210Pb sediment profiles and geochronology in shallow lacustrine systems under marked hydrological variability in the Argentinean Pampas

CÓRDOBA F.; PIOVANO E.; GUERRA, L.

Ushuaia - Tierra del Fuego

Congreso; VI Congreso Argentino de Cuaternario y Geomorfología.; 2015

Asociación Argentina de Cuaternario y Geomorfología

High-precision dating and accurate chronological models are essential to interpret the record of Late Quaternary environmental changes. Particularly, the success in performing consistent reconstructions based on lake sediments primarily depends upon factors such as the development of reliable age models in order to calibrate the recent multi-proxy record against instrumental data. 210Pb radiochronology is a widely-used technique for dating sediments spanning the past 100-150 years (Jones et al. 2009, von Gunten et al. 2012). We report here the results of detailed 210Pb analysis of two sequences of lake sediments from Lagunas Encadenadas del Oeste (LEO), southern Argentinean Pampas (37°S, 62°W). Non-exponential and non-monotonic 210Pbuns depth-profiles in the sedimentary record of LEO system indicate the interaction of complex processes behind radionuclide fluxes and lake sedimentation. The chronology of depositation in the lakes has been modeled using different 210Pb-based mathematical models: a) Constant Rate of Supply (CRS), b) Constant Initial Concentration (CIC), c) Constant Flux Constant Sedimentation rate (CFCS) and d) Sediment Isotope Tomography (SIT). The use of independent chronostratigraphic markers along the sedimentary records were critical to assess and validate derived ages and therefore to select the most appropriate model to establish the chronological framework of the most recent sediments of the LEO system. The southwestern Argentinean Pampas, where the lake system is located, is characterized by a marked precipitation variability pattern during the period AD 1888-2008. The instrumental record, for instance, indicates the beginning of a wet phase after the 1970s, while a pronounced lake level drop began in year AD 2003. Rainfall variability is assumed to be an important factor in controlling the direct atmospheric fallout of 210Pbuns onto the lake surface, as suggested by the significant correlation between precipitation and 210Pbuns activity along cores. Non-steady 210Pb flux along time suggests that the use of classical numerical models (i.e., CRS, CIC, CFCS) to derive chronologies should be carefully considered (Córdoba et al. in review). Since no assumptions need to be established, the SIT-model appears to be an adequate technique for deriving ages in the LEO system and other lacustrine systems in the Argentinean Pampas. These results highlight that hydroclimatic data and sedimentary features must be carefully analyzed in order to develop a reliable 210Pb chronological framework along sediment records from shallow lakes under high inter-annual and inter-decadal precipitation variability.