The Tethys (Cismon core) and Pacific (DSDP Site 463) Ocean record of OAE1a: a taxonomic and quantitative analyses of planktonic foraminifera and their biological response across the Selli Level equivalent

The early Aptian (121 to 118 million years ago) represents a case history of excess CO2 derived from a major volcanic episode, namely the emplacement of the Ontong Java Plateau. During this time-interval, oceans experienced a global phenomenon of widespread deposition of organic carbon-rich sediments under oxygen-depleted conditions, called the Oceanic Anoxic Event 1a (OAE1a). The sedimentary expression of the OAE1a is represented by the Selli Level, a regional marker-bed identified in the Umbria-Marche area (central Italy) consisting of laminated black shales rich in organic matter and low carbonate content, alternated with radiolarian silts. This thesis is aimed at investigating the OAE1a across the Selli Level equivalents at DSDP Site 463 (Mid-Pacific Mountains) and in the Cismon core (Southern Alps, northern Italy) in order to provide a detailed and quantitative study of the planktonic foraminiferal assemblages, because a quantitative documentation in terms of species composition, variation in shell sizes and absolute abundances is still lacking. This study is performed on pelagic sediments and both sections are well-dated through bio-, magneto- and chemostratigraphy. The project was divided in four distinct phases: 1) taxonomic and biostratigraphic analyses at the stereomicroscope, and SEM (Scanning Electron Microscope) to identify genera and species; 2) morphometric analyses of foraminifera to measure the variations in population size in washed residues (measure of the maximum diameter of each species using SEM images); 3) quantitative analyses to evaluate fluctuations in abundance at genera level (and species where possible) in thin sections and washed residues; 4) integration of results with geochemical, nannoplankton and sedimentological data and interpretations. Taxonomic and biostratigraphic analyses reveal that planispiral (genus Globigerinelloides), trochospiral (genus Hedbergella), globigeriniform with globular chambers (genera Gorbachikella and Gubkinella) and pseudo-planispiral taxa with very elongate chambers (genus Leupoldina) occur at both studied sites, whereas trochospiral and planispiral taxa with elongate chambers (genera Lilliputianella and Globigerinelloides) are found only in the Cismon core. Shell size measurements show a general increase from the base to the top of the studied interval at both sites (much higher at Cismon core) due to an increase of calcification. The group with the highest range of variation in shell size is represented by the Globigerinelloidids, followed by Lilliputianellids and then by Hedbergellids. Planktonic foraminiferal abundances and comparison with the nannoconid abundance data reveal that both groups responded to variations in surface-water fertility, temperature and CO2-induced acidification. In particular, when nannoconids underwent a biocalcification decline and crisis (started ~1 Ma before OAE1a), planktonic foraminifera decreased too. At the onset of the OAE1a, both calcareous groups show a marked decline that can be related to widespread meso- to eutrophic conditions, combined with excess CO2 in the ocean - atmosphere system. No extinctions within nannoconids and planktonic foraminifera have been observed in the OAE1a interval, and both calcareous groups can be considered as virtually absent. At Cismon core after the decline, nannoconids resume in the middle part of the Selli Level equivalent and continue to increase even above it, mirroring the δ13C trend, but never become so abundant as before the OAE1a. Planktonic foraminifera remain stable and start to increase significantly just above the Selli Level equivalent, where they become more abundant than before the OAE1a. The different timing recovery between the two calcareous groups could be justified by the higher susceptibility to dissolution of planktonic foraminifera than nannoconids. Ecological interpretations of planktonic foraminifera reveal that: at both sites the genus Hedbergella is the most common component of the foraminiferal fauna across the studied interval and, thus, considered the most opportunistic taxon. The genus Globigerinelloides is a typical meso-oligotrophic taxon, while Lilliputianella and Leupoldina are meso-eutrophic taxa that likely proliferated in oxygen - depleted, deep and cool waters associated to a possible expansion of the OMZ (Oxygen Minimum Zone). The high values in abundances of the 3 groups of planktonic foraminifera, and their fluctuations in abundance above the Selli Level equivalent, can be related to the ecological competition among them, where planispirals resulted to be the most efficient and successful competitors compared to the pseudo-planispiral and trochospiral taxa. The data and results presented here contribute to the understanding of the evolution of early Cretaceous (Barremian-Aptian) planktonic foraminifera, to the improving of the biostratigraphic correlations on global scale, and provide additional and new interpretations of the response of planktonic foraminifera to ocean acidification. My results emphasize the planktonic foraminiferal ability to evolve and diversify in a time-interval of complex and profound perturbation of the ocean-atmosphere system without undergoing extinction and, on the contrary, showing significant capacity of adaptation.