Triple Oxygen and Deuterium Isotopes in Gypsum Hydration Water for Quantitative Paleo-humidity Reconstructions

Variations in atmospheric relative humidity (RH) and precipitation may have driven major ecological and sociocultural changes during the Quaternary but quantitative proxies for RH are scarce and difficult to calibrate. The isotopic composition of lake water (δ17O, δ18O and δD, and derived d-excess and 17Oexcess) is sensitive to changes in atmospheric RH and temperature. Because 17Oexcess is less sensitive to temperature effects than the d-excess during evaporation, combining 17Oexces and d-excess provide information about the relative effects of humidity and temperature change in the hydrological cycle. Here we demonstrate how the isotope ratios of hydration water measured in gypsum from lake sediments can be used to reconstruct past changes in RH. We present stable isotopes of gypsum hydration water from two lake systems across the last deglaciation. In Lake Estanya (NE, Spain) the 17Oexcess and d-excess of the paleo-lake water indicates that evaporation of water during the Younger Dryas (ca. 12 kyr BP) occurred under RH conditions of 40-45%. Environmental humidity gradually increased over the Preboreal period and stabilised at 70-75% during the Holocene until present. In Lake Peten-Itza (Guatemala), the isotopic values of the paleo-lake waters during the Late Glacial can be explained by a lowering of atmospheric RH by 10% and cooling of temperature by 5oC compared with modern conditions. Our results demonstrate that the coupled measurement of 17Oexcess and d-excess of gypsum hydration water in lake sediments can provide a useful quantitative proxy for paleo-humidity.