Calcareous nannofossil quantitative data from the Tithonian-Berriasian interval: implications for the Jurassic/Cretaceous boundary

The Jurassic/Cretaceous (J/K) boundary time interval is characterized by a major calcareous nannofossil speciation episode: several Cretaceous genera and species appear and rapidly evolve. Nannoliths, and especially nannoconids, show a progressive increase in diversity, abundance and degree of calcification through time, which have been documented in both Atlantic and Tethys oceans (Nannofossil Calcification Event – NCE, Bornemann et al., 2003; NCEs, Casellato, 2009). Calcareous nannofossil biostratigraphy, relative and absolute abundances have been investigated on selected Tethyan land sections (Southern Alps, Northern Italy) in order to integrate calcareous nannofossil events with the polarity chron sequence and, partly, with calpionellid biostratigraphy. Analyses have been performed on un-heated magneto-core end-pieces, using both simple smear slides and ultra-thin sections (7-8µm thick). Calcareous nannofossil absolute abundances, performed on ultra-thin sections, have been also investigated on the DSDP Site 534 from Central Atlantic Ocean, in order to compare nannofossil patterns in different paleogeographic settings and to point out their supraregional importance. All calcareous nannofossil zones and subzones proposed for the Tithonian-Berriasian interval (NJ-19; NJ-20a, NJ-20b; NJK-A, NJK-B, NJK-C, NJK-D; NK-1, Bralower et al. 1989) have been recognized. Quantitative studies indicate that nannolith taxa (firstly F.multicolumnatus, then C.mexicana, finally P.beckmannii) increase significantly in abundance, size and calcification degree gaining lithogenetic proportion (NCE1, Casellato 2009): the abundance acmes are reached in discrete steps between calcareous nannofossil Zones NJ-20 and NJK-A. Nannoconids rapidly evolve across the J/K boundary, reaching lithogenetic abundances from calcareous nannofossil Subzone NJK-C upward (NCE2, Casellato 2009), when highly calcified conical morphotypes (N.wintereri, N.steinmannii minor, N.kamptneri minor) appear. This event correlates with the middle CM19n to the CM18r interval and the ‘explosive’ appearance of small globular Calpionella alpina (C.alpina “acme”). Calcareous nannofossil quantitative studies permit to identify additional potential events, characterized by an increase in size, calcification degree and abundance of nannoliths. Calibration with magnetostratigraphy indicates that these trends could be very useful as new bio-horizons in identifying the lower Upper Tithonian and for locating the J/K boundary. The speciation of highly calcified nannoconids and their remarkable increase in volume and abundance, increase the stratigraphic resolution of the J/K boundary time interval. Bornemann, A., Aschwer, U. and Mutterlose, J., 2003. The impact of calcareous nannofossils on the pelagic carbonate accumulation across the Jurassic-Cretaceous boundary. Palaeogeography Palaeoclimatology Palaeoecology, 199(3-4): 187-228. Bralower, T.J., Monechi, S. and Thierstein, H.R., 1989. Calcareous nannofossil Zonation of the Jurassic-Cretaceous Boundary Interval and Correlation with the Geomagnetic Polarity Timescale. Marine Micropaleontology, 14: 153-235. Casellato C.E. (2009) - Causes and consequences of calcareous nannoplankton evolution in the Late Jurassic: implications for biogeochronology, biocalcification and ocean chemistry. PhD Thesis, 116 pp., Università degli Studi di Milano, Scuola di Dottorato “Terra, Ambiente e Biodiversità”, Dottorato di ricerca in Scienze della Terra, Ciclo XXI.