Large and rapid salinity fluctuations affected the eastern Mediterranean at the Tortonian–Messinian transition

Restricted marine basins are highly sensitive to climatic fluctuations, yet, paleoenvironmental responses to gateway restriction frequently remain unclear. Here, we investigate the Mediterranean Sea that experienced gradually restricted conditions starting during the late Tortonian and culminating with complete isolation from the Atlantic during the peak of the Messinian Salinity Crisis. We established sea surface temperature and salinity records during the Tortonian to Messinian transition (7.52–7.20 Ma) through coupled analysis of organic biomarkers and oxygen and carbon isotope ratios of planktonic foraminifera from the eastern Mediterranean basin (Potamida section, Crete Island, Greece). We further contrast these sea surface data with stable isotope records of benthonic foraminifera to identify periods of increased density-driven deterioration of bottom water ventilation in the basin. The combined results show that normal marine conditions, expressed by relatively warm (27.5 °C) Tortonian surface waters with normal salinity (38) prevailed until 7.36 Ma. The 7.36–7.32 Ma interval is characterized by pulses of increased bottom water salinity, that contrast the lasting normal salinity (39) at the sea surface, indicating strengthened water column stratification provoked by sluggish water circulation already prior to the Tortonian–Messinian boundary. A brief return to marine conditions, similar to the time prior to 7.36 Ma, re-appeared between 7.32 and 7.31 Ma. Between 7.31 and 7.28 Ma, a rebound to enhanced stratification took place accompanied by a high-amplitude, stepwise decrease in both sea surface temperature and sea surface salinity. Ultimately, the Tortonian–Messinian boundary was characterized by lower sea surface temperature and salinity accompanied by increased salinity and/or colder bottom waters. A ∼ 10 °C cooling episode (at 7.212 Ma) highlights the regional importance of the global Late Miocene cooling event.