Haloturbation in the northern Atacama Desert revealed by a hidden subsurface network of calcium sulphate wedges

Abstract. While the formation of periglacial wedges and polygonal patterned grounds has been extensively studied and many of the processes involved have been understood, knowledge on the formation of similar post-sedimentary features found in arid to hyperarid environments remains largely rudimentary. Our contribution to fill this gap is the investigation of a network of vertically laminated calcium sulphate-rich wedges in the subsurface of the Aroma fan in the northern Atacama Desert. The subsurface wedges are characterised by high anhydrite contents and hence differ from the wedge and polygon structures of other study sites in the Atacama Desert, which appear to have been predominantly formed by thermal contraction or desiccation processes. By contrast, haloturbation mechanisms are thought to be a main driver of wedge formation at the Aroma fan site. Haloturbation requires moisture input, and hence Aroma fan wedge formation is likely to be associated with meteoric water received from sporadic rain events and episodes of arid climate characterised by slightly wetter conditions than prevailing at present. The polygonal patterned ground is covered by a stratigraphically younger gypsum-dominated surface crust cover. The presence of the surface crust could indicate an environmental change towards drier conditions, which favoured surface accumulation of calcium sulphate and other salts by means of atmospheric deposition. Such a climatic shift could have caused a deceleration of haloturbation and other wedge formation processes in the subsurface, although modern sediment conveyance from the surface towards its interior still appears to occur along cracks within the crust. In order to gain comprehensive insights into the complex mechanisms involved in wedge formation and formation rates, the establishment of a geochronological framework directly obtained from wedge and crust material remains indispensable. The temporal resolution of wedge growth stored within the succession of vertical laminae promises a high potential for the calcium sulphate wedges to be used as palaeoclimate archives, potentially helping to unravel wedge and polygonal patterned ground formation in other water-limited environments, such as Mars.