Oxygen Isotopes as Indicators of Cenozoic Hydroclimate Change in Europe

Comparison of modern and Cenozoic oxygen isotopes provides insight into changes in past terrestrial paleoclimate and hydroclimate. In Europe, moisture transport across the continent is governed by many processes, including the position of the westerly jet and return of water vapor to the atmosphere via plant water use, both of which may change in a warmer, higher CO2 climate. To elucidate hydroclimate changes across Europe in the Cenozoic, we compile previously published analyses of oxygen isotopes, recorded in authigenic materials such as paleosol, lacustrine, and speleothem carbonates, and mammal tooth enamel, to generate a dataset of over 6,500 18O datapoints spanning the European continent. Spatial compilations are useful in the geologic record to distinguish between the competing isotopic effects on the 18O of water resulting from changes in climate, topography, and moisture sources, which are otherwise difficult to disentangle from a single site. Compiled Quaternary 18O data across Europe indicate that different proxy materials reliably record the same or similar local meteoric water signature. Across the continent, these Quaternary data capture the decrease in 18O with increasing longitude that is observed in modern waters, indicating that the same proxies can be applied to reconstruct meteoric 18O during the Cenozoic. Preliminary results from pre-Quaternary Cenozoic proxy data indicate that the longitudinal 18O gradient is not markedly reduced or steepened relative to the modern, even during globally warmer periods such as the Miocene. This result suggests that westerly moisture transport across Europe during the Cenozoic resembled modern-day moisture transport processes, despite large changes in atmospheric CO2. Although this first-order isotopic trend appears throughout the Cenozoic record, many sitesparticularly those nearer to the Paratethyshave elevated estimated paleo-precipitation 18O relative to modern. Disparities between the Cenozoic record and modern data may reflect elevation changes due to multiple small orogens that developed during the Cenozoic along the Tethyan margin, changes in moisture sources as the Paratethys shrank, differences in the seasonality of authigenic mineral formation, and changes in atmospheric CO2 that affect hydroclimate parameters.