THE END OF THE MESSINIAN SALINITY CRISIS IN THE MEDITERRANEAN: new data on the Miocene-Pliocene boundary.

The Miocene-Pliocene transition (MPT) in the Mediterranean area represents one of the unresolved geological riddles of the Neogene. The MPT coincides with the end of the Messinian salinity crisis (MSC, Hsü et al., 1977), an event that led to the deposition of massive volume of evaporite on the Mediterranean seafloor. The final stage of the MSC started at ~5.53 Ma and its uppermost part corresponds to the “Lago-Mare” phase, characterized by the occurrence of brackish shallow water ostracods of Parathethyan origin. The “Lago-Mare” deposits are sharply overlain by Zanclean (earliest Pliocene) marine sediments, astrocronologically dated to start at 5.33 Ma (Van Couvering et al. 2000).

The interpretation of this abrupt environmental change is strongly debated. One scenario assumes a catastrophic flooding of all Mediterranean sub-basins that were previously disconnected from the Atlantic Ocean and from each other (Caruso et al., 2020). An alternative scenario invokes a gradual refilling started during the Lago-Mare phase and continued during the basal Pliocene (early Zanclean) (Roveri et al., 2008; Stoica et al., 2016; Merzeraud et al., 2018).

To investigate the paleoenvironmental conditions across the MP transition, we investigated six sections along a west to east transect of the Apennines foredeep, using an integrated approach that merge the traditional stratigraphic, palaeontological, geochemical, and petrographic data with the analysis of molecular fossils (lipid biomarkers).

The top of the Messinian sediments is marked by a bioturbated dark layer in all six studied sections. The presence of glauconite at the top of the dark layer and of firm ground burrows of the Glossifungites icnofacies filled with Zanclean sediments suggest starved sedimentary conditions and the partial lithification of the sea floor during the earliest Zanclean. In addition, the benthic foraminifera indicate an increase of bottom oxygen content and a deepening of the basin across the MPT. Preliminary results of over 40 samples indicate excellently preserved molecular fossils both in the “Lago-Mare” sediments and in the Zanclean open marine deposits with a predominance of terrestrially-derived higher-plants long chain n-alkanes (LC_alk) and of glycerol dialkyl glycerol tetraethers (GDGTs) of both marine and terrestrial origin. Future analyses will focus on the compound specific carbon and hydrogen isotopes of LC_alk to further constrain precipitation and vegetation changes associated to the MPT. Changes in seawater (via isoprenoidal GDGTs) and land temperatures (via branched GDGTs) will be also reconstructed.