Terracettes in the hyperarid Atacama Desert – fog-driven landforms of Holocene age?

Terracettes are quasi-contour parallel step-like microtopographic features consisting of repetitious platform-type benches and slope-type risers and are documented from hillslopes in a range of climates. While a number of studies emphasize their formation by trampling of livestock and grazing animals (cat steps or stock trails), it has been shown that terracette formation may be explained by a number of natural processes, including solifluction or freeze-thaw processes, slumping, soil creep, or vegetation control. Despite this variability and the controversy about their origin, these micro-terraces may alter hillslope soil moisture and vegetation patterns, infiltration and surface hydrology, as well as downslope sediment flux, potentially disconnecting downslope conveyance processes of surface runoff. Given the process mechanisms discussed in these previous studies, the extremely hyperarid climate of the central Atacama Desert in northern Chile may be regarded as unfavourable for terracette formation; here, livestock and grazing animals are absent, moisture availability is extremely limited, and frost processes in elevations below ~1000 m asl are rare. Nevertheless, here we report on terracette-covered slopes in the central Atacama Desert located close to the Rio Loa canyon in the Coastal Cordillera that represents an important inland pathway for coastal fog. Based on sedimentological, geochemical (e.g., micro-X-ray fluorescence) and geomorphological investigations, thin section analyses, UAV-derived aerial photos, soil moisture monitoring as well as post-infrared infrared stimulated luminescence (post-IR IRSL) dating, we present geomorphological, chronostratigraphical and soil hydrological characteristics of the terracettes and discuss potential drivers of terracette formation. Our observations suggest a combination of wind and fog-related moisture supply, particularly during several day-long periods of sustained high relative humidity and fog occurrence, as the key driver for terracette formation, adding to the various processes discussed in previous studies. Post-IR IRSL dating of terracette platform sediments suggests a late Pleistocene to Holocene formation of the terracettes, thereby illustrating the role of fog in driving hillslope dynamics and shaping the desert landscape in the Atacama under past and present hyperaridity.