Palaeoenvironmental and palaeoceanographic changes during the lower to middle Jurassic (Toarcian-Aalenian, ~183-171 Ma). : new evidences from calcareous nannofossils of the Lusitanian Basin

Tese de doutoramento (co-tutela), Geologia (Paleontologia e Estratigrafia), Faculdade de Ciências da Universidade de Lisboa, Université Claude Bernard Lyon 1, 2016

At ~183 Ma when large amounts of organic matter was dumped onto the ocean beds (known as the Toarcian Oceanic Anoxic Event, T-OAE), the Early Toarcian marine biota experienced one of the most important biological crises in Earth history. Calcareous nannoplankton was deeply affected and started only to recover from the end of the Early Toarcian. After the T-OAE, as phytoplankton communities dwelling the oceans photic layer recover from the previous disturbing conditions, the ecological niches once occupied by calcareous nannoplankton are again replenished. Located in the westernmost part of the Tethyan ocean, the Lusitanian Basin acted as a seaway between the NW Tethys and the Mediterranean Tethys provinces, therefore been actively influenced by these two different water masses. Hence such a region stands as the perfect spot to study the calcareous nannoplankton community, as its fossil remains bears species typical of both provinces. In order to appraise the nannoplankton demise and subsequent recovery, absolute and relative abundances of nannofossils were determined from pelagic marl-limestone couplets from three sections of the Lusitanian Basin. Also brachiopods shells were investigated for stable carbon and oxygen isotopes analysis. In order to independently assess the morphologic evolution of Lotharingius coccoliths, a Toarcian section from south France was studied. Different statistic methodologies were used throughout this work, in order to screen for robust and significant information from the numerous datasets that were built. New data from Portugal and France allowed for a new biostratigraphic scheme to be herein suggested for the western Tethys. Robust and distinct major bioevents are acknowledged at three major periods. The uppermost part of the lower Toarcian is acknowledged through the First Occurrence (FO) of Watznaueria colacicchii. The middle-upper Toarcian boundary is consistently recognized through the FO of Discorhabdus criotus whereas the Toarcian-Aalenian limit is clearly identified by the FO of Watznaueria contracta and Hexalithus magharensis. Sedimentological interpretation and carbon and oxygen isotopic data, together with the nannofossil record, allowed to characterized both the biotic and abiotic changes that occurred in the water column of the Lusitanian Basin over a period of ~10 myr. Three distinctive palaeoenvironmental and palaeoceanographic settings are inferred. (1) The basal Toarcian was characterized by very unstable environmental conditions. Across this period both Boreal and Mediterranean water masses freely mixed as supported by the high abundance of nannofossils with geographic affinities from both Tethyan realms. The nannofossil record together with isotopic data upholds the trophic character of the water column across this warm and transgressive period as mainly mesoeutrophic. (2) During the long-term regressive period that started from the middle Toarcian, as the water masses gradually cooled down, the dominant water mass in the basin has its origin in the Boreal province and the deepening of the nutricline provides the ecological niches for deep dwelling coccolithophores. This period is interpreted as being mainly stable and oligotrophic. (3) Starting from the upper Toarcian and although not as stressful as before, the water column again acquires a predominant meso-eutrophic character as inferred from the carbon isotopic data and nannofossil record. This was a period where the connection between the two Tethyan water masses were at a minimum if not lost at all, and mainly Mediterranean waters filled the basin. These environmental changes are mirrored by the size variation in the coccoliths of the Lotharingius genus. Biometry results show a double influence of environmental factors (acting on the short-term) and evolution (mainly noticeable on the long-term) in two discrete western Tethys sections. Small coccoliths of Lotharingius are more abundant in strata here interpreted as likely deposited during stressed and meso-eutrophic environmental conditions where nutrients would be more abundant, whereas larger coccoliths dominate the assemblages in stable and stratified waters masses. However, a major size change is observed during the middle Toarcian, when the average size of Lotharingius coccoliths goes from smaller to larger than 4 μm. This size change likely illustrates both the Cope’s rule and the Left Wall model. Calcareous nannofossils proved to be a privileged proxy in documenting long-term environmental changes occurring in the T-OAE aftermath, either through the analysis of their assemblage composition either through their size changes. They are a particularly useful palaeoceanographic tool, which allowed infer on the water mass exchanges that occurred across the Lusitanian Basin seaway. Calcareous nannofossil biostratigraphy acknowledged in this work renders a clear and unequivocal recognition of main Toarcian substages boundaries, susceptible to be integrated in the future Geological Time Scale.