Microstratigraphy and palaeoenvironmental implications of a Late Quaternary high-altitude lacustrine record in the subtropical Andes

High-mountain lake records in semiarid foreland settings, such as the central Andes of North-western Argentina, are highly restricted and often deprived of well-preserved microstratigraphic information to analyze palaeoenvironmental changes and their causes, particularly for periods prior to the Last Glacial Maximum. Laguna La Salada Grande (23 degrees S/65 degrees W, 4063 metres above sea-level) is a closed shallow lake located at Cordillera Oriental, North-western Argentina with a unique depositional record, including geomorphic and stratigraphic evidence of palaeoenvironmental changes since the Late Pleistocene. In order to understand the depositional dynamics of this mountain lacustrine system at different timescales, limnogeological multiproxy analyses together with a radiocarbon and Pb-210-based chronology were applied on massive and laminated sediments from La Salada Grande. Laminated deposits were further analyzed using novel sub-centimetric mineralogical, textural and geochemical automated methods (including a combination of micro-X-ray fluorescence and quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN (R)). Thick laminated microfacies at the beginning of the sequence record a deep and organic matter productive palaeolake prior to ca 34 ka (1 ka = 1000 years before 1950), that changed into a highly fluctuating shallower-palaeolake with frequent detrital influxes after ca 34 ka. Microstratigraphy of the coarser-grained detrital laminae between 31 ka and 25 ka reveals irregular and probably sub-centennial high-energy discharge events, pointing to convective atmospheric activity as the main trigger. After ca 21 ka the lake evolved to deeper water-level conditions resulting in fine-grained deposits with limited delivery of coarse-grained sediments, followed by an abrupt lake level drop and incision. The multi-millennial palaeoenvironmental reconstruction of La Salada Grande, and its correlation with other palaeoclimate records, shows a close link between the lacustrine depositional processes and palaeoclimate changes associated with the South American summer monsoon dynamics. This multifocal research in such an understudied environment provides key knowledge about lacustrine functioning and discharge events-climate interactions of mountain lakes of semi-arid climates.

Automated summary

This study analyzed the sediment and stratigraphic records of Laguna La Salada Grande in the central Andes of Northwestern Argentina to explore the paleoenvironmental changes and their causes. The results showed that the lake experienced a phase of deep water with organic matter enrichment since the Late Pleistocene, followed by a highly fluctuating shallower phase with frequent detrital influxes. Irregular high-energy discharge events, possibly triggered by convective atmospheric activity, were observed between 31,000 and 25,000 years ago. Furthermore, the study found a close link between the lake and the paleoclimate changes associated with the South American summer monsoon dynamics.