Foraminiferal assemblages of the Bric della Muda laminites (Nizza Monferrato, Piedmont): Proxies of cyclic paleoenvironmental changes in the Lower Messinian of North-western Italy

Foraminiferal assemblages in laminites cropping out at the Bric (i.e. Hill) della Muda (Nizza Monferrato, Piedmont, Northwestern Italy) document deep marine conditions during the early Messinian, spanning a time-interval approximating, or slightly wider than the stratigraphic range of Globorotalia nicolae (6.82–6.72 Ma). Fluctuations in abundance of planktonic warm-water oligotrophic taxa and cold-water eutrophic species show a cyclical pattern, very comparable to variations measured in cyclically-bedded formations of the pre-evaporitic Messinian in the Mediterranean area. A subtropical climate with cyclic warmer and cooler episodes, characterized by surface water stratification and vertical mixing in the water column, respectively, is also documented. A depositional depth close to or larger than 1000 m is inferred on the basis of planktonic assemblages yielding also the deep planktonic Hastigerinella digitata and of very high P/(P + B) ratios. Anoxic or strongly dysoxic bottom conditions are indicated by the absence or rareness of benthic foraminifers, which are represented by taxa tolerant of low-oxygen or high organic content (Bolivina pseudoplicata, Chilostomella oolina, Globobulimina affinis). A slight decrease in P/(P + B) ratios, and the presence of rare shallow-water forms in the uppermost layers suggest a mild dysoxia and a probable shallowing, when compared with coeval events in Northern Apennines and Southern Greece sections.