Calcareous nannoplankton response to the ocean-atmosphere perturbation associated with the latest Cenomanian OAE 2

The latest Cenomanian Oceanic Anoxic Event (OAE) 2 was a time of severe perturbations caused by natural CO2 emissions related to climate change, ocean fertilization and acidification probably associated with the Caribbean Plateau emplacement. This study was performed on pelagic sediments from different localities: Eastbourne, Clot de Chevalier, Novara di Sicilia and two Western Interior sections. Morphometric analyses were performed on selected calcareous nannofossil taxa and indicate a calcification decline of calcareous nannoplankton in the form of a reduction in size of the produced coccoliths. B. constans appears to be the most sensitive species: its decrease in size goes along with deformation (increased ellipticity). Size fluctuations through OAE2 are observed, with similar and synchronous changes in all the analyzed sections at distant sites and with comparable preservation state. Specifically, coccolith size fluctuations match with paleoceanographic reconstructions: • an increase in coccolith size at δ13C isotopic peak A is correlated with a pCO2 drawdown • minimum size is coeval with δ13C isotopic peak B where a trace metal peak occurs along with an increase in sea surface temperature. Morphometric data obtained for OAE2 were compared with the OAE 1a datawhen the submarine volcanism of Ontong Java Plateau induced excess CO2 that was detrimental to marine calcifiers, with temporary failure of nannoconids and production of dwarf/malformed coccoliths. Coccolith size and shape variations possibly represent an adjustment to survive lower pH. Furthermore hydrothermal plumes during construction introduced biolimiting/toxic metals that might have disturbed the functioning of some coccolithophorid species affecting biocalcification. These data suggest that ocean chemistry related to the amount of CO2 concentrations, played a central role in coccolith secretion by B. constans with a repetitive reduction in size during OAE 1a and 2.