Evidence from Paleosols for a Middle Miocene Transition to Hyperaridity In the Northern Atacama Desert, Chile

The initiation of hyperaridity within the Atacama Desert is poorly constrained, with age ranges spanning from Jurassic to Pliocene time. Here we present morphological and isotopic characteristics of paleosols from the northern Atacama (19-20ºS) to identify the spatial extent and timing of Miocene climate change in the Atacama Desert and determine its relation to Andean uplift. Paleosols in the Azapa Formation are Calcic Vertisols with root traces, well-developed soil carbonate, argillic horizons and slickensides. Isotopic analysis of soil carbonate identified d18O values of -7.88 to -2.83‰ and d13C values of -9.46 to -4.70‰. These paleosols are firmly dated by Ar/Ar dating of overlying Oxaya Ignimbrite and paleomagnetic stratigraphy to 22 Ma, and provide evidence for semiarid conditions with precipitation likely between 200-400 mm/yr at this time. In the Lower to Middle Miocene El Diablo Formation along Rio Suca all paleosols contain numerous root voids that are infilled with gypsum cement. Gradual loss of bulk soil carbonate and soil carbonate nodules upward in section likely reflect enhanced aridity during this time. Firm evidence for hyperaridity is present in extremely mature gypsic paleosols interbedded in the top of the El Diablo Formation at several localities. The mature gypsic paleosol yielded d18O values of 0.21% to 2.95% and d13C values of -1.99% to -1.42% and is overlain by a lava adjacent to Miñi Miñi that is thought to be ~ 8 Ma in age. This paleosol is thought to represent several million years of soil development, suggesting a minimum age for hyperaridity of ~10 Ma. These results are comparable to those from paleosols in the Calama Basin and suggest a similar age to the initiation of hyperaridity in the northern Atacama, supporting the hypothesis that the development of a Central Andean rainshadow played a key role in the formation of the Atacama Desert.