Foraminiferal assemblages and paleoenvironmental reconstruction in the Messinian ofTertiary Piedmont Basin (Northwestern Italy)

The Messinian Salinity Crisis was a major event in the Mediterranean geological history and has been preceded by progressive changes in water chemicophysical parameters as well as in marine assemblages, culminating in the deposition of evaporitic deposits. Multidisciplinary studies are in progress on the Messinian succession cropping out in the surrounding of Alba (Tertiary Piedmont Basin, NO Italy). The analyses allow the correlation of the TPB Messinian deposits with the well known coeval successions of the Mediterranean area. Foraminiferal assemblages of the pre-evaporitic Sant’Agata Fossili Marls (SAF), represented by a cyclic alternation of homogeneous marls and laminated euxinic shales, are referable to the Early Messinian Globorotalia conomiozea zone and part of them pertain to the distribution interval of Globorotalia nicolae (FO 6.82 Ma – LO 6.72 Ma).They document the transition from upper epibathyal and oxigenated bottoms to a shallower, restricted basin. The water column shallowing and increasing eutrophy are suggested by the progressive decrease of the P/(P+B) ratio, of the planktonic and benthic specific diversity, by the disapperance of benthic deep taxa and by the abundance increase of both inner neritic (Ammonia etc.) and stress-tolerant infaunal taxa (Bolivinids, Buliminids). Upper SAF cycles are progressively devoid of planktonic and benthic foraminifers. Quantitative data on planktonic foraminifers show cyclical variations in abundance of warm water oligotrophic forms (Orbulina universa and Globigerinoides sp.) and of cold watereutrophictaxa (mainly fitophagous Neogloboquadrinids). Moreover, an abundance peak of Neogloboquadrina atlantica (Mediterranean Messinian FCO 6.65 Ma) occurs in the middle-upper SAF and documents an influx of Atlantic cool waters, as well as eutrophic conditions, or seasonal upwelling, just before the basin isolation. Calcareus nannofossils add other data about the paleoenvironment and appear less affected by environmental stress than foraminifers. Cyclic eutrophic conditions are also supported by peak of “small” Reticulofenestra and Helicosphaera carteri and by the progressive decrease of oligotrophic Discoaster spp. The sharp increase in surface salinity is testified in the uppermost SAF by the Sphenolithus abies peak abundance.