Evidence for the development of the Andean rain shadow from a Neogene isotopic record in the Atacama Desert, Chile

Varying ages from Triassic to Pliocene have been proposed for the onset of hyperaridity in the Atacama Desert. The exact timing for the initiation of hyperaridity is critical for determining potential causes, which range from regional effects of global cooling to Andean uplift above elevations conducive to extreme rain shadows. Analysis of the stable isotopic composition of lower Miocene–Quaternary (21–0.015 Ma) palustrine and lacustrine carbonates in the Calama Basin reveals extreme changes in their oxygen and carbon isotopic composition during the Miocene. Limestone δ 18O values increased by ∼ 5‰ from middle to late Miocene, ranging from − 5.5‰ at 12 Ma to − 1‰ at ∼ 6 Ma. Carbon isotopic values increase by 9‰ over the Neogene, from average values of − 3‰ at 21 Ma to + 3‰ at 12 Ma, and reaching a maximum of + 6‰ at 5 Ma. The increase in oxygen isotopic values occurred over a time span in which the catchment area of the basin experienced significant uplift, causing the δ 18O value of precipitation to become more negative. We attribute the shift towards higher δ 18O values to enhanced evaporative enrichment both of soil water or snow prior to infiltration, and within shallow lakes or wetlands prior to carbonate precipitation. The large increase in δ 13C values was likely caused by a transition from a vegetated landscape influenced primarily by soil-respired CO 2 to a landscape largely devoid of vegetation and influenced by atmospheric and volcanic CO 2. Isotopic values of palustrine carbonates therefore indicate that hyperaridity commenced in the Calama Basin during the middle to late Miocene, in agreement with other paleoclimatic records from the basin. The cause for the onset of this climate change is thought to be due to the development of a strong Andean rain shadow associated with the uplift of the Andes to mean elevations > 2 km.