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During Messinian time, the Mediterranean underwent hydrological modifications culminating 5.97 Ma ago with the Messinian salinity crisis (MSC). Evaporite deposition and alleged annihilation of most marine eukaryotes were taken as evidence of the establishment of basin-wide hypersalinity followed by desiccation. However, the palaeoenvironmental conditions during the MSC are still a matter of debate, chiefly because most of its sedimentary record is buried below the abyssal plains of the present-day Mediterranean Sea. To shed light on environmental change at the advent and during the early phase of the MSC, we investigated the Govone section from the Piedmont Basin (NW Italy) using a multidisciplinary approach (organic geochemical, petrographic, and carbon and oxygen stable isotope analyses). The Govone section archives the onset of the crisis in a succession of organic-rich shales and dolomite-rich marls. The MSC part of the succession represents the deep-water equivalent of sulphate evaporites deposited at the basin margins during the first phase of the crisis. Our study reveals that the onset of the MSC was marked by the intensification of water-column stratification, rather than the establishment of widespread hypersaline conditions. A chemocline divided the water column into an oxygen-depleted, denser and more saline bottom layer and an oxygenated, upper seawater layer influenced by freshwater inflow. Vertical oscillations of the chemocline controlled the stratigraphic architecture of the sediments pertaining to the first stage of the MSC. Accordingly, temporal and spatial changes of water masses with different redox chemistries must be considered when interpreting the MSC event.

<p>In the late Miocene, the Mediterranean Basin became a restricted basin because of its progressive tectonic isolation from the Global Ocean. The almost complete halt of the Atlantic-Mediterranean water exchange about 6 Ma ago triggered the deposition of the Mediterranean Salt Giant during the Messinian salinity crisis (MSC; 5.97-5.33 Ma). The environmental conditions, which developed at the onset and during the MSC, are still debated since the evaporites buried beneath the modern Mediterranean seafloor are mostly inaccessible and the marginal successions contain scarce or no body fossils. Aiming to improve our knowledge on the environmental conditions at the onset of the MSC, we investigated the sedimentary record of intermediate palaeobathymetric settings (200-1000 m) from the Piedmont Basin (NW Italy) through a multidisciplinary approach (petrography, organic geochemistry). Shale/marl couplets deposited after the MSC onset are lateral time equivalents of shallow water (<200 m) shale/gypsum couplets deposited during the first phase of the crisis (5.97-5.60 Ma). Our results suggest that the MSC onset coincided with an intensification of water column stratification, most likely favoured by enhanced freshwater input due to moister climate conditions. No evidence of hypersaline conditions was found at the onset of the crisis, but rather normal marine conditions seem to have persisted at least in the upper water column, influenced by freshwater discharge. A stable chemocline apparently separated an upper water layer from a stagnant deeper-water body typified by reducing conditions. These physicochemical changes in the water column governed the sedimentary facies distribution during the first phase of the MSC.</p>