Middle Miocene climate and stable oxygen isotopes in Europe based on numerical modeling

The Middle Miocene (15.99-11.65 Ma) of Europe witnessed major climatic, environmental, and vegetational change, yet we are lacking detailed reconstructions of Middle Miocene temperature and precipitation patterns over Europe. Here, we use a high-resolution (similar to 0.75 degrees) isotope-enabled general circulation model (ECHAM5-wiso) with time-specific boundary conditions to investigate changes in temperature, precipitation, and delta O-18 in precipitation (delta O-18(p)). Experiments were designed with variable elevation configurations of the European Alps and different atmospheric CO2 levels to examine the influence of Alpine elevation and global climate forcing on regional climate and delta O-18(p) patterns. Modeling results are in agreement with available paleobotanical temperature data and with low-resolution Middle Miocene experiments of the Miocene Model Intercomparison Project (MioMIP1). However, simulated precipitation rates are 300-500 mm/yr lower in the Middle Miocene than for pre-industrial times for central Europe. This result is consistent with precipitation estimates from herpetological fossil assemblages, but contradicts precipitation estimates from paleobotanical data. We attribute the Middle Miocene precipitation change in Europe to shifts in large-scale pressure patterns in the North Atlantic and over Europe and associated changes in wind direction and humidity. We suggest that global climate forcing contributed to a maximum delta O-18(p) change of similar to 2 parts per thousand over high elevation (Alps) and similar to 1 parts per thousand over low elevation regions. In contrast, we observe a maximum modeled delta O-18(p) decrease of 8 parts per thousand across the Alpine orogen due to Alpine topography. However, the elevation-delta O-18(p) lapse rate shallows in the Middle Miocene, leading to a possible underestimation of paleotopography when using present-day delta O-18(p)-elevation relationships data for stable isotope paleoaltimetry studies.