Calcareous plankton and shallow-water benthic biocalcifiers: Resilience and extinction across the Cenomanian-Turonian Oceanic Anoxic Event 2

Oceanic Anoxic Event 2 (OAE 2), spanning the Cenomanian/Turonian boundary (93.9 Ma), was anepisode of major perturbation of the global carbon cycle. Its geochemical signature is a synchronouspositive δ13C excursion in both carbonates and organic matter that resulted from the net burial oflarge amounts of organic carbon in deep-sea and hemipelagic settings.Causes for OAE 2 are still the subject of investigations; however, several studies postulate thatmassive submarine volcanic activity emitted greenhouse gases and provided biolimiting metals inmarine ecosystems, leading to the onset of the Cenomanian-Turonian Thermal Maximum and to theenhancement of ocean fertility. Ocean temperature, sea-surface stratification, nutrient availability, and carbonate ion saturation were subject to variations during OAE 2 that resulted in fluctuations indiversity abundance and calcification of species.We analyzed the record of the main biocalcifiers of pelagic-hemipelagic settings (planktonicforaminifera and calcareous nannofossils) and of low-latitude carbonate platforms (larger benthicforaminifera and rudist bivalves) by looking at well-dated sections. Carbon isotope stratigraphyallowed precise correlation from shallow to deep water and tied the biotic response to the record ofgeochemical proxies of paleoenvironmental changes. The main extinction event, severely affectingthe shallow-water benthic biocalcifiers and to a minor extent the calcareous plankton, occurredwithin and after the Plenus Cold Event. Fluctuations in surface seawater temperature and extremewarming were probably the main cause of extinction, with contributions from decreased seawatercarbonate saturation and disruption of ocean stratification. Overall, calcareous plankton fared muchbetter, showing a greater resilience than carbonate-platform biocalcifiers to paleoenvironmentalperturbations across the OAE 2.