Size variations in selected coccolith species through Mesozoic oceanic anoxic events: implications for paleoceanographic reconstructions

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. A total of 30 specimens of B. constans and 50 of W. barnesiae have been analyzed in each sample performing 4,870 measurements of B. constans and 7,500 of W. barnesiae. In addition, the coccolith ellipticity and surface area were calculated. This study was carried out on samples coming from five sections situated at different paleolatitudes including: Eastbourne, Clot de Chevalier, Novara di Sicilia, Pueblo, and Cuba. 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). The data collected document a decrease in size or “dwarfism” of B. constans observed in all sections during OAE 2, here 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 morphometric data collected for OAE 2 were compared with similar data available for B. constans and W. barnesiae across the early Aptian OAE 1a and the late Albian OAE 1d. Although absolute values differ, very similar trends were observed: in particular, B. constans coccoliths decrease in size in the core of the OAE 1a perturbation, becoming dwarf. For the Toarcian OAE, morphometric analyses were conducted on Biscutum and Lotharingius, both showing dwarfism in the anoxic interval. We speculate that, even if different degrees (and maybe types) of paleoenvironmetal perturbations were acting during the T-OAE, OAE1a, and OAE2, biocalcification of specific coccoliths reacted analogously with dwarfism when threshold conditions of excess CO2 and warming were reached.