Climatic and hydrologic variability in the northern Mediterranean across the onset of the Messinian salinity crisis

The youngest and one of the largest saline deposits of Earth history formed in the Mediterranean during the Messinian salinity crisis (MSC; 5.97–5.33 Ma, Late Miocene), when global climate was experiencing a trend of cooling and aridification. However, recent paleoclimate reconstructions indicate strong climate gradients across the Mediterranean Basin and the persistence of humid conditions in the northern Mediterranean during the earliest phase of the MSC. To characterize climatic and hydrologic variability across the onset of the MSC, we studied inorganic geochemical proxies, comprising Si, Ti, and Zr element distributions and organic geochemical proxies, such as sedimentary plant wax contents and their carbon (δ13Cn-alk) and hydrogen (δ2Ηn-alk) stable isotope compositions in deep-water (> 200 m), alternating shale and marl deposits of the Piedmont Basin (Govone section; NW Italy). Our results suggest that deposition was controlled by precession-driven climate fluctuations before and after the onset of the MSC, with shales deposited under moister conditions (lower element/ Al ratios and δ2Ηn-alk values) and marls when drier conditions prevailed (higher element/Al ratios and δ2Ηn-alk values). The observed δ2Ηn-alk pattern further suggests changes in the relative contributions of the sources of moisture for precipitation. At precession minima, shales were deposited and moisture was mainly sourced from the North Atlantic, as reflected by lower δ2Ηn-alk values. At precession maxima, marls were deposited and higher δ2Ηn-alk values point to a – probably western – Mediterranean source of moisture. The decrease of element/Al ratios as well as the coeval increase of plant wax abundances after the onset of the MSC indicate enhanced humidity and increased net precipitation over the northern Mediterranean. These changes coincided with a restriction of the Atlantic-Mediterranean gateways and resulted from an enhanced contribution of deuterium-enriched moisture sourced from seawater evaporation in the Mediterranean, evidenced by an increase of δ2Ηn-alk values.