Early and middle Holocene floodplain environment and vegetation dynamics at the Atuel - Diamante distributary fluvial system, Mendoza, Argentina

Abstract

The Atuel - Diamante distributary fluvial system (AD - DFS; ∼34.5–37.5°S, 66.5–68.5°W) is a late Quaternary major landscape feature in the distal dry Andean piedmont of Argentina. A widely extended fluvial terrace in the proximal setting of the fluvial system records the Early and Middle Holocene (E − MH). Considering that climate reconstructions from central-western Argentina have suggested an arid Middle Holocene scenario in the region, three main objectives were proposed to be achieved based on the analysis of the AD - DFS fluvial terrace, 1) to infer E − MH palaeoenvironments, 2) to disentangle the E − MH vegetation dynamics and 3) to contribute to the understanding of control factors at the AD - DFS during the E− MH Holocene. We present results of sedimentary successions logging and description, and lithofacies and palynological analyses constrained by radiocarbon dating at four localities at the AD - DFS proximal setting. Sedimentary analysis reveals an E − MH fluvial aggradational plain recording mainly floodplain environments and secondarily fluvial channel elements. The floodplain is interpreted as an unconfined vertical-accretion sandy floodplain that shifts to a confined cut and fill floodplain type after a fluvial incision likely at the late Middle Holocene. The fossil pollen records a fluvial system with major water availability at the earliest Early Holocene (EH). Then, a major environmental change after 10,230–10,487 cal yrs. BP is marked by a deficit in water availability affecting the algae, macrophytes and riparian communities (Poaceae). After 9133–9535 cal yrs. BP, and before 7163–7422 cal yrs. BP, the Monte and Monte-Espinal elements increased and reached the highest values of Prosopis (60–80%) linked to low values of Poaceae and Ephedra spp. This condition is hypothesized to be associated with driest and warmest conditions during the MH in the Andean piedmont. The reconstructed paleoenvironmental scenario, higher flow at the fluvial system at the earliest EH and aridity in the piedmont during the MH, is in agreement with the regional climatic framework established for the Andean piedmont region.