Palaeoenvironmental reconstructions of the Messinian/Zanclean boundary using ostracods and forams: the Montepetra borehole (North Apennine, Italy)

The 80 m thick Montepetra core (Romagna Apennine, Italy) spans the stratigraphic interval comprised between the upper Messinian lago–mare (p–ev2 unit) and the lower Zanclean marine deposits. The Messinian deposits consist of the superimposition of at least two lithological fining upward cycles yielding brackish ostracods with Paratethyan affinity, referable to the Loxocorniculina djafarovi Zone, and generally scarce and partially reworked benthic and planktonic foraminifers. The Pliocene is represented by the very base of the Zanclean stage as suggested by the presence of the Sphaeroidinellopsis acme, the shift of Neogloboquadrina acostaensis sinistrally coiled and the base of the Thvera magnetic Subchron. The results of a multivariate approach, using the cluster analysis and the Detrended Correspondence Analysis on the in situ ostracods and the study of the foraminifer assemblages suggest that the salinity was the main parameter driving the environmental changes of the Messinian interval. It spanned cyclically from high mesohaline (assemblages dominated by Ammonia tepida and Cyprideis agrigentina) to low mesohaline in the lower part of the core, and from middle mesohaline to oligohaline in the upper part. Depth variations are less remarkable, except for two short deepening events in correspondence of the occurrence of “pointed candonids”. At the M/P boundary the palaeoenvironmental change is abrupt and sharp, evidenced by a complete change in the fossiliferous content.