No abstract

<p>Polygonal patterned grounds on the Earth’s surface are typically associated with subsurface-wedge structures in periglacial environments. The presence of such wedges is usually taken as an indicator for cryogenic processes in the subsurface, which form a characteristic vertically laminated sequence. However, similar structures can be found in the subsurface of the Aroma fan in the Central Depression of the Atacama Desert in northern Chile. Within the salt-bearing deposits of the alluvial fan, the calcium-sulphate wedges appear to be preliminary formed by haloturbation and may represent the hyperarid equivalent to periglacial wedge structures. The characteristic vertical lamination of the wedges contains calcium-sulphate phases accompanied by clastic minerals, as found by X-ray diffraction and X-ray fluorescence analyses. Hence, the calcium-sulphate phases in the wedges are assumed to be potential drivers for salt dynamics causing subsurface wedge-growth and surface polygonal patterned ground formation. Due to varying water availability in a generally extremely water-limited environment, these salt dynamics possibly led to significant volumetric changes in the deposits induced by dissolution and (re)precipitation of salts from infiltrating solutions and phase transitions of calcium-sulphate phases.</p> <p>The subsurface-wedge network of the Aroma-fan outcrop is covered by a ~ 20 cm thick calcium sulphate-bearing surface crust, which potentially covered a polygonal patterned ground. The formation and preservation of the surface crust might indicate an amplification of arid conditions leading to the inhibition of wedge growth in the subsurface. To unravel the mechanisms and governing environmental conditions of calcium-sulphate wedge and crust formation at the Aroma site, we present various mineralogical, geochemical, and sedimentological data of wedge and crust material.</p> <p>Furthermore, we applied geochronological methods to resolve wedge-growth phases and episodes of local moisture supply. We tested meteoric <sup>10</sup>Be dating and post-infrared infrared stimulated luminescence (post-IR IRSL) dating on wedge material to gain information on the evolution and activity of wedge growth under arid to hyperarid conditions. Such geochronological data is indispensable for using the wedges as terrestrial proxy record for the palaeoclimate in the northern Atacama Desert.</p>

<p>Polygonal patterned grounds are common surface expressions of subsurface dynamics in periglacial and Martian environments. In the Periglacial these structures are typically associated with vertically laminated wedges in the subsurface being the product of cryogenic processes. These landscape features similarly occur in arid to hyperarid environments, such as in the Atacama Desert. Due to the salt-dominated soil of the Atacama Desert, haloturbation and salt heave mechanisms control the formation of wedges and polygons under arid conditions. We present x-ray diffraction and x-ray fluorescence analyses of wedges from the central Atacama Desert that contain various calcium-sulfate phases as potential drivers for the wedge-growth activity. The formation of these wedges is connected to varying water activity. Hydration- and dehydration-induced phase transitions of calcium-sulfate phases result in significant volumetric changes in the soils. In combination with crystallisation pressure of (re-)precipitated salts from infiltrated solutions, these processes significantly contribute to the subsurface stress field. The upward stress release is assumed to express in a microtopographic signature of the surface, such as the polygonal patterned ground. In order to investigate the polygon-wedge system under arid conditions, we will present a combination of sedimentological, mineralogical, and geochemical analyses of subsurface wedges from the central Atacama Desert. We also present data constraining the formation age of calcium sulfate-dominated wedges that formed within the El Diablo Formation of Miocene age.</p>

The growth of vertically laminated calcium-sulphate wedges in the Atacama Desert is assumed to be driven by the interaction of moisture supply and salt dynamics in the subsurface. Geochronological data of these wedge laminations is yet sparse but indispensable to resolve wedge-growth phases and episodes of moisture supply and to use these deposits as a palaeoclimate archive in the hyperarid environment. Our pilot study presents a first approach of dating a calcium-sulphate wedge from the Atacama Desert using coarse-grain feldspar luminescence dating. Our results show a widespread and clustered equivalent-dose distribution of two wedge samples from ~20 Gy up to saturation. Optically stimulated luminescence (OSL) of quartz revealed unsuitable properties for dating wedge deposits. Consequently, we applied post-infrared infrared stimulated luminescence (post-IR IRSL) to coarse-grained feldspars. Since feldspar single-grain measurements yielded a low number of luminescent grains, we used 1 mm aliquots as reliable single-grain proxies for genuine single-grain measurements. Data from energy-dispersive x-ray spectroscopy (EDX) showed that the feldspar single grains have large differences in their internal K content, resulting in an averaged internal K content of 3.9 ± 1.0 % for all luminescent grains. This result was subsequently used for dose rate and age calculations. Our results of equivalent-dose distributions and palaeodoses derived from the minimum age model reveal most recent wedge-growth activities at 10.6 ± 2.2 ka and 7.9 ± 1.8 ka for the two wedge samples.