Late Pleistocene paleowetlands in semiarid dunefields, Western Pampas of southern South America

TRIPALDI, ALFONSINA; OZÁN, IVANA L.; HEIDER, GUILLERMO; FORMAN, STEVEN L.

Roma

Congreso; XXI INQUA; 2023

International Union for Quaternary Research

Hydromorphic features, like rhizoliths and lithological properties of rocks/sediments, are indicators of special hydrological conditions in past environments, like variations in soil saturation, drainage, water table depths. They can be related to distinct landscapes and/or climatic conditions. The presence of these features in the Quaternary record of today drylands are key elements to infer wetter than present conditions. Among these features, rhizoliths are conspicuous megascopic structures that appear as root traces of massive or concentric-like mineral precipitations. They result from a total or partial replacement of root organic matter and indicate a paleolandscape with a high colonization of vascular plants. The Western Pampean Dunefield of central Argentina (34ºS, 65.3ºW), southern SouthAmerica, is characterized by blowout dunes, several of them with km-long diameters, and showing superimposed smaller dunes over their depositional lobes. This eolian landscape is, at present, mostly stabilized by a savannah-like vegetation cover and agriculture fields. It exposes a record of profuse eolian activity along different periods of the late Pleistocene (~50-17 ka) and during most of the Holocene (~12-1 ka). A today notorious aspect of this dunefield is the hydrological state of the deflation basins of the blowout dunes, while in the eastern flank these basins commonly host shallow lakes, formed during, at least, the last millennium, in the western area the blowout dunes are dry, vegetated landscape. The sedimentary record at some blowout dunes includes hydromorphic features, including carbonate and ferruginous rhizoliths and calcareous crusts. They appear at a paleoshoreline position of current perennial shallow lakes, but also in blowout basins without a vestigeof any kind of wetland during, at least, the last ~100 yr. The integration of geomorphology,sedimentology, micromorphology, and chronostratigraphic data allow defining/interpreting the sedimentary facies hosting the rhizoliths and related deposits, in the context of a dominated eolian landscape, and to discuss the geomorphological, climatic, and ecological conditions which allowed the formation of wetlands (lakes?) in such inland dunefields during, we infer, the deglaciation times.