Identifying the onset of the Messinian salinity crisis: a reassessment of the biochronostratigraphic tools (Piedmont Basin, NW Italy)

At the end of the Miocene, water exchange between the Mediterranean Sea and the world ocean was severely reduced, leading to deposition of huge volumes of evaporites during the Messinian salinity crisis (MSC). The onset of this event has been precisely dated at 5.971 Ma by means of magneto-, bio- and cyclostratigraphic tools, but clear paleobiological proxies for its recognition are so far missing, especially in those basins in which evaporite deposition is delayed. The disappearence of calcareous microfossils was the only paleobiological signal used to approximate the beginning of the crisis, but recently calcareous plankton has been recorded above the onset in several sections. Calcareous nannofossil and planktonic foraminifer data from the Piedmont Basin indicate that the cycle recording the beginning of the crisis is characterized by a peculiar succession of bioevents. These bioevents are (i) a calcareous nannofossil Sphenolithus abies abundance peak, followed or accompanied by minor peaks of Helicosphaera carteri, Umbilicosphara rotula and Rhabdosphaera procera, and (ii) the planktonic foraminifer Globorotalia scitula and G. suterae influx. The same sequence of bioevents has been recorded in sections from the Eastern and Central Mediterranean (Pissouri and Tokhni sections in Cyprus; Fanantello section in the Apennines) within the same age range. We thus propose that the S. abies and U. rotula peaks (often accompanied by minor peaks of H. carteri and R. procera) provide a reliable tool for the identification of the onset of the MSC independently from the occurrence of evaporites. This is particularly useful when studying successions deposited in intermediate- and deep-water basins, where evaporites are absent or their deposition is delayed. Our findings can potentially provide a reliable proxy for the identification of the MSC onset in deep-sea cores.