Holocene evolution of a floodplain wetland in the dryland piedmont of central-west Argentina

Abstract

In arid central-west Argentina, South America, many wetlands have developed in association with rivers draining the Andean piedmont and are vital hotspots for resources. However, knowledge about their long-term evolution is generally scarce. The Bañados del Atuel wetland, a low gradient fluvio-aeolian plain linked to the Atuel-Diamante fluvial system, is analyzed to depict its geomorphological and sedimentological Holocene evolution. The study area comprises a fluvial fill terrace with fine-grained alluvial deposits (~ 4186?4419 cal years BP), deposited by a palaeodistributary fluvial system, that is covered by SW?NE oriented aeolian dune complexes. The present-day wetland, which developed after formation of the fill terrace, is characterized by: (1) a NW?SE oriented floodplain with distributary channels and fine-grained, massive to laminated deposits of low organic matter content, dated to 2755?2864 and 729?895 cal years BP, and includes SW?NE oriented dune systems and salt flat depressions; and (2) a transfer area with active headcutting channels, entrenched in the fill terrace, that connects the NW?SE oriented floodplain with (3) a saline endorheic depression (salt lake) with active deflation. The fill terrace distribution suggests much more extensive floodplain environments prior to the late Holocene; the present-day wetland is not older than the last 2?3 millennia and records a late Holocene trend of floodplain size reduction. Fluvial processes of avulsion were likely driven by mid and late Holocene El Niño Southern Oscillation (ENSO) events. Late Holocene arid conditions favoured aeolian dune formation and fluvio-aeolian interactions. Furthermore, anthropogenic river modifications, starting ~ 200 years ago, also have promoted severe changes in the Atuel-Diamante fluvial system, deepening the aridity in the wetland.