QUANTIFYING THE ISOTOPIC CONTINENTAL EFFECT IN PRECIPITATION AND IMPLICATIONS FOR RECONSTRUCTING CLIMATE AND TOPOGRAPHY FROM ISOTOPIC GRADIENTS

Since the establishment of the IAEA-WMO precipitation-monitoring network in 1961, it has been observed that isotope ratios in precipitation (δ²H and δ¹⁸O) generally decrease from coastal to inland locations, an observation described as the continental effect. While discussed frequently in the literature, there have been few attempts to quantify the variables controlling this effect despite the fact that isotopic gradients over continents can vary by orders of magnitude. In a number of studies, traditional Rayleigh fractionation has proven inadequate in describing the global variability of isotopic gradients due to its simplified treatment of moisture transport and its lack of moisture recycling processes. In this study, we use a one-dimensional idealized model of water vapor transport across a storm track to investigate the dominant variables controlling isotopic gradients in precipitation across terrestrial environments. We find that the sensitivity of these gradients to progressive rainout is controlled by a combination of the amount of evapotranspiration and the ratio of transport by advection to transport by eddy diffusion, with these variables becoming increasingly important with decreasing length scales of specific humidity. A comparison of modeled gradients with global precipitation isotope data shows that these variables account for the variability in observed isotopic gradients between coastal and inland locations. The mathematical framework presented here provides a means to quantitatively test hypotheses of climatic and tectonic change based on reconstructions of isotopic gradients in the past. Furthermore, the strong observed dependence of the continental effect on moisture recycling provides the potential for separating the effects of changing topography and climate encapsulated in isotopic records and examining the coupling between them.