MID-MIOCENE NITRATE PALEOSOLS FROM THE ATACAMA DESERT: IMPLICATIONS FOR THE ANTIQUITY OF THE ATACAMA DESERT

The Atacama Desert in northern Chile is the driest location on Earth. Soils, with high concentrations of soluble salts including nitrates, chlorides, sulfates, and perchlorates, provide direct evidence for this extreme aridity. Here we present evidence of the morphological characteristics, stable isotopes 13C & 18O , and salt chemistry 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. The petrogypsic horizon (Bym) 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 (Bzm) 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.