Activity and stability of surfaces and soils in the Atacama Desert, Chile

The hyperarid conditions of the central Atacama, characterized by extremely low precipitation and high evaporation rates, create a unique environment where soil stability is generally thought to be exceptionally high due to the widespread gypsum and salt enrichment. Terrestrial cosmogenic nuclide-based surface exposure ages suggest that many surfaces underwent limited to no changes since the Neogene or early Pleistocene. However, a number of recent studies also underline the younger landscape-scale geomorphodynamic activity, as evidenced by e.g., the incision of the Rio Loa canyon during the late Pleistocene, or by growth of calcium-sulphate wedges and associated patterned grounds in the Central Depression at the onset of the Holocene. Despite this discrepancy, there is a limited understanding of past and present soil dynamics under this extreme hyperaridity, including subsurface turbation processes driven by both biological and salt dynamics (bioturbation, haloturbation). So far, no geochronological framework exists for these important subsurface soil processes, and the factors controlling these processes are still unknown.

Our study aims at providing new insights into the dynamics of subsurface soil processes in the hyperarid Atacama Desert. We use feldspar single grain luminescence dating techniques combined with sedimentological and geochemical analyses to decipher the activity or inactivity of soil material conveyance processes. We present results from investigations of four soil profiles. All profiles are situated in alluvial (fan) deposits along a west-to-east climatic transect stretching from the fog-affected western slopes of the Coastal Cordillera near sea level to the hyperarid core of the Atacama Desert at approximately 2000 m above sea level. Even though all studied profiles are situated in alluvial (fan) deposits, the geomorphic setting and thus the (sub)recent sedimentation dynamics differed considerably between the profiles. Soil dynamics in the form of vertical grain transport as well as material exchange and mixing were only detected in the coastal profiles where sufficient moisture supply supports the presence of vegetation and associated soil fauna. In these lower elevations, alluvial (fan) surfaces appear geomorphologically stable since their deposition, but our profiles exhibit evidence of significant post-depositional soil material reworking. In the hyperarid region above fog occurrence, that is only affected by rare episodic rain, post-depositional turbation processes seem to be absent or restricted to the surface layer. However, in these hyperarid regions, sediment (re)deposition seems to have taken place on relatively recent time scales, thereby adding more data on late Pleistocene to Holocene surface activity in the driest non-polar desert on Earth, that are likely driven by aeolian dust and/or episodic alluvial processes.