Radiogenic Isotope Provenance of Dry Deposition in Hyperarid Soils of the Atacama Desert, Chile.

The Atacama Desert, Chile, represents an extreme in dust deposition and accumulation, due to its longstanding arid to hyperarid conditions. Long term dry deposition has led to thick salt layers, including unusual nitrate deposits in some locations. We present strontium (Sr) isotope data from three soil profiles, ranging from hyperarid to arid (~2-20 mm annual rainfall), with the goal of delineating atmospheric dry deposition sources and understanding post-depositional redistribution of cations in the profile. Strontium can be used as a proxy for the provenance of calcium and other soil cations. Likely sources of Sr are Andean volcanics (87Sr/86Sr = 0.705-0.708), regional salars possibly derived from weathering of these rocks, marine aerosols from occasional fog events (0.7092), and local Mesozoic granitoids. Samples from multiple horizons at each site were leached with water (for soluble salts, primarily gypsum) and acetic acid (for carbonate), and selected residues were completely dissolved using hydrofluoric acid. All profiles show a general trend of decreasing 87Sr/86Sr with depth for both water and acetic acid leachates. The profile from the site that is intermediate both geographically and in annual rainfall yields 87Sr/86Sr ratios of 0.7054-0.7066, significantly lower than those of the other profiles (0.7068-0.7075). Insoluble residue (silicate) isotope data from all three sites show little correspondence with the trends in the water and acetic acid leachates. Pseudo-isochrons in the silicate portion are suggestive of a Mesozoic granitoid source, possibly a reflection of locally derived silicate dust being decoupled from regional sources of dust rich in soluble components.