The effects of excess CO2 on calcareous nannoplankton biocalcification: the case history of the latest Cenomanian Oceanic Anoxic Event 2

The latest Cenomanian Oceanic Anoxic Event 2 (OAE 2, ~ 94 Ma) represents a profound perturbation of the ocean-atmosphere system caused by natural CO2 emissions related to the emplacement of the Caribbean Plateau inducing climate change, ocean fertilization and acidification. This study was performed on pelagic sediments from five localities: Eastbourne (Sussex, United Kingdom), Clot de Chevalier (France), Novara di Sicilia (Sicily, Italy) and two Western Interior sections (Pueblo, Colorado and Cuba, Kansas, USA). These five sections have been chosen based on availability of integrated stratigraphy. In fact, they all have a good time control, especially C isotopic stratigraphy and biostratigraphy, that offers the opportunity to correlate data from the different localities, discriminating between local, regional and global changes. The work was aimed at the identification of possible changes in coccolith size/shape as a response to paleoenvironmental perturbations associated with Oceanic Anoxic Event 2 (OAE 2). Biometric analyses were performed on selected coccolith species Biscutum constans and Watznaueria barnesiae across the Cenomanian –Turonian time interval. For each species, length and width of coccoliths have been measured on digitally captured images. The data collected document a decrease in size of B. constans observed in all sections during OAE 2, interpreted as due to excess CO2 that might have induced species-specific dwarfism. On the other hand, any changes in size of W. barnesiae were detected, suggesting that this taxon was less sensitive to stressed environmental conditions. The comparison of our morphometric data with those available for the early Aptian OAE 1a and latest Albian OAE 1d, indicates that B. constans repeatedly underwent size reduction and malformation possibly suggesting that the same paleoenvironmental factors controlled calcification of B. constans during OAEs. The analyses also pointed out a progressive reduction of the mean size of B. constans through time, with generally larger specimens in the early Aptian, intermediate in the late Albian and smaller in the Cenomanian-Turonian boundary interval, here potentially ascribed to different degrees of paleoenvironmental changes. The data available for OAE 1a, OAE 1d and OAE 2 suggest that ocean chemistry related to the amount of CO2 concentrations, played a central role in coccolith secretion by B. constans. We speculate that, even if different degrees (and maybe types) of paleoenvironmetal perturbations were acting during the Cretaceous OAEs, biocalcification of specific coccoliths reacted analogously with dwarfism when threshold conditions of excess CO2 and warming were reached.