Reliability of calcareous nannofossil events in the Tithonian-early Berriasian time interval : implications for a revised high resolution zonation

The latest Jurassic was a crucial time for calcareous nannoplankton as a major speciation episode took place with the appearance and rapid evolution of several new genera and species, particularly of highly calcified nannoliths at low latitudes: this origination pulse provides the opportunity to achieve high-resolution biostratigraphic data amplifying the possibility of dating and correlating. A thorough revision of published as well as newly updated nannofossil biostratigraphies across the Tithonian–lowermost Berriasian interval was achieved to quantitatively evaluate the reliability of individual nannofossil events. Our database comprises 95 land and oceanic sites from different paleogeographic settings and latitudes (Europe, Atlantic Ocean, America, South Asia). We excluded sections with hiatuses or characterized by uncertain calcareous nannofossil biostratigraphy due to scarcity of data and/or poor preservation, as well as taxonomic incongruities. For each stratigraphic section, the critical assessment took into account sampling rates, nannofossil preservation, abundance and taxonomy: biohorizons based on single samples, questionably determined, or based on ambiguous/mismatching taxonomic concepts were excluded. We first performed calibration of nannofossil events against magnetostratigraphy to highlight reproducibility and time variability through the CM22–CM17 interval. The time uncertainties of individual nannofossil events were calculated, considering sampling and sedimentation rates derived from magnetostratigraphy. The reproducibility and variability of nannofossil events were also estimated relative to calpionellid biostratigraphy, as the latter is the primary tool selected by the Berriasian Working Group (ICS) for the definition of the Cretaceous base. We distinguished highly reliable events characterized by maximum reproducibility, shorter time variability, common and continuous occurrence; moderately reliable events are distinguished by intermediate reproducibility and time variability; unreliable events have minimum reproducibility and longest time variability. The quantitative evaluation of 37 events against magnetostratigraphy in the CM22–CM17 interval allowed to discriminate among 9 highly reliable first occurrences, 4 reliable first occurrences, 10 moderately reliable events (7 first occurrences, 2 last occurrences and 1 last common occurrence), 6 poorly reliable events (5 first occurrences and 1 last occurrence) and 8 unreliable first occurrences. The evaluation of 18 events against the calpionellid zonation resulted in 4 highly reliable first occurrences. The revisited taxonomy and reliability assessment of calcareous nannofossil events were used to revise the calcareous nannofossil zonation of the Tithonian–early Berriasian time interval: one zone (NC 0 Zone) and four subzones (NC 0a, NC 0b, NJT 14a, NJT 14b Subzones) are newly defined here. Moreover, one zonal definition is emended (NJT 17 Zone) and the age of top/bases of four zones (NJT 14, NJT 15; NJT 16, NC 1 Zones) are revised. The first appearance datum of Nannoconus wintereri, showing the highest reproducibility and lowest time variability, correlates with magnetochron 19n-2n and results to be the calcareous nannofossil event best approximating the base of the Calpionella alpina Zone equated to the Jurassic/Cretaceous boundary.