500-year-record multiproxy paleolimnology study of a shallow pampean lake and GCR flux.

LAPRIDA, C. COMPAGNUCCI, R., CHAPARRO, M., SINITO, A.M., VALERO GARCÉS, B. AND NAVAS, A. M.

Mendoza, Argentina

Simposio; Reconstructing Past regional Climate Variations in South America over the Late Holocene: a new PAGES intiative.; 2006

PAGES

Short sediment cores from Chascomús lake, a shallow lake in the pampas of Buenos Aires province (35º 36"S -50º 00" W) were investigated to reconstruct high-frequency climatic variations for the last 500 years. Our analysis of cores were focalized in changes in sedimentological, paleontological, geochemical and rock-magnetic parameters in order to characterize the physical and chemical paleohydrology and riverine influence on the lake, and allow for the correlation among cores for lake-level reconstruction. The chronology is constrained by AMS 14C dating (/1460 AD at 35 cm depth). As a working hypothesis, we consider a constant sedimentation rate since there is no drastic changes in sedimentary facies. Two pulses of increasing sea-level were recognized related with pulses of fluvial input around 1725 AD and 1880 AD. Clastic sediment flux as indicated by grain size remained minimal during low lake-level stages, when Limnocytehre-dominated assemblages reflects high alcalinity and groundwater input. Thereafter, Cyprideis-dominated assemblages, low TOC content, increase grain size and concentration/grain size magnetic parameters indicate increased fluvial influence to the lake, and the presence of dilute, low salinity waters. We compare the timing of these lake-level fluctuations with the galactic cosmic ray flux (GCR) calculated from Taylor Dome dataset. Low lake levels coincide with intervals of unusually high GCR flux (= low solar activity), probably related with Maunder and Dalton Solar Minima. This allows to infer that lake levels in Argentine pampas contain valuable paleoclimatic information related with high-frequency climatic variability.