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The marine system of the Mediterranean-Paratethys region in the Middle Miocene was influenced by the global climatic changes corresponding to the Miocene Climate Optimum. The latter was characterized by global warming of deep oceanic waters succeeded by a decrease of wind activity and ocean water circulation together with a decline in oceanic productivity. This study provides a detailed paleoenvironmental and depositional history of the Middle Miocene deposits from Mt. Požeška Gora (the east part of Croatia). Stable carbon and oxygen isotope data with foraminiferal fauna coupled together with palynological data indicate both seasonal and long-term changes in surface water temperatures, nutrient contents, and salinity. Furthermore, it was possible to determine two phases of Middle Miocene flooding in the studied marine system: (a) a shallow, high nutrient marginal sea strongly affected by freshwater inputs corresponding to the thermal maximum and periods with enhanced precipitation during the Miocene Climate Optimum, and (b) a younger deeper marginal sea with a developed seasonal stratification, rather intermediate nutrient availability and reduced fresh-water inputs. Based on the microbiostratigraphy and strontium isotope stratigraphy (87Sr/86Sr) coupled with previously published high-precision geochronology, the timing of the first and second flooding can be correlated with the Langhian age - ≤15.4 Ma and 14.6 Ma, respectively Possibly, the properties of the surficial waters that were described as a part of the flooding phases in individual time intervals can indicate changes in circulation patterns as a result of potential influence of the open-sea water masses. • Determination of two transgressive pulses of the Middle Miocene flooding in the Paratethys. • (1) shallow marginal sea strongly affected by freshwater input. • (2) deeper marginal sea with a developed seasonal stratification. • Plankton biostratigraphy coupled with geochronology/SIS (87Sr/86Sr) - the timing of individual floodings. • Flooding phases - indicative of changes in circulation + sea-level changes. [ABSTRACT FROM AUTHOR]