The Sensitivity of Terrestrial δ 18 O Gradients to Hydroclimate Evolution

Abstract

Terrestrial gradients in the oxygen isotopic composition of meteoric water (δ 18 O), 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 δ 18 O 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.