The Sensitivity of Terrestrial δ18O Gradients to Hydroclimate Evolution

Terrestrial gradients in the oxygen isotopic composition of meteoric water (d 18O), as reconstructed through proxies, reflect characteristics of ancient hydrologic conditions. These gradients are primarily influenced by the atmospheric transport of water vapor and the balance of precipitation and evapotranspiration, which are linked to climate and topography. We incorporate these effects into a one-dimensional model that predicts the spatial evolution of d 18O based on local topography and the regional water-energy budget. Specifically, we build on existing reactive transport models by incorporating parameterizations of orographic precipitation and energetic constraints on evapotranspiration following the Budyko water balance framework. We test our model on three modern transects that represent topographically distinct environments. These are the Amazon Basin (lowlands), the Cascade Range (mountains), and the eastern Himalayan Range (lowlands and mountains). Comparisons among these gradients demonstrate that the topographic regime determines how sensitive isotope records are to hydroclimate evolution. As a result, isotope records differentially express signatures of topography and the regional water balance, and we present a quantitative framework to predict this trade-off. Finally, we link these effects to climate evolution and discuss how our model may help disentangle topographic and climatic signals through Earth history. Plain Language Summary Isotopic tracers of the water cycle are commonly used in paleoreconstructions to infer ancient climatic and topographic conditions. With a growing record of proxy data, spatial analysis for a given interval in Earth history is becoming more common, presenting opportunities for more quantitative interpretations. We present a model that predicts the oxygen isotopic composition of precipitation over space. The model requires relatively few input parameters so that it can be readily applied to paleoclimate problems. The main controls on the isotopic composition of precipitation are topography, vapor transport, and the water-energy balance. We also discuss how quantifying the energy balance presents new opportunities for the interpretation of oxygen isotope paleoclimate records. Finally, we predict how continental isotope gradients may evolve in different climate states.