State of the Hydrological Cycle during the Eocene: Model-Data Comparison

The early Eocene was much warmer compared to modern conditions and represents the warmest time interval of the Cenozoic. In addition to determining the character of regional temperature change during globally warm conditions, a clear understanding of how the hydrological cycle was impacted is a fundamental pursuit. The isotopic composition of precipitation is a fundamental signal that relates to the character of the hydrologic system - dependent on distance of transport, number of rainout events, amount of rainfall, and evapotranspiration. Terrestrial biomarkers, such as higher plant n-alkanes can be used to track the hydrogen isotopic composition (δD) of precipitation and have been applied to interpret hydrological changes during the Paleocene Eocene Thermal Maximum. That work concluded that rapid, global warming was associated with increased rainout at the poles with the probability of relative drying across the mid-latitudes. Several other n-alkane δD records for the early Eocene have already been generated including Cicogna (Italy), MAR-2X (Venezuela), and Tawanui (New Zealand). In this study, we present results from the water isotope enabled version of National Center for Atmospheric Research (NCAR) Community Atmospheric Model version 3, the atmospheric component of Community Climate System Model (CCSM) with Eocene boundary conditions, two different pCO2 levels (2240 and 4480 ppm). Modeling results are evaluated and compared with existing n-alkane δD records. Preliminary results suggest that the model qualitatively reproduces the latitudinal trend observed in the data, with the most D-enriched values observed at the tropics and depletion towards the poles. However, the model predicts values that appear more D-enriched than the proxy records, by up to 40 per mil in the high latitudes. Reasons for this discrepancy along with uncertainties in the proxy records and modeling results are discussed. These results will be useful for validating models and interpreting proxy records for major changes in the climate system in the geological past.