Calcium sulphate-rich wedges in the subsurface indicating salt dynamics in alluvial-fan deposits from the Atacama Desert

Subsurface wedges are common geomorphological structures in periglacial environments typically associated with polygonal patterned grounds on the surface. Their presence is an indicator for cryogenic processes in the subsurface resulting in vertically laminated sequences from which the wedges are formed. Similar geomorphological features occur in arid to hyperarid environments, such as in the Atacama Desert, where haloturbation processes are assumed to control subsurface-wedge formation. X-ray diffraction and x-ray fluorescence analyses of wedges from the central Atacama Desert revealed various calcium-sulphate phases accompanied by clastic minerals in the laminae sequences. Calcium-sulphate phases in the wedges are thus thought to be potential drivers for salt dynamics and wedge-growth activity. In combination with varying water availability, these salt dynamics lead to significant volumetric changes in the deposits due to phase transitions of calcium-sulphate phases, as well as dissolution and (re)precipitation of salts from infiltrating solutions. As water is scarce in the Atacama Desert, geochronological data of calcium-sulphate wedges is essential to resolve wedge-growth phases and episodes of local moisture supply, indispensable for using the wedges as palaeoclimate archives. We applied post-infrared infrared stimulated luminescence (post-IR IRSL) dating to coarse-grain feldspars extracted from a calcium-sulphate wedge outcropped on the Aroma fan in the Central Depression of the Atacama Desert. Our post-IR IRSL results from two wedge subsamples showed a widespread equivalent-dose distribution indicating multiple phases of wedge formation. Equivalent-dose distributions and palaeodoses based on the minimum age model revealed the most recent wedge-growth activities at 10.6 ± 2.2 ka and 7.9 ± 1.8 ka.