Record of Mid-Eocene Warming Events in the Istrian Paleogene Basin, Neotethys (Outer Dinarides, Croatia)

Several short sections from Istria (northern Adriatic Sea) were studied to determine the impact of short-term climate variability on pelagic assemblages from the northern mid-latitudes during a critical period of evolution in the mid-Eocene greenhouse climate. Two important warming events were documented in this interval—the Late Lutetian Thermal Maximum (LLTM) and the Middle Eocene Climate Optimum (MECO). The warmings were characterised by the highest calcareous nannofossil species richness and an eutrophic environment. Our records of calcareous nannofossil index species allowed the standard Zones NP16–NP17 (Mediterranean Subzones MNP16A–MNP17A) to be applied. Common occurrences of the planktonic foraminiferal genera Subbotina and Turborotalia indicate eutrophic-to- mesotrophic conditions between the mixed layer and the shallow thermocline waters in the basin. Episodes of eutrophication from periodic upwelling under an accelerated warming event (in MNP16A) are indicated by the subdominant Coccolithus pelagicus. According to the isotope data, the maximum negative δ13C occurred at 45° N in Alano, with a prominent second peak at 39.66 Ma, which is in agreement with our possible second peak, suggesting higher insolation in the North during the warming event. The warm water Reticulofenestra producta dominated in Subzone MNP17A, suggesting a relatively shallow mesotrophic environment with decreased species richness characteristic of the post-MECO phase.