The eastern Mediterranean pre-MSC brine pool as an analogue for future subtropical hydroclimate

During the Late Miocene the Mediterranean Sea experienced severe disruption of its connectivity to the Atlantic Ocean, highlighted by a rapid sea-level drop, culminating to the Messinian Salinity Crisis (MSC; 5.97-5.33 Ma). Such a paleoceanographic change, triggered by the cumulative effect of climate and tectonics, caused high-amplitude fluctuations in the hydrology of the entire basin, and further influenced the geological history of the Mediterranean Sea. Although a consistent pattern of the paleoclimate has started to emerge, we currently lack a continuous sea surface salinity (SSS) record linking the timing of sea surface temperature (SST) variations, sea-level fluctuations, and the overall environmental change, particularly for the pre-evaporitic period. Initial viewpoints of a linear and gradual salinity increase prior to the onset of the MSC have been recently revised and replaced by highly variable salinity-related patterns representative of the stepwise restriction of the Mediterranean Sea. Here we use the combined Tetra Ether (TEX₈₆-) and/or alkenone unsaturation ratio (U^K′₃₇) based SSTs and oxygen isotopes (δ¹⁸O) from the cyclic marl-sapropel sedimentary succession of Agios Myron section (north-central Crete, Greece) to assess hydroclimate changes during that time, and we finally present the first record of SSS in the eastern Mediterranean Sea for the earliest Messinian (7.2–6.5 Ma). The relatively stable marine conditions after the Tortonian/Messinian boundary, expressed through a cool and fresh upper water column, significantly changed at ∼6.9 Ma, when an important reversal in the heart of the Messinian cooling trend supplemented by a coherent hypersaline water column took place. The observed SST and SSS patterns provide context for a two-fold evolution of this event (centered at 6.9–6.8 and 6.72 Ma), which finally led to the onset of a brine pool into the eastern Mediterranean basin. The transitional character of the following time interval (6.7–6.5 Ma) marks another important step in the basin restriction with a wider range of salinity fluctuations from highly saline to diluted conditions and enhanced water column stratification prior to the deposition of evaporites. Overall, this evolution supports the concept of a stepwise restriction of the Mediterranean Sea associated with substantial hydroclimate variability and increasing environmental (thermal and salinity) stress, and further confirms its position as a preferred laboratory for developing new conceptual models in paleoceanography, allowing the investigation and scale assessment of a phenomenon with high chances of representing a future analogue scenario.