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| Author(s): |
Valentina Cesari
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Instituto de Geociências (IG/BT) |
| Defense date: | 2025-11-18 |
| Examining board members: |
Luigi Jovane;
Milene Freitas Figueiredo;
André Pires Negrão;
Fernanda Quaglio
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| Advisor: | Luigi Jovane |
| Abstract | |
The climatic history of the Earth is characterized by long- and short-term variations, including extreme conditions that have profoundly impacted ecosystems. This study aims to reconstruct palaeoceanographic changes and paleoenvironmental conditions in two stratigraphic successions that record major events of intense climatic and carbon-cycle perturbations. The first case study presents new integrated geochemical, magnetic, and mineralogical data from a core drilled through late Albian to early Turonian sediments in the Sergipe-Alagoas Basin, off northeastern Brazil, which includes one of the few records of the Oceanic Anoxic Event 2 (OAE2) from the South Atlantic Ocean. The obtained results show a major change in sedimentation in the middle Cenomanian when mixed carbonate and siliciclastic deposits were replaced by chalk. This transition reflects a major change in palaeoceanographic conditions toward a more stable circulation mode, with better connection between the different oceanic basins, persistent currents, and bottom ventilation. Cyclicity in the ln(K/Al) record indicates insolation-driven fluctuations in aridity and humidity. A new age model, based on the stable 405 kyr-long eccentricity cycles, places the onset of the OAE2 carbon isotope excursion at ~94.74 Ma. Sedimentation during the OAE2 seemed to be mainly controlled by long-term processes and local effects rather than global perturbations. Variations in ln(Sr/Ca) indicate increased primary productivity persisting for two long eccentricity cycles, while transient reducing conditions, inferred from loss in magnetic susceptibility compared to Fe, persisted across the earliest long eccentricity maximum phase, at least. The second case study focuses on a detailed petrographic analysis of bioclastic carbonate turbidites from the middle Eocene Baskil section of the Neotethyan Elaz Basin, eastern Turkey. The Baskil section is a global reference for the Middle Eocene Climatic Optimum (MECO), a transient (~500 ka) warming event interrupting a long-term cooling trend. In terms of duration and isotopic signature this event is more similar to an OAE than a hyperthermal. Turbidites in the Baskil section are mainly composed of intrabasinal carbonate clasts sourced from carbonate-ramp factories. Bioclastic detrital modes document changes in sediment sources triggered by the combination of local factors and global palaeoceanographic and climatic changes. The progressive shallowing of the intrabasinal source area is related to a regression which occurred in the Neotethys from ~41 Ma until the Eocene/Oligocene boundary (~34 Ma). The permanent decrease in abundance of the orthophragminids parallels the long-term climatic trend towards cooling conditions after the MECO. However, their abrupt reduction, coinciding with increased water column stratification, was also influenced by regional changes in ocean circulation after ~40 Ma. These changes negatively impacted the specialized orthophragminids thorough increased hydrodynamism in the meso-oligophotic zone, resulting in decreased carbonate production at this depth. The preponderance of encrusting foraminifera coincided with the deterioration of environmental conditions and basin restriction within the Neotethys. In conclusion, these two case studies highlight the crucial role of regional changes in ocean circulation in modulating both sedimentary and ecological responses during periods of climatic and carbon-cycle perturbations and allow for a better understanding of the interplay between global and local palaeoceanographic and paleoclimatic processes. (AU) | |
| FAPESP's process: | 18/23294-3 - Facies characterization of Barremian deposits in the Atlantic Ocean |
| Grantee: | Valentina Cesari |
| Support Opportunities: | Scholarships in Brazil - Doctorate (Direct) |