Relict Soil Evidence for Profound Quaternary Aridification of the Atacama Desert, Chile.

We examined a Mio-Pliocene aged relict soil at the southern periphery of the Atacama Desert in Chile. The soil presently lies in a very arid climate, between absolute desert to the north and more semi-arid conditions to the south. We found that the profile contains a robust and detailed record of profound drying over the course of its history. First, the gravelly alluvium has been weathered and up to 35% clay has accumulated in the Bt horizons. We found that rare earth elements are very good immobile index elements in alluvial deposits, and that they help quantify the losses of Si, Na and other major elements during this initial weathering phase. Following the silicate weathering phase, carbonate accumulation ensued (only partially engulfing the initially carbonate-free profile). Carbon isotope ratios in carbonate on laminations on bottoms of gravels show a highly reproducible record of increasing ¹³C with decreasing age, reflective of decreasing plant density. O isotopes in the carbonate show that rainfall sources remained constant over the accumulation phase. The present climate is too arid to leach carbonate dust from the soil surface. In total, the changes in soil chemistry and isotopes indicate that the soil likely started forming in a climate with more than 50 cm/y MAP, and has continuously (though possibly with hiatuses) become more arid, with today’s climate likely being the driest (MAP = 2.6 cm/yr). While the geochronology of the soil is somewhat uncertain, the strong signal of post Tertiary aridification is consistent with marine records that show the southern Pacific was in a permanent El Nino state until the end of the Pliocene.