MID-MIOCENE HYPERARIDITY IN THE ATACAMA DESERT, CHILE: EVIDENCE FROM THE GYPSIC BARROS ARANA PALEOSOL

The mid-Miocene Barros Arana paleosol outcrops in several drainages along the southeastern margin of the Calama Basin in northern Chile. The paleosol is ~3 meters thick and is composed mainly of angular fluvial gravels and pedogenic gypsum. Other pedogenic salts present include soil carbonate, halite, and nitrate (ranging from trace amounts to up to 4%). Soil carbonate is generally found at the base of the paleosol and is surrounded by gypsum. The Barros Arana paleosol is an extremely well-developed paleosol, similar to modern soils in the hyperarid core of the Atacama. The paleosol contains pedogenic salts within almost all voids, has a bulk density of ~2.4 g.cm3, and contains large vertical fractures up to 30cm wide and 3m deep. These fractures are thought to form by the expansion and contraction of pedogenic salts over significant periods of time. The Barros Arana paleosol is overlain by the Sifon ignimbrite, which has been dated to 8.3 Ma. The duration of time represented by the paleosol is unknown, but is thought to be on the order of 1 to 5 million years.

The accumulation of different soluble salts in soils is directly related to precipitation. The modern relationship between pedogenic salts and precipitation in the Atacama indicates that soil carbonate forms in wetter (>20mm/yr), vegetated environments; soil gypsum accumulates in drier (20-10mm/yr), unvegetated locations; and soil nitrate collects in extremely dry (<10mm/yr) locations. Therefore, The Barros Arana paleosol suggests a period of increasing desiccation in the Atacama prior to 8.3 Ma; with soil carbonate precipitating, then soil gypsum overprinting the soil carbonate, followed by the accumulation of nitrate. We suggest that the cause for this desiccation was most likely the uplift of the Andes to elevations of ~3km, which blocked the majority of easterly air masses from entering the Atacama.