THE ANTIQUITY OF THE ATACAMA DESERT AND ITS IMPLICATION FOR THE PALEOELEVATION OF THE CENTRAL ANDES

Over the last four years our research group has examined Neogene paleosols along an ~850 km north-south transect (19-26ºS) in the Atacama Desert to reconstruct climate change and understand the relationship between climate change and Andean uplift along the western flank of the Andes. We have documented paleosol morphology, chemistry, mineralogy, and isotopic composition and compared these features with modern soils in Chile today. Paleosols from the Upper Oligocene and Lower Miocene were described from the Yalqui Fm. (~24 Ma) in the Calama Basin (24ºS) and from the Azapa Fm. (~22 Ma) near Río Camarones (19ºS). Paleosols from both localities are calcic vertisols containing soil carbonate nodules, argillic horizons, root traces, and gley mottling. These types of soils are not present in the Atacama today, but are found in central Chile where precipitation is ~250 to 350 mm/yr. In Middle-Upper Miocene strata, we identified extremely well developed gypsic paleosols all along our transect. These gypsic paleosols are generally 2 to 4 m thick, are 20 to 40% SO₄, and contain only trace to minor amounts of soil carbonate. These extremely well-developed gypsic paleosols are thought to represent several million years of pedogenesis and are capped by ignimbrites or stratigraphic units that that have been dated to ~9-8 Ma. Gypsic soils occur throughout the modern Atacama in areas that receive ~10-30 mm/yr rainfall.

We interpret that the formation of the observed Atacama gypsic paleosols occurred following a mid-Miocene reduction in precipitation amounts from >200 mm/yr to <30 mm/yr. This drastic reduction in precipitation likely resulted from regional uplift of the central Andes to a threshold elevation of ~2 km that blocked summer monsoon moisture from entering the Atacama.