THE FORMATION AND EVOLUTION OF AN ATACAMA NITRATE SOIL, BAQUEDANO NITRATE DISTRICT, CHILE

Soils in the hyper-arid core of the Atacama contain high concentrations of soluble salts, including nitrates, sulfates, chlorides, and perchlorates. Although previous studies have analyzed surficial soil samples and bulk nitrate ores in the Atacama, there are currently few detailed geochemical soil descriptions for nitrate ore (>7 % NO₃^-) soils of known age. This study examines the morphology, mineralogy, geochemistry, and stable isotopic composition of soil carbonates (δ¹³C and δ¹⁸O) and sulfates (δ³⁴S) from a well exposed nitrate soil at Oficina Ercilla, just north of the town of Baquedano. An age model for the soil is constructed based on a 3 m depth profile of cosmogenic Be¹⁰. The nitrate soil at Oficina Ercilla is polygenetic, with an early phase of calcic horizon formation and subsequent formation of more soluble salt horizons (gypsic, halic). Soil bulk density changed from a likely initial value of 1.6 to 1.8 g/cm³ to 2.4 g/cm³ today. On average, a 1m x 1m x 3m deep soil profile contains 570 kg of sulfate, 631 kg of nitrate, 416 kg of chloride, and 1.89 kg of perchlorate. At depth, soil carbonate δ¹³C values are relatively homogenous (~ -3.5 ‰) and reflect pedogenic carbonate formed under a mixed C3/C4 paleo-environment. The δ¹³C values in the upper, less dense, portion of the soil range from -1.81 to +3.21 ‰, indicative of eolian carbonate dust from various sources as well as carbonate precipitated in equilibrium with the atmosphere. Cosmogenic age constraints for the nitrate soil from Oficina Ercilla suggest that it started forming in the late Pliocene. If this soil is representative for other nitrate soils in the Atacama Desert, than the nitrate soil ores likely formed as a result of strengthening of the temperature inversion along the Pacific coast after the Pliocene Warm Period, and are younger than the Miocene gypsic paleosols in the Atacama.