Orbitally Forced Hydrological Balance During the Messinian Salinity Crisis: Insights From Strontium Isotopes (87Sr/86Sr) in the Vena del Gesso Basin (Northern Apennines, Italy)

The 87Sr/86Sr ratio is a proxy of the hydrologic structure of marginal basins characterized by the mixing between marine and continental waters. This reliable paleoceanographic and paleoclimatic tool is considered a key to the reconstruction of high-amplitude hydrologic changes that governed the evolution of the Mediterranean region during the Messinian salinity crisis (MSC), when reduced connections with the global ocean resulted in the deposition of the most studied Earth's salt giant. Paleohydrological dynamics leading to the deposition of the Primary Lower Gypsum during the first stage of the MSC were revealed by the study of the 87Sr/86Sr composition of 114 samples from the Vena del Gesso basin (Northern Apennines, Italy). The analysis of seven high-resolution 87Sr/86Sr profiles from individual gypsum beds suggests a climatic influence of precessional-scale orbital parameters during evaporite deposition. The variations of Sr isotope ratios over time indicate that gypsum precipitation started from a seawater body dominated by continental contributions, followed by a relative increase of marine input. The overall results show a gradual upward detachment from the global ocean 87Sr/86Sr curve, with crucial steps corresponding to the most extreme eccentricity minima, suggesting an orbital influence on the hydrological balance of the basin. Given the sedimentological and geochemical analogies shared by the Mediterranean marginal basins that hosted the Primary Lower Gypsum deposition, the 87Sr/86Sr evolution of the Vena del Gesso basin may be considered as indicative of Mediterranean-scale hydrologic dynamics, driven by gradual reduction of exchanges with the Atlantic and variable freshwater input.