Quantitative reconstruction of past climate mean states in the Atacama Desert using hydrogen and triple oxygen isotopes of gypsum hydration water

Gypsum crystals capture the isotopic composition (δ²H, δ¹⁷O, and δ¹⁸O) of ambient water in their structurally bonded water and may serve as a useful tool to reconstruct paleoclimate. Relative humidity, water temperature, wind speed, along with the isotopic composition of atmospheric vapor and inflowing water control, to a variable degree, the relative proportion of equilibrium and kinetic isotope fractionation during evaporation, and, thus, ultimately determine the d-excess and ¹⁷O-excess of gypsum-bonded water. Here, we demonstrate that the respective best fit of these variables through measured gypsum-bonded water isotope data using the classic Craig-Gordon evaporation model provides apparent absolute values for the fundamental climate mean state variables humidity and temperature and an empirical wind speed parameter of the geologic past.

In this proof-of-concept study, we sampled gypsum crystals within individual stratigraphic units of Pliocene lacustrine deposits from the Atacama Desert, extracted their structurally bonded water, and analyzed the hydrogen and triple oxygen isotope composition. The spread of measured isotope data within each sampled stratigraphic unit suggests variable degrees of evaporation between individual gypsum samples along a common evaporation trajectory. We used the Craig-Gordon evaporation model together with a Monte Carlo simulation to determine the limits of climate mean state variables that fit the measured isotopic data.

Our results demonstrate that primary isotope signatures of marine and continental source waters are preserved in structurally bonded gypsum waters. The data coherently suggest a slightly warmer (18-35°C), less windy and much more humid (50-75%) climate for the Pliocene Atacama, which is consistent with marine records and global circulation climate models that agree on “permanent El Niño” conditions for the Pliocene in the equatorial East Pacific.

Under the assumption that mixing of different brines or multiple sources is insignificant - as would be evident from scattering of isotopic data below the evaporation trajectory in ¹⁷O-excess over δ¹⁸O – the combined hydrogen and triple oxygen isotope analyses of gypsum-bonded water provides a powerful tool to quantify past mean states of humidity and temperature, and to estimate paleo-wind conditions.