Late Pleistocene-Holocene paleoenvironments and paleoclimate of a subtropical distributive fluvial system at the eastern Andean piedmont of South America

TRIPALDI, ALFONSINA; MEHL, ADRIANA; HESSE, PAUL

Roma

Congreso; XXI INQUA; 2023

International Union for Quaternary Research

The eastern Andean piedmont of southern South America shows several distributive fluvial systems (DFS) formed by antecedent rivers that carry seasonal meltwater from the Andes Cordillera. One of them is formed by the Atuel and Diamante rivers after crossing the San Rafael tectonic block in deep canyons. Ongoing projects are studying the landscape and deposits of this DFS to reconstruct the late Quaternary landscape evolution, provide paleoclimatic inferences, and improve DFS facies models. Methodology includes geomorphologic mapping by remote sensing, field survey, facies analysis of pedosedimentary successions, coring and multiproxy analysis of wetlands, luminescence and AMS chronology. The fluvial dynamics is closely related to aeolian processes as the Atuel-Diamante DFS is under semiarid condition (rain shadow of the Andes Cordillera). The megafan is bounded by dunefields, and the fluvial landforms and deposits are modified and reworked by wind activity. Today, the Atuel-Diamante DFS is a ‘misfit’ fluvial system due to current interglacial conditions and anthropic intervention on the drainage network. In 1886 the Diamante river waters were artificially diverted to the east, later, in the last 70 years, hydroelectric power dams affected the fluvial regime of their lower basins.Also, the proximal part of the DFS was turned into an agricultural irrigated oasis, by introducing fluvial networks to supply water for irrigation, and soils became strongly salinized. Several landscape features reveal variable geomorphological and climatic conditions in the late Pleistocene-Holocene, like channel incision and fill terraces,wider paleochannels than present channels, a profuse aeolian cover over most the DFS, among others. The chronology shows a late Pleistocene-early Holocene record in an upper-most, 8 m thick, fluvial terrace, suggesting important fluvial aggradation during the last glacial termination and a significant change in the system with extensive incision at the early-mid Holocene. OSL ages show late Pleistocene dune formation and successive stages of dune building in the Holocene. These are associated with fluvial channels and surfacesformed and successively abandoned throughout the Holocene. Although the sand dunes are currently wellvegetated, our record shows that Holocene climate fluctuations have been sufficient to change the hydrological balance, activating sand dunes and diminishing river discharges.