Lago-mare – A new scenario for the final stage and the end of the Messinian salinity crisis

A critical review of the contrasting scenarios proposed for the final evolutionary stage of the Messinian Salinity Crisis, the "Lago-mare", suggests that each hypothesis may capture certain aspects of the puzzle. Integrating all the scientific elements into a coherent framework remains a significant challenge. This impasse may stem from the foundational assumptions of the prevailing crisis paradigm, which posits a negative hydrological budget for the Mediterranean, its isolation from the Atlantic, and a dominant influx of low-salinity waters from the Paratethys. The homogeneity of Sr-87/Sr-86 values, the absence of marine mollusks, the lack of unquestionable evidence for both shallow water indicators in deep basins and for high-amplitude water level changes suggest a new, comprehensive scenario, implying a positive Mediterranean hydrological budget and a continuous, albeit reduced and possibly intermittent, inflow of deep to intermediate Atlantic water. The Lago-mare is interpreted as a single, sustained event spanning the entirety of stage 3 (5.54-5.33 Ma), during which the Mediterranean remained full of water, possibly maintaining a connection with the Atlantic through the paleo-Gibraltar Strait, exhibited strong density stratification, and developed an estuarine circulation. We propose a tripartite water column structure, comprising: i) a surface brackish lid (< 200 m thick); ii) an intermediate marine layer sustained by Atlantic inflow; iii) a deep, dense, likely hypersaline layer formed during stages 1 and 2. The low salinity of the brackish lid is evidenced by the absence of mollusk taxa typical of fully marine and/or marineconnected shallow-water environments. The surface layer resulted from an increased freshwater input, not necessarily derived only from the Paratethys, but also driven by a shift in the regional precipitation patterns linked to the onset of the African Monsoon and the simultaneous uplift and emergence of the ApennineMaghrebian orogen in the middle of the ancient Mediterranean. Such a hydrological configuration led to oxygen-depleted conditions in deep settings, as indicated by the apparent absence of benthic fauna and bioturbation. The presence of euryhaline organisms, especially near the top of stage 3 and close to the Messinian-Zanclean boundary, may reflect episodic marine incursions or brief hydrological shifts. A mostly permanent Atlantic connection, along with a sustained marine underflow, could account for the erosional features observed at Gibraltar because of flow action throughout the whole Messinian, as well as for the Mediterranean base-level rise documented in the upper part of stage 3. This rise expanded shallow-water habitats and may have facilitated the observed progressive diversification and proliferation of Paratethyan hypohalinemesohaline (anomalohaline) biota, i.e. those inhabiting water bodies of variable salinity, but disconnected from the marine setting. Under these conditions, the salinity crisis ended as the brackish surface layer gradually thinned, permitting the inflow of Atlantic surface waters, inducing the shift of the circulation pattern to an antiestuarine mode, the activation of deep-water circulation and the development of the Mediterranean outflow. The return to fully marine conditions was rapid yet gradual, typically marked by a transitional interval and likely progressing in a slightly diachronous fashion along a west-to-east gradient.