Upper Jurassic-lowermost Cretaceous biocalcification events derived from calcareous nannofossil and calpionellid paleo-fluxes: an example of envinromental stability promoting pelagic biocalcification?

The Tithonian-Berriasian time interval is characterized by a major calcareous nannofossil speciation episode and by the appearance of the calpionellid group: several genera and species first appear and rapidly evolve, showing an increase in diversity, abundance and particularly in calcification degree. This time interval is also characterized by a significant increase in carbonate-rich sediments. Our aim is to reconstruct calcareous nannofossil and calpionellid calcite paleo-fluxes to monitor the impact of biocalcification on pelagic sedimentation and to point out any linkages between calcareous plankton evolution and palaeoclimatic and/or palaeoceanographic conditions. Calcareous nannofossil and calpionellid absolute abundances have been achieved on Monte Pernice and Torre de’ Busi sections (Southern Alps, Northern Italy) and DSDP Site 534 (Central Atlantic Ocean). Paleo-fluxes have been obtained integrating absolute abundances with single specimen mass-weight (10-12gr of CaCO3) per area unit (cm2) per time unit (yr). During the Early Tithonian a first calcification event is characterized by nannolith (F.multicolumnatus, C.mexicana, P.beckmannii) increase in abundance, size and calcification degree, followed by the occurrence of first calcified calpionellids (Tintinopsella). A second, larger calcification event occurs across the Jurassic/Cretaceous boundary: it is characterized by a dramatic increase of nannoconid abundance and calcification degree, concomitant with a moderate abundance increase in calcified calpionellids (genera Crassicollaria, Calpionella, Remaniella). The calcareous nannofossil and calpionellid contributions to paleo-fluxes have been compared: despite the minute sizes, nannofossils, particularly nannoliths, determine the sedimentation of biogenic carbonates, with calcite paleo-fluxes of 2 orders of magnitude higher than calpionellids. The Late Jurassic-earliest Cretaceous time interval is characterized by generally stable environmental conditions, with relatively cool climate, low pCO2 and low oceanic Mg/Ca ratio. It is here inferred that the Tithonian appearance and mass occurrence of strongly calcified plankton and its evolutionary expansion were mostly induced by stable environmental conditions, favouring diversification of highly calcified specimens. Biomineralization rates by highly calcified plankton produced a major increase in pelagic carbonate sedimentation, affecting the carbonate system at the short- and long- term C-cycle.