Microstratigraphic insights of a mountain closed-lake: A Late Pleistocene laminated record of the Central Andes, Northwestern Argentina

GUERRA, LUCIA; MARTINI, MATEO A.; HAJDAS, IRKA; ARIZTEGUI, DANIEL; DE HALLER, ANTOINE; VOGEL, HANDRIK; MOSCARIELLO, ANDREA

Dublin

Congreso; INQUA Congress 2019; 2019

International Union for Quaternary Research (INQUA)

Rhythmical laminationis preferentially formed in deep lakes with small surface areas without bioturbationin wind protected basins. A contrasting setting characterizes present mountain lacustrinebasins of Eastern Cordillera, at the Argentinean Central Andes. In thissemi-arid area, seasonal (December-March) precipitation is below 400 mm yr-1,mostly supplied by the South American summer Monsoon (SASM). Under theseclimatic conditions few shallow ephemeral lakes can be found at high-altitudes(˃4000 m a.s.l.). Laguna Salada Grande (LSG) (23ºS/65ºW, 4100 m a.s.l.) is asmall (5.8 ha) shallow (approximately 1 m depth), intermittent closed-lake. Thepresence of a laminated section from ~47 14C kyr and ~22.3 calkyr BP underlying massive sediments, together with paleoshorelines situatedseveral meters above the present lake level, provided the basis for ahigh-resolution study of the Late Pleistocene paleoenvironment in this area. U singhigh-resolution techniques, the laminated section of a short core has beenanalyzed in detail with magnetic susceptibility, XRF scanning and micro-XRFmapping, and dated with AMS. The application of automated scanning electronmicroscope, QEMSCAN?, in impregnated thin sections was explored to picturedifferential mineralogical distributions. Grain size analysis in discretelaminae have shown that mixed laminae are formed by alternation of silty sands withcarbonate shells, white carbonate muds, siliciclastic (illite) clays and yellowochre Fe-rich clays.The elemental mapping of Ca/ Fe ratios indicates changes indetrital input to the lake pointing towards runoff, slope wash and rainfallprocesses controlling the development of lamination. Biogenic sediments alsoindicate seasonal changes in productivity. Ochre laminae indicate variableredox conditions and subsequent diagenetic processes. The combination of XRFgeochemistry, mineralogy and image analysis at different scale resolutionallows establishing a high-frequency event stratigraphy for this high-altitudelake system during a deep perennial paleolake stage. Differences within thelaminated sequence suggest a shift at ~34 ka from productive conditions in arelatively deep perennial productive paleolake with water column stratificationto more variable lake-stands in a smaller paleolake. These stages can beregionally correlated in time with Late Pleistocene wet cycles on theAltiplano/Puna plateau tied to the SASM activity.