's model, with coherent constraints by a re-equilibration of isotopic fluxes in lake steady-state. Based on an evaporative lake model and a simple water stable isotopic balance between two potential moisture sources for Sajama precipitation (eastward advected moisture and lake evaporation), we show that total or partial (from 5 to 60%) evaporation of the lake during the Tauca regression phase could explain the extra 5 ppm Sajama isotopic excursion. These results suggest that local hydrological cycle of lacustrine areas could substantially affect the interpretation of near-by archives of past isotopic composition of precipitation (e.g ice cores or speleothems). [Copyright Author(s) 2013. CC Attribution 3.0 License: https://creativecommons.org/licenses/by/3.0/legalcode]