Micropalaentological evidences of high productivity episodes in the Zanclean of Piedmont (Early Pliocene, Northwestern Italy). Boll. Soc. Paleont. Ital., 50 (2): 111-133.

ABSTRACT - Quantitative and semiquantitative analyses carried out on calcareous (foraminifers, ostracods, calcareous nannofossils = CN) and siliceous (diatoms, radiolarians) microfossil associations preserved in diatomaceous silts and clays from Monferrato (Piedmont, Northwestern Italy), forming part of the Argille Azzurre (Blue Clay) Formation, document episodes of enhanced productivity during the Early Pliocene (Zanclean). This study represents the first documentation in Piedmont of Pliocene mixed calcareous and siliceous microfossil assemblages, generally reported from areas of upwelling and high productivity conditions. The assemblages collected from the sections of La Torretta, Castelcebro and Calliano allow a nearly coincident time interval to be proposed for their deposition, in the MPL3 foraminiferal zone (Globorotalia margaritae - Globorotalia puncticulata Zone), the MNN13 and/or MNN14-15 CN zones, the Nitzschia jouseae diatom Zone and RN11 (Stichocorys peregrina Zone) radiolarian zone. High productivity in the water column is documented by the following: the frequent occurrence of diatoms, dominated by Chaetoceros RS, Thalassionema group and locally by Paralia sulcata, all related to upwelling conditions; the common to frequent presence of radiolarian; opportunistic phytophagous or dominantly phytophagous cool-water planktonic foraminifers (Globigerina bulloides, Neogloboquadrina acostaensis, Globigerinita glutinata and Turborotalita quinqueloba); benthic infaunal taxa, phytodetritus feeders or exploiting different trophic resources (Cassidulina carinata, Bulimina spp., Stainforthia complanata); the common ostracod Costa edwarsii and also by common Calcidiscus leptoporus, Helicosphaera carteri and H. sellii, also related to mesotrophic to eutrophic cool waters. The absence of the oligotrophic discoasterids may be another signal of high nutrient levels. Productivity was rather high in comparison with that suggested for coeval assemblages in Piedmont, devoid of siliceous microfossils and generally characterized by more diversified foraminiferal and ostracod assemblages. Nevertheless, the rather low diatom numbers suggest a moderate productivity in comparison with recent oceanic upwelling areas. The low foraminiferal number (FN) and benthic foraminifer and ostracod diversity may result from dilution of tests in the generally very abundant fine-grained terrigenous fraction, but could be influenced also by seasonal upwelling conditions. Similar very low FN characterized the MPl3 assemblages in the deeper, epibathyal succession of Moncucco Torinese and suggest an increase of deposition rate, due to the warm, moist Early Pliocene climate. Seasonal successions of cool eutrophic (G. bulloides, N. acostaensis, T. quinqueloba) and warm oligotrophic taxa (Globigerinoides spp., Orbulina spp.) are suggested in the upper La Torretta section by their contemporary common occurrence.