A unique look into European middle Eocene weather; Comparing a new synoptic-scale proxy record to simulated daily conditions.

In addition to improving our general understanding of the climate system, the study of past warm climates is often stated relevant due to the possible resemblances to various future scnenarios. The comparison between proxy records and climate model simulations offers opportunities for validation beyond the boundaries of present climatic conditions. Besides their scarcity in both spatial and temporal coverage, existing proxies pre-dating the Pleistocene only provide an integrated signal typically over 1000 years or more. Climate model simulations provide much more data beyond that which can be validated using proxies, that is therefore often not considered in palaeoclimate studies.A unique opportunity presents itself with a new ultra-high resolution record of the middle Eocene, obtained from the giant marine gastropod Campanile giganteum. Found in the Paris Basin (palaeolatitude 40-45N), these gastropods lived in a shallow marine environment and reached growth rates of over 600mm/year. Well-preserved fossils, in combination with such high growth rates, provide the first record to our knowledge resolving weather-timescale variability in the Eocene.In this study, we interpret this snapshot of European middle Eocene weather and compare our findings to daily model fields using the CESM 1.0.5. We continued existing 38Ma simulations using 4 times pre-industrial CO2 and CH4, which were shown previously to be a good match with the middle Eocene climate (see: Baatsen et al. 2020, Climate of the Past, doi: 10.5194/cp-16-2573-2020). The proxy record shows distinctly different seasonalities of temperature and salinity over central Europe in the middle Eocene. We can interpret these patterns as indications of a monsoonal climate with notably high variability in summer precipitation. Such a climatic regime is supported by the model, which also shows monsoonal conditions over central Europe as well as northern Africa. The agreement between the proxy record and the simulations, on both seasonal and synoptic scales, thus promotes further interpretation of these model results on an entirely new scale.