Multi-proxy investigation of the post-evaporitic succession of the Piedmont Basin (Pollenzo section, NW Italy): a new piece in the Stage 3 puzzle of the Messinian Salinity Crisis

Paleoenvironmental reconstruction of the Mediterranean Basin at the end of the Messinian Salinity Crisis is contentious. One section that records this final phase (Stage 3) is the Pollenzo Section in the Piedmont Basin (NW Italy). Here, we present new stratigraphic, sedimentological, petrographic, micropaleontological (ostracods, calcareous nannofossils, foraminifera, dinoflagellates) and geochemical (87Sr/86Sr ratios) data from the Cassano Spinola Conglomerates (CSC) and interpret the paleoenvironment of this northernmost tip of the Mediterranean Basin. The CSC comprise three depositional units: members A and C, which were deposited subaqueously, and the intervening member B, which is continental. The CSC is topped by a ~50 cm-thick black layer, which is directly overlain by the open marine Argille Azzurre Formation of early Zanclean age. Our investigation reveals that member A is largely barren of autochtonous microfossils, except for an almost monospecific ostracod assemblage of Cyprideis torosa at the top, which indicates shallow-water (<30 m) conditions. Paratethyan ostracods and, possibly, taxa of calcareous nannofossils adapted to low-salinity water first occur in member C. 87Sr/86Sr ratios measured on ostracod valves from the member A/B transition (0.708871-0.708870) and member C (0.708834-0.708746) are lower than the coeval Messinian seawater values (~0.709024) and the 87Sr/86Sr ratios of a hypothetical lake filling Piedmont (>0.7090) estimated by means of the present-day 87Sr/86Sr signature of the Po river, the main drainage system of Northern Italy that receives the weathering products (including ions) of the Alps and Apennines. These values are likely to reflect the mixing of local high 87Sr/86Sr river water with low 87Sr/86Sr from the Mediterranean, which at the time was dominated by inputs from Eastern Paratethys, circum-Mediterranean rivers and Atlantic Ocean. Our results suggest that, at times during the final stage of the Messinian Salinity Crisis, the Piedmont Basin was hydrologically connected with the main Mediterranean Basin. At regional scale, this implies that the water level in the Mediterranean Basin was relatively high.