Evaluating hydrological influences on mid-latitude δ 18 O p in the Middle East

The oxygen isotope ratio of precipitation (δ18Op) in the mid-latitudes varies in response to multiple climate influences imposing significant challenges to the interpretation of climate proxies such as the oxygen isotope ratio of lake and speleothem calcite (δ18Oc) that incorporates an isotopic finger print of precipitation. This challenge is particularly acute for pre-historic time periods when climate forcings differed significantly from modern and consequently, internal feedbacks altered the transport of moisture as well as the rate of fractionation that determine the isotopic composition of atmospheric moisture. Here we investigate how δ18Op from the mid-Holocene was influenced by internal feedbacks in the Holocene with an isotope-enabled climate model that simulates the atmospheric response to changing boundary conditions and different climatic forcing. We find that δ18Op during the Mid-Holocene was lower than present day consistent with published proxy records. However, this lower value cannot be simply explained by basic isotope drivers such as precipitation amount or seasonality. Rather, we find that the combination of changes in local surface temperature, precipitation amount, upstream isotopic composition of vapor as well as the season can quantitatively explain the isotopic differences between the mid-Holocene and present day.