The Atacama Desert in northern Chile is the driest location on Earth. Soils, with high concentrations of soluble salts including nitrates, chlorides, sulfates, and even perchlorates, provide direct evidence for this extreme aridity (Ericksen, 1981, Rech, 2003). However, there is little agreement among research as to the age, and cause, for the onset of extreme aridity. Dunai et al., 2005, suggest that this extreme aridity began 25 million years ago (Mya) and was the result of global climate change. Alpers and Brimhall, 1988, argue that hyperaridity began around 14 Mya and was primarily the result of the uplift of the Andes Mountains to a paleoelevation capable of blocking Amazon air masses, whereas Hartly and Chong argue that the initiation of hyperaridity dates to 3 Mya, and was related to global climate change.Here we present evidence of the morphological characteristics, salt chemistry, and mass independent fractionation anomalies (D17O values) in dated paleosols to reconstruct a Middle Miocene (17-13 Mya) climatic transition from semi-aridity to hyper-aridity in the Atacama Desert. Paleosols along the southeastern margin of the Calama Basin change from Vertisols with root traces, slickensides, gleyed horizons, and calcium carbonate nodules to an extremely well-developed Aridisol with a massive (3m) petrogypsic horizon (Fig. 1). The petrogypsic horizon has a high bulk density, >2.0 g/cm2, and contains 10-45 wt. % SO4, or ~20-90% gypsum. Petrographic analysis of the Bym horizon shows a complex history of precipitation and dissolution of pedogenic salts, analogous to modern soils in the Atacama. Between 1.5-3.5m depth is a petrosalic horizon that contains up to ~2.5% NO3, 0.5% Cl, and trace amounts of perchlorate. We interpret this transition from Vertisols to nitrate Aridisols, which occurred between ~17-13 Ma, to represent a change in precipitation from >200 mm/yr to <20mm/yr based on the relationship between these soils and precipitation in northern Chile today. This drastic reduction in precipitation likely resulted from uplift of the Andes to elevations >2km, which blocked moisture from the South American Summer Monsoon from entering the Atacama.