Assessing the last “large scale fluvial modification” across the hyperarid Atacama Desert, northern Chile

Abstract

During the last decade, a consensus has been widely established about an Early Miocene onset of hyperaridity characterizing the Atacama Desert located in the Andean forearc region in northern Chile. Prevailing hyperarid conditions were interrupted by many pluvial episodes of varying duration and spatial extent. In contrast to the onset of hyperaridity, highly discrepant interpretations persist regarding the last “large scale fluvial modification” (Ritter et al., 2018) of the Atacama Desert. While terminal aggradation of the coastal alluvial fans is constrained at several sites to the Last Glacial Maximum (MIS 2), followed by Holocene progradation to the shore, chronological constraints for last major fluvial activity throughout the inland desert scatter between the Late Miocene and Middle Pleistocene. The types of investigated sedimentary archives differ and many studies have a local focus. Instead, a systematic assessment from the hyperarid coast (W) to the footslopes of the Precordillera (E) is yet lacking. We therefore mapped 84 alluvial fan systems with small (<25 km²) source areas along a latitudinal transect at ~21°S. The last-abandoned, widely distinguishable fan surface generations (S1) and subrecent incised channels of five alluvial fan systems was further systematically sampled for 10Be cosmogenic nuclide exposure dating of surface pebbles. Morphometric results indicate a significant positive dependence of both fan area and fan slope on the catchment area and can further be interpreted as the result of (palaeo)climatic effects on the fan systems. Initial 10Be exposure dating reveals Middle Pleistocene terminal aggradation of the S1 fan generation, confirming the younger interpretations of the last “large scale fluvial modification” of the Atacama Desert. Forthcoming additional 10Be results will provide further insights into the function of the alluvial fans as buffers in the sediment cascade under prevailing hyperaridity, spatial patterns in Pleistocene fan morphodynamics and implications for a potential spatio-temporal gradient in (palaeo‑)precipitation.ReferenceRitter, B., Stuart, F.M., Binnie, S.A., Gerdes, A., Wennrich, V., Dunai, T.J. (2018). Neogene fluvial landscape evolution in the hyperarid core of the Atacama Desert. Scientific Reports 8, 13952. doi:10.1038/s41598-018-32339-9