Quantifying processes in Earth’s Critical Zone in the Atacama Desert by combined luminescence and sedimentological approaches

The central Atacama Desert is generally considered the driest non-polar desert on Earth. Given the fog-related moisture availability along the northern Chilean Coastal Cordillera and the increasing precipitation towards the Andean Cordillera, it is an ideal area to explore the transition of biotically to abiotically driven subsurface soil processes (e.g., soil turbation and weathering) in the Earth’s Critical Zone (ECZ). So far, no geochronological framework exists for these subsurface soil processes, and the factors controlling these processes are still unknown. Here we combine feldspar single grain luminescence dating with detailed sedimentological and geochemical analyses to improve our understanding of factors, processes, and time scales involved in subsurface soil processes in the Atacama Desert, ultimately contributing to decipher geomorphodynamics and landscape evolution under hyper arid conditions. While single-grain luminescence dating has successfully been applied to infer sediment transport and mixing processes in various geological settings [e.g., Reimann et al., 2017], luminescence dating of Atacama Desert sediments has proven to be challenging and time-consuming. It has been shown that establishing a standardised growth curve (SGC) for single-grain feldspar post-infrared infrared stimulated luminescence measurements reduces the measurement time considerably [Li et al., 2018]. In this regard, we previously showed that SGCs are only suitable for Atacama Desert feldspars if special modifications are made [Maßon et al., under review].

Based on a combination of the sedimentological and geochemical analyses of samples from nine sediment profiles of 35-180 cm depth, four dust traps and luminescence dating techniques using the modified SGC approach of Maßon et al. [under review] we explore biotic and abiotic subsurface soil processes in the ECZ along two W-E-oriented climatic transects in the north and south of the central Atacama Desert. The northern transect focusses on the transition from the Central Depression to the Precordillera, where biotic components in the ECZ increase with increasing humidity and elevation. The southern transect reflects the transition from the fog-influenced Coastal Cordillera to the Central Depression, where biotic components in the ECZ decrease with decreasing fog-frequency and increasing elevation. We present first results of the successful combination of the refined SGC method from Maßon et al. [under review] and detailed sedimentological and geochemical results to disentangle and infer the processes and rates of sediment deposition and surface evolution, as well as post-depositional subsurface soil processes in both transects. Furthermore, we test if different soil formation processes (e.g., incorporation of aeolian dust vs. in-situ weathering), identified using a combination of sedimentological and geochemical analysis of samples from both sediment profiles and dust traps, can be traced by specific luminescence vs. depth fingerprints. Our preliminary results indicate surface and soil activity during the Late Pleistocene and Holocene even in the abiotic sections of the transects. This suggests that Earth surface dynamics and soil processes such as vertical particle transport and the incorporation of aeolian dust in the most hyperarid parts of the Atacama Desert – virtually independent from flowing water and plant activity - are more active than previously expected, although acting on long time and subtle spatial scales.