The stable isotope composition of halite and sulfate of hyperarid soils and its relation to aqueous transport

Halite (NaCl) and gypsum or anhydrite (CaSO4) are water-soluble minerals found in soils of the driest regions of Earth, and only modest attention has been given to the hydrological processes that distribute these salts vertically in soil profiles. The two most notable chloride and sulfate-rich deserts on earth are the Dry Valleys of Antarctica and the Atacama Desert of Chile. While each is hyperarid, they possess very different hydrological regimes. We first show, using previously published S and O isotope data for sulfate minerals, that downward migration of water and sulfate is the primary mechanism responsible for depth profiles of sulfate concentration, and S and O isotopes, in both deserts. In contrast, we found quite different soluble Cl concentration and Cl isotope profiles between the two deserts. For Antarctic soils with an ice layer near the soil surface, the Cl concentrations increase with decreasing soil depth, whereas the ratio of 37Cl/35Cl increases. Based on previous field observations by others, we found that thermally driven upward movement of brine during the winter, described by an advection/diffusion model, qualitatively mimics the observed profiles. In contrast, in the Atacama Desert where rare but relatively large rains drive Cl downward through the profiles, Cl concentrations and 37Cl/35Cl ratios increased with depth. The depth trends in Cl isotopes are more closely explained by a Rayleigh-like model of downward fluid flow. The isotope profiles, and our modeling, reveal the similarities and differences between these two very arid regions on Earth, and are relevant for constraining models of fluid flow in arid zone soil and vadose zone hydrology.