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Earth history is punctuated by phases of extreme global stress of concurrent warming, ocean fertilisation and acidification that impacted biologic diversity and function. Under excess CO2 and greenhouse conditions, the Mesozoic deep ocean became temporarily depleted of oxygen, promoting the accumulation of massive amounts of organic matter during Oceanic Anoxic Events (OAEs). Although global anoxia and enhanced organic matter burial are the most striking and intriguing palaeoceanograhic phenomena, OAEs can be studied also to decipher the oceanic ecosystem response to CO2 pulses. In Jurassic and Cretaceous oceans, calcareous nannoplankton were already common from coastal to open oceanic settings and of enough abundance and diversity to produce calcareous oozes. Indeed, Jurassic and Cretaceous pelagic micrites mainly consist of coccoliths and nannoliths, in addition to variable amounts of diagenetic carbonate. Therefore, pelagic limestones are ideal for epitomising variations in abundance and composition of calcareous phytoplankton at large scale to understand their response to global change.

Italian pelagic successions are a reference for the Tethys Ocean and, in general, for low to middle latitudes. We consider herein well-dated sections with quantitative nannofossil data across OAEs to synthesise changes in abundance of the dominant, micrite-forming, nannofossil taxa and species-specific variations in size to trace the response of calcareous nannoplankton as expressed by biocalcification across the early Toarcian T-OAE, late Valanginian Weissert-OAE, early Aptian OAE1a and latest Cenomanian OAE2. In general, a major decrease in nannofossil abundance is recorded for the highly calcified dominant forms, evidenced by the "Schizosphaerella crisis", the "nannoconid decline" and the "nannoconid crisis" during the T-OAE, Weissert-OAE and OAE1a, respectively. An even more dramatic drop in coccolith/nannolith abundance characterises OAE2, with a nannoplankton biocalcification "blackout" through the Bonarelli Level in Italian sections. Despite these abundance crises, calcareous nannofloras recovered soon after the paleoenvironmental perturbation terminated, although the return to pre-OAE conditions occurred rather slowly and assemblage composition was renewed across the event. Species-specific changes in size were detected for Schizosphaerella across the T-OAE and for Biscutum constans (Gorka, 1957) in the intervals of maximum perturbation within OAE1a and OAE2. Size of Nannoconus steimannii Kamptner, 1931, conversely, does not show variations across the Weissert-OAE and OAE1a. The T-OAE and OAE1a were preceded and accompanied by a few million-year-long origination phase, indicating the calcareous nannoplankton ability to positively respond to and overcome stressing oceanic conditions, as further evidenced by absence of extinctions. Calcareous nannoplankton reacted differently during the Weissert-OAE and OAE2 as the Valanginian "nannoconid decline" is gradual and followed by a symmetric increase in abundance, while the late Cenomanian nannofossil drop in abundance was as sudden as its recovery. In both cases, extinctions are paralleled by entry of new taxa, at a slower rate across the Weissert-OAE and at faster rates in the case of OAE2. The influence of palaeoenvironmental stress on calcareous nannofloral abundance and composition during the early Toarcian T-OAE, late Valanginian Weissert-OAE, early Aptian OAE1a and latest Cenomanian OAE2, are clearly recorded in Italian pelagic sections and at supraregional to global scale. However, the effects on nannoplankton evolution, if any, was differentiated and resulted in overall originations. Calcareous nannofossil patterns underline the resilience of this phytoplankton group during OAE perturbations.