THE ROLE OF EXTREME PALEOENVIRONMENTAL CONDITIONS ON NANNOPLANKTON CARBONATE PRODUCTION: CASE STUDIES FROM THE CRETACEOUS

The mid Cretaceous represents a period characterized by unstable and dynamic environmental-climatic conditions, including short-term events with extreme environmental conditions known as Oceanic Anoxic Events (OAEs). These events, triggered by a significant increase in CO2 resulting from submarine/subaerial volcanic activities, leading to global oceanic anoxia and the deposition of carbon- rich organic sediments on a global scale. Coccolithophores algae are the most important phytoplankton group in the oceans, and they have played an important role since their first appearance in the Triassic. These organisms are calcifying microalgae covered by a coccosphere consisting of small calcite platelets, named coccoliths. They are primary producers and through two processes, calcification and photosynthesis, they influence the global carbon cycle in the short- and long-term. In the last decades, coccolithophores and fossil coccoliths have been intensively studied to reconstruct surface water (paleo)environmental conditions and the impact of (paleo)climatic perturbation on their ability to calcify. The geological record allows to explore historical cases marked by profound changes in the ocean-atmosphere system which are ideal to trace short- and long- term effects on coccolith average size. Yet, our understanding of the combined impact of environmental factors on these organisms remains incomplete. This PhD thesis aims to shed light on the environmental factors influencing the calcification process of coccolithophores. To achieve this goal, here we selected and analysed four different nannofossil species based on their paleoecological affinity: Watznaueria barnesiae (oligotrophic species), Biscutum constans, and Zeugrhabdotus erectus (mesotrophic species), and Rhagodiscus asper (temperature-related species). We highlighted species-specific responses through long- and short-term morphometric analyses in different sites at different latitudes (low latitude: Cismon Core, Piobbico Core, Monte Petrano, ODP Site 1049 and high latitude: DSDP Site 511) and paleoenvironmental contexts. The study interval corresponds to the latest Barremian-early late Cenomanian, covering approximately 27 million years, during which high-resolution climatic-environmental information is already available in the literature. This interval encompasses interludes of extreme environmental conditions, such as OAE 1a, OAE 1b, and OAE 1d, as well as more stable environmental conditions, such as those observed in the Albian. The latter period is also characterized by a high-resolution study of cyclic black shales. Our results documented noticeable size variations in the considered species although more pronounced in B. constans and Z. erectus and moderate in W. barnesiae being more tolerant. Furthermore, it was observed for the first time that R. asper, a less-studied species, also displayed size responses, but only in correspondence with the negative carbon isotope excursion of OAE 1a. All of these variations potentially could be attributed to the influence of different environmental factors with different combination. Notably, the long-term morphometric dataset of W. barnesiae and B. constans, showed that in the Albian both species average size was affected by fertility changes in combination with temperature variations, although in opposite ways. W. barnesiae displayed larger coccoliths under low fertility in combination with higher temperature, while B. constans exhibited the opposite trend under these conditions. Also, looking at OAE 1a and OAE 1b –the latter being studied for the first time in terms of morphometric responses– confirmed that all nannofossil species showed changes in size during these extreme events that happened in mostly the same environmental conditions. Specifically, a size reduction was observed during OAE 1a, and variable changes were recorded during OAE 1b in different sites, likely influenced by local factors. In conclusion, the findings of this PhD thesis identify an influence of environmental factors on different coccolithophore species, indicating that these factors act in combination. Importantly, it is highlighted for the first time that each factor has a certain degree of effect. This study contributes to the understanding of the complex interplay between environmental factors and coccolithophore species, emphasizing the importance of considering these factors in the context of paleoclimatic and paleoenvironmental reconstructions.