Quantifying the climatic and topographic controls of precipitation isotopes in continental interiors: applications to unraveling isotopic records of climate in Cenozoic Central Asia

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 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 through evapotranspiration (ET). We use a one-dimensional idealized model of water vapor transport along 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 primarily by ET with secondary controls exerted by eddy transport. A comparison of modern isotopic gradients within high elevation continental interior regions shows that the effects of seasonal changes in ET are of the same order of magnitude as the effects of rainout due to orographic precipitation. This implies that changing climate and associated changes in ET rates may amplify or completely negate isotopic signals of uplift. We further apply the model to a spatial compilation of Cenozoic isotopic records throughout Central Asia. Over the past 50 Ma, extensive recycling of water via ET has likely masked isotopic signals of the uplift of the northern Tibetan Plateau, Tian Shan, Altai, and Hangay ranges as revealed by complimentary methods of measuring uplift timing and rates. Our results highlight the importance of the coupling between topography, atmospheric circulation, and biological processes in controlling isotopic records of past climate.