Magnetostratigraphy and rock-magnetism of Paleogene marine section from the Venetian Alps (Italy)

In this thesis, I present the magnetostratigraphy and the rock-magnetic properties from three selected early Paleogene hemipelagic sections (namely the Ardo, the Cicogna, and the Forada sections) from the Venetian Southern Alps (NE Italy). The contribution of this thesis to the present-day knowledge of the early Paleogene time is twofold and is discussed in two parts. In Part 1 the implications and the improvements of the presented dataset to the current early Paleogene time scale are presented. The magneto-biochronology of the early Paleogene, especially the late Paleocene–early Eocene, is in fact still poorly defined; this is essentially because of the controversial and ambiguous relationships between paleontologic and magnetic events due to the presence of unconformities in several deep-sea and shallow water sections from the literature around the Paleocene–Eocene boundary, and the lack of (magnetic) resolution in the long Chron C24r interval. I present in Part 1 a detailed magnetostratigraphy across the early Paleocene part of the Ardo section, as well as across the late Paleocene–early Eocene Cicogna section. The sediments of both the Ardo and the Cicogna sections preserved a well-defined record of the geomagnetic reversals, respectively from Chron C29r to C26r, and from Chron C25r to C23r, which allowed the construction of a robust age model by means of magnetostratigraphic correlation to the CK95 (Cande and Kent, 1995) geomagnetic polarity time scale. The dataset of the Cicogna section is also integrated with a detailed calcareous nannoplankton biostratigraphy; several new and potentially useful biohorizons that help refining the chronology of this critical interval of the current time scale of Berggren et al. (1995) were found. In Part 2, rock-magnetic data from the Cicogna and the Forada sections are described. In particular, the data from Cicogna gave me the opportunity to better understand key aspects of the late Paleocene–early Eocene Earth’s climate evolution. The late Paleocene–early Eocene climate was characterized by a general warming trend that eventually culminated with the early Eocene climatic optimum (EECO, ~52–50 Ma), which was characterized by globally warm climatic conditions, even at extremely high latitudes. This warming trend was punctuated by several short-lived hyperthermal events, the most prominent of which was the Paleocene–Eocene thermal maximum (PETM), globally marked by a 2‰–4‰ global negative carbon isotope excursion and extensive carbonate dissolution of deep-sea sediments. The early Eocene and PETM warm and humid climates should be characterized by enhanced land weathering rates, which would promote the sequestration of excess the greenhouse CO2 by chemical weathering of silicates followed by the deposition of carbonates, in accordance with the negative feedback mechanism for the long-term stabilization of the Earth’s surfaces temperature proposed by various authors. Enhanced land weathering rates of Fe-bearing silicates should be accompanied by the production of Fe3+-oxides (e.g. hematite) as residuals of the weathering reactions, which then may be mobilized, transported, and deposited in adjacent sedimentary basins. The rock-magnetic properties of the Cicogna sediments generally indicate a relative increase in abundance of detrital hematite during times of enhanced warming (i.e. PETM and early Eocene warming trend), as revealed by a statistical correlation with the benthic oxygen isotopes record from the literature. These results confirm the existence of the silicate weathering machine as a buffer of climate warming on both short (105 kyr -PETM-) and long (106 kyr -EECO-) time scales. To find a confirmation of this mechanism, I studied in detail the 8 m-thick Forada section across the PETM; the dataset from Forada, however, revealed that the sediments are pervasively contaminated by post-depositional chemical reduction processes that partially masked the primary, climate-controlled, rock-magnetic variability.