Orbital forcing and astrochronology of Early-Middle Eocene carbon cycle alterations from the Umbria-Marche Apennines (Italy) and the Negev desert (Israel)

Abstract (English) In this PhD thesis we apply cyclostragraphy and astrochronology to a series of marine sedimentary successions recording the early and middle Eocene in the northern and southwestern Tethys. Key aspects of the short- and long-term climatic variations that characterize this time interval are addressed. The lower Eocene “hot-house” world reached the highest temperature of the entire Cenozoic era and experienced a highly dynamic climate, being punctuated by extreme short-term climate variations, which also had important repercussions on global marine geochemical cycles, manifested by hyperthermals events. In addition, during the lower Eocene, peculiar sedimentation environments were established in the oceans that favored an exceptional accumulation of siliceous sediments, preserved in the form of hard chert and porcelanite layers, whose formation mechanisms and relationship to climate and orbital variations are still poorly understood. Moreover, since hyperthermal events caused periodic intervals of dissolution, and hard chert horizons often hindered complete and undisturbed core recovery by ocean drilling, the astrochronological time scale for this period has not yet been established with certainty. We focused on studying sedimentary successions from the Tethys because its position relative to the Atlantic and Pacific oceans served as connection between the two oceans, potentially influencing global atmospheric and oceanic zonal circulation and becoming a crucial factor in climate variability. Each chapter focuses on one of the aspects mentioned above. In Chapter 2, we present a cyclostratigraphic study of a core obtained in the Negev Desert, Israel. We construct an astrochronological framework and identify and characterize each Carbon Isotope Excursion (CIE) preserved in the record. The identification and correlation of the main hyperthermals and CIEs allowed, for the first time, to have a description of the geochemical features of these events in a sub-tropical marginal site of the Tethys. In Chapter 3, we provide the first high-resolution analysis of the distribution of chert horizons in the northern Tethys, analyzing two successions, Contessa Road-Bottaccione (CR-BTT) composite section and Smirra core 1 and 2 (S1-S2), in the Umbria-Marche Apennines. We provide an intersite calibration that allows to determine a mutual correspondence between the logged chert layers at the CR-BTT and the Si/Al record from the composite S1-S2, along with a chemostratigraphic calibration to other carbon isotope records and discuss the occurrence of chert comparing it with δ13C trends, in particular respect to a persistent ~1% positive shift in δ13C values (here termed Early Eocene Carbon Shift or EECS), at the climax of the Early Eocene Climatic Optimum. We propose an interpretation of these events relating them to major changes in the marine carbon and silica cycles and in the nannofossil and diatoms communities. Finally, a cyclostratigraphic analysis allowed for the identification of a strong astronomical control on chert distribution, with the primary forcing driven by obliquity cycles. In Chapter 4, we conducted a study on a recently exposed section in the Umbria-Marche Apennines, presenting magnetostratigraphic data. A detailed analysis of the phasing of the magnetic susceptibility signal with respect to the astronomical periodicities allowed for the revision C21n/C21r magnetochron boundary age and confirmed the ages and phasing of C20n/C20r and C20r/C21n magnetochron boundaries as accepted in the most recent GPTS contributing to the refinement of the Eocene Astronomical Time Scale.