High relative humidity in the hyper-arid late Miocene Atacama Desert quantified by triple oxygen and hydrogen isotopes in gypsum

In the Chilean Atacama Desert, alternation of lacustrine evaporites and diatomites suggest that hyper-aridity has prevailed since the early Miocene, but was interrupted by recurring pluvial phases. Changes in lacustrine deposition, however, can be either climate- or tectonically-driven, which challenges a unique interpretation. Here, we demonstrate that an isotope model-data approach combining triple oxygen and hydrogen isotope data of paleo-lake water derived from gypsum hydration water with the Craig-Gordon evaporation model can provide quantitative information on local hydroclimate conditions in the past Atacama Desert.

We analyzed the isotope composition (δ17O, δ18O, δ2H) of structurally bonded water extracted from lacustrine and marine gypsum deposits in the Atacama Desert of Late Miocene and Pleistocene age, respectively. We used a Monte Carlo simulation to obtain the best-fit solution of input variables of the Craig-Gordon equation – notably, relative humidity, the isotope composition of source water and atmospheric water vapor – through isotope data of multiple gypsum samples from a single unit of the paleo-lacustrine formations. Gypsum ages were obtained by U-Pb dating.

Our results demonstrate that a generally more humid climate persisted over northern Chile 8.9 Ma ago, caused by distal vapour advection with significant rainout along the vapour advection pathway. The regional atmospheric circulation likely changed as a result of an overall weaker sub-tropical subsidence over the Miocene SE Pacific Ocean. Marine lagoon gypsum with an age of 1.8 ± 0.2 Ma were deposited under near-similar hyper-arid conditions in comparison to the present-day coastal climate.