Abstract
Laguna Pastos Grandes (Bolivia), nesting in a volcanic caldera, is a large, palustrine-to-lacustrine system fed by meteoric and hydrothermal calco–carbonic fluids. These different fluid inputs favor a complex mosaic of depositional environments, including hydrothermal springs, pools, and an ephemeral lake, producing abundant present-day carbonates developing over a Holocene carbonate crust dated by U–Th. Present-day carbonates (muds, concretions, and microbialites) recorded a large range of isotope variations, reaching 13.9‰ in δ13C and 11.1‰ in δ18O. Sedimentological and geochemical data indicated that the main processes influencing the isotope record were: (i) rapid CO2 degassing and temperature decreases along hydrothermal discharges; (ii) strong evaporation favored by the arid high-altitude Andean climate, locally enhanced by capillary water rise within microbial mats or by wind-induced spray falling on vadose concretions. Unlike past or present perennial lake systems in Central Andes, the short residence time of brine waters in the ephemeral central lake prevents enrichment of lacustrine carbonates in 13C and 18O. The very low fraction modern F14C in these present-day carbonates demonstrates that incorporation of fossil magmatic carbon related to the volcanic context also prevents any radiocarbon dating. The use of isotopes for the interpretation of ancient continental series should always be accompanied by a thorough characterization of the environmental setting.
Highlights
In continental settings, carbonate mineralization can occur in many environments, fed by different types of parent waters
Laguna Pastos Grandes can be divided into two main domains with: (i) the upper and lower palustrine carbonate platforms covered by hydrothermal springs and pools [14]; (ii) a shallow, ephemeral saline lake (Figure 2A; Figure S1)
Laguna Pastos Grandes is a palustrine-to-lacustrine site developing in a quiescent volcanic caldera with abundant ongoing carbonate sedimentation, diversified macrofacies, and several parent waters
Summary
Carbonate mineralization can occur in many environments, fed by different types of parent waters. The Central Andes (South America, Andean Cordillera) is a region where the absence of long series of peat, continental ice, or speleothems, encourages the use of lacustrine carbonate series to reconstruct climatic and hydrological changes during the Quaternary. The most remarkable example of ancient lakes in the Central Andes is the Altiplano Basin, where several lake highstands, covering a large part of the watershed, occurred in wet periods during the Late Pleistocene. These lake highstands have been identified by the presence of abundant calcitic bioherms that mark paleoshores, several meters above the modern shoreline of Salar de Uyuni [7]. Previous studies documenting δ18 O and δ13 C in carbonates from ancient Andean lake deposits provide evidence of changes in the hydrological
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