Salts as proxies for water-soil interaction in the hyperarid Atacama Desert

The Atacama Desert is the oldest and driest non-polar desert on Earth, where salts have accumulated through atmospheric deposition over millions of years of hyperaridity. These salts can serve as an indicator to provide an understanding of the interaction between water and soil in changing environmental conditions. Therefore, four soil profiles were studied for their mineralogy, abundance of salts, and stable isotopic composition of sulfate. In all soil profiles, sulfates are the predominant salts showing a downward transition from gypsum to anhydrite accompanied by an increase in highly soluble salts and a decrease in δ³⁴S and δ¹⁸O values of sulfate. These trends are consistent with downward water infiltration during rare rain events causing salt dissolution and subsequent precipitation in the deeper soil column. This conclusion is also supported by our Rayleigh fractionation model. The presence of anhydrite at >40 cm depth is attributed to their association with nitrate and chloride salts, which reduces water activity during sulfate precipitation and thus stimulates anhydrite formation. Along the elevation transect, the total salt inventories of the individual profiles show a tendency for nitrate and chloride concentrations to decrease with elevation. This observation, together with the stable isotopes of sulfate, suggests a fog-independent source and points to the remobilization of soluble salts by enhanced erosion of the hillslopes. These findings are essential for understanding pedogenetic processes and long-term regional habitability of hyperarid environments.