Geomorphological activity and stability of surfaces and soils formed in hyperarid alluvial deposits (Atacama Desert, Chile)

Abstract. The hyperarid conditions of the central Atacama Desert, characterized by extremely low precipitation and high evaporation rates, create a unique environment where geomorphic stability is generally considered to be exceptionally high. Terrestrial cosmogenic nuclide-based surface exposure ages suggest that many surfaces underwent limited to no geomorphic changes since the Neogene or early Pleistocene. However, a number of recent studies reveal more recent landscape-scale geomorphic activity and link this to slightly wetter episodes during the Quaternary. In order to determine drivers of geomorphic activity, we performed a multi-proxy analysis of five profiles situated in alluvial deposits along a climatic transect from the coastal plain to the upper reaches of the Coastal Cordillera (~ 0–2000 m a.s.l.), combining single-grain feldspar luminescence dating with geochemistry, granulometry, and field observations. Alluvial deposits are prone to heterogeneous bleaching; therefore, we tested the bleachability of the feldspars and found a shallow effective bleaching depth (< 2 cm) and high near-surface bleachability. The five profiles studied could be grouped into geomorphological active and stable sites, based on our multi-method approach. Profiles located on geomorphologically active surfaces show evidence of recent sediment deposition and, in one case, vertical grain transport. In contrast, stable surfaces preserve reworking signals relating to bioturbation at the coast and desert pavement formation in the hyperarid Coastal Cordillera. While no clear chronological trend along the west–east climatic gradient could be found, two phases of widespread geomorphic surface activity – ~ 50 ka and during the last ~ 5 ka – coincide with regionally wetter intervals compiled from other studies. Our findings highlight the value of single-grain luminescence data for reconstructing local sediment dynamics and soil reworking in arid environments, and the need to account for both depositional and post-depositional processes in paleoenvironmental interpretations.