Pedogenic Processes and Paleoclimatic Significance of Sulfate Soils in the Atacama Desert, Chile

Soils in the Atacama Desert are well known for their high accumulations of nitrate, yet based on our observations, sulfates are the predominant pedogenic salt in regions of the Atacama that receive <~20 mm yr-1 precipitation. Sulfate soils average ~3m in thickness, yet can obtain maximum thicknesses of up to 8 m. Sulfate minerals typically comprise 30 to 50 weight percent of these soils, generally totaling between 200 to 1000 kg SO4 per m2 of soil profile. The age of these sulfate soils is likely highly variable, but widespread sulfate soils on the Middle Miocene Tarapacá pediplain landscape surface and in situ soils that are overlain by lavas and ignimbrites dated to 8 Ma indicate that some of these soils represent >8 Ma of pedogenesis. Gypsum is the most abundant sulfate mineral, yet high concentrations of anhydrite, bassanite, bloedite, glauberite, mirabilite, and thenardite are also present, especially in the hyperarid core. Soil carbonate is generally restricted to the surface crust and within large vertical fractures. In other areas, soil carbonate is typically <0.001 weight percent. However, other climatic archives clearly indicate wetter intervals within the Atacama when plants covered the landscape and soil carbonate formed in younger soils. We suggest that soil carbonate, with its lower solubility, precipitates out of solution first in sulfate soils in the upper Av soil horizon during these wet intervals. Since the Av soil horizon is continuously being deflated and re-deposited, we believe that it is unclear whether the lack of soil carbonate in well-developed sulfate soils can be used as an indicator of permanent hyperaridy in arid lands. However, this idea has to be tested by examining soil carbonate concentrations in other locations that have experienced pluvial events.