Estuarine Lago Mare fauna from the Tertiary Piedmont Basin indicates episodic Atlantic/Mediterranean exchange during the final stage of the Mediterranean Salinity Crisis

Abstract

The geochemical signature of fossils from Lago Mare deposits at Moncucco Torinese (NW Italy) indicates a temporary marine ingression into the Tertiary Piedmont Basin just before the Miocene/Pliocene boundary. Isotope (δ18O, δ13C, 87Sr/86Sr) and trace element (Sr/Ca, Ba/Ca) data from mollusk shells of multiple species and croaker otoliths indicate an upper estuarine environment with considerable seasonal variability in evaporation and freshwater input. Reconstructed salinities<10psu and low 87Sr/86Sr values indicate the close proximity of a watershed draining off of Mesozoic ophiolites and carbonates of the Alps and/or Apennines. Communication with an open marine environment is supported by otoliths of fully marine fishes such as lanternfishes, codlets, and slimeheads found in the same assemblage that carry a euhaline signature close to open ocean values. A thorough taphonomic evaluation renders reworking of the otoliths from pre-evaporitic deposits unlikely; instead we explain their presence in the Lago Mare assemblage as related to large predatory marine fish that periodically migrated into marginal marine and estuarine environments.The presence of a paralic paleobiotope with thalassogenic water in the Tertiary Piedmont Basin contradicts ideas of the Mediterranean as a brackish lake fed by the Paratethys during the latest stage of the Mediterranean Salinity Crisis. Instead, our data support previous notions of temporary Atlantic/Mediterranean exchange during sea-level highstands preceding the permanent re-connection at the Miocene/Pliocene transition. The establishment of short-lived and disjunct estuarine and lagoonal environments such as in the Tertiary Piedmont Basin during marine ingressions also puts highly variable 87Sr/86Sr values and occurrences of shallow water benthic foraminifera in Lago Mare deposits of other Mediterranean basins into a new perspective.