Evidence of Holocene climate and sea-level change in palynological, 13C, and 18O records from an alluvial sequence at Arroyo Claromecó, Buenos Aires

PRIETO, A.R.; VILANOVA, I.; HAJ, A.; BETTIS III, A.; CLAUDIO GERMAN DE FRANCESCO

Bahía Blanca

Simposio; 13° Simposio Argentino de Paleobotánica y Palinología; 2006

Palynological and gastropod isotopic records (d13C and d18O) from an alluvial sequence from Arroyo Claromecó (38º50´S; 60º05´W) illuminate changes in vegetation and paleoenviromental conditions during mid-late Holocene. Both d13C and d18O records show a similar trend and support major environmental changes reflected in the palynological data. Between 6950 and 6540 yr BP sedges, associated with halophytic vegetation, suggest the development of extensive, shallow, fkshwater-brackish wetlands. Light d13C and d18O values suggest humid conditions. At 6540 yr BP Poaceae-Chenopodiaceae communities replaced sedges and phytoplankton developed indicating deeper and more extensive brackish wetlands. These conditions prevailed until ca. 6000 yr BP. During this period, d13C variability is high and d18O values have a positive trend (0 to +2%) related to the influence of marine-brackish water associated with the Holocene sea-level highstand. The presence of Heleobia australis d´orbigny at this time offers further paleontological evidence of tidal influence. Between ca. 6000 to 5300 yr BP fluctuations in both Poaceae-Chenopodiaceae proportions and Botryoccocus, Spirogyra and acritarchs values suggest the lateral extent, salinity, and chemical conditions of wetlands were highly variable. The trend to heavier d13C values and the shift of d18O to + 2% indicate a highly evaporative environment, but marine influence cannot be dismissed. From 5300 to 2500 yr BP the highest fluctuations in the palynological spectra and isotopic values suggest an unstable environment. After 2500 yr BP, the presence of halophytic and psammophytic vegetation and lighter and variable d13C and d18O trends reflect the wetlands desiccation. Contribution to Project "Comparison of the Late Glacial and Holocene climate, vegetation, and landscape change between the Argentinean Pampas and the mid-continent of North America". PIP CONICET 5667105.