Mudanças climáticas durante o Paleógeno da Antártica: A utilização de macroinvertebrados da Formação La Meseta como indicadores de paleotemperturas do Eoceno

Abstract

Current scenario of icehouse geoclimate of our planet is the result of intense global changes during the Cenozoic, including geotectonic, geochemical and even orbital factors, which were marked by a drop in global temperatures and the extensive glaciation of Antarctica. In this context, the La Meseta Formation, outcropping in the northern half of Seymour/Marambio Island, Antarctic Peninsula, records part of this history, as it preserves rocks aged between 56 and 40 Ma, encompassing the time interval with intense fluctuations in global temperatures, predating the start of the formation of ice sheets in Antarctica. The isotopic composition of carbon and oxygen (C12, C13, O16 and O18) in fossil macroinvertebrate shells reflects the temperature conditions at the time of biomineralization. The verification of originality in the isotopic signal is essential for the truthful evaluation of data, since diagenetic alterations and isotope selection by some organisms during the mineralization of their shells can markedly influence the results. Regardless of taxonomic origin, it is not uncommon for a single shell to present more than one category of microstructure, which results in greater complexity in the multifunctional properties of the organism. In this project, four taxa collected in the La Meseta Formation had their isotopic compositions analyzed by Mass Spectroscopy, and their microstructures verified by Scanning Electron Microscopy and Raman Spectroscopy. The main objective was to evaluate their relevance as paleoclimatic proxies of the Eocene of Antarctica, including the characterization of the elemental content to verify the occurrence of diagenetic minerals that might interfer in the paleotemperatures interpretations. Our analyzes indicated that microstructure of the carbonate shells is prone to be considered as a potential taxonomic character. However, anomalous results (outliers) in the isotope analysis can be observed both in recrystallized samples and in samples with preserved microstructure. The La Meseta Formation dataset shows a trend of δ18O increase in rates, indicating a possible cooling towards the top of the unit. Furthermore, the bivalves of the genera Cucullaea and Lahillia presented isotopic values consistent with those already existing in the literature, which confirms that these taxa can be considered as good paleoclimatic proxies. The integration of these data helps to indicate the originality of the isotopic signal more accurately in order to characterize the microstructure of fossil macroinvertebrates.