Tempo and dynamics of the peri-tyrrhenian quaternary explosive volcanism inferred from distal archives

The study of volcanic ashes (or “tephra”) has an enormous potential for volcanic stratigraphy and explosive activity reconstruction. Indeed, tephra layers deposited and preserved in sedimentary records, located sufficiently far away from the volcanic sources, can document deposits of minor and/or older volcanic events not recorded in proximal (i.e., near-vent) sections. This allows a more rigorous and complete reconstruction of the past activity and recurrence time evaluation of a volcano, with important implications for hazard assessment in volcanically active regions. The central Mediterranean area, and in particular central-southern Italy, represents an ideal location for tephra investigations. Indeed, the presence of numerous volcanoes, active over long periods of time, combined with that of several sedimentary (i.e., sub-aerial, lacustrine and marine) successions hosting rich tephra sequences, represents an ideal condition for the development and application of the tephrostratigraphic method. In central Italy, the lacustrine succession of Fucino Basin recorded the deposition of tephra layers from the peri-Tyrrhenian potassic Quaternary volcanic systems over a period of at least the last ~430 kyr, representing a key site for tephrochronological investigations in the area. The study of Fucino tephra layers allowed the identification of explosive events previously undocumented (or undescribed) in proximal sections, but also provided new and more reliable chronological constraints of previously known eruptions. However, not all the Fucino tephra have been geochemically characterised and associated to their source volcano, so that the eruptive history of the Italian volcanoes during the Middle Pleistocene still remains partly unknown and unexplored. Reconstructing this activity is fundamental, as, during the Upper-Middle Pleistocene, the Quaternary potassic peri-Tyrrhenian Italian volcanoes were intensely active and dispersed several tephra layers documented all over the central-eastern Mediterranean region and acting as marker horizons. Among these, the tephra layers known in marine records as C-22, X-5, and X-6, due to their peculiar geochemical composition, their distribution over wide regions and precise ages, are among the most valuable for tephrochronological studies. Based on their geochemical composition, the volcanic origin of these tephra was postulated to be in the Campanian area, likely from Campi Flegrei. However, so far, they were not documented in proximal settings, leaving their specific source yet to be determined. Furthermore, in order to confidently and undoubtedly correlate tephra layers of unknown origin to their volcanic sources and specific equivalent eruption, geochemistry of the proximal near-vent deposits must be available. Unfortunately, glass composition for the peri-Tyrrhenian potassic volcanoes is still limited, as the available literature mainly consists in whole-rock bulk analysis that can be potentially affected by the presence of crystals (phenocrysts and microlites), especially in highly porphyritic samples. Furthermore, most of the available glass analysis for the Quaternary peri-Tyrrhenian potassic volcanoes derives from studies of tephra in distal settings, resulting scattered in the literature in several papers. Also these studies are incomplete, as they cover only specific time intervals and report analysis only of selected volcanoes, whilst for others (e.g., Roccamonfina) the available glass data is completely lacking. Thus, an organized glass-based geochemistry database of proximal pyroclastic units is almost lacking and needed for future tephra investigations in the area. This Thesis represents an attempt at resolving the open issues mentioned above, which are presented as three case studies in an equivalent number of scientific articles. The first two case studies - and related scientific articles - address two tephra sequences from the F4-F5 Fucino Basin’s succession, covering the 430-365 ka and 250-170 ka time intervals, broadly corresponding to Marine Isotope Stages (MIS) 11 and 7 respectively. In both cases, the Fucino tephra layers have been lithologically, geochemically (i.e., major, minor, and trace elements, Sr and Nd isotopes) and geochronologically (i.e., 40Ar/39Ar dated) characterised. Furthermore, deposits of near-vent eruptive units emplaced during these time intervals from the Vulsini, Vico, Sabatini, and Colli Albani volcanoes have been similarly characterised with the same set of analyses to provide a glass-based geochemistry reference database. This allowed to backtrack the Fucino tephra to their specific volcanic sources, which include the Vulsini, Vico, Sabatini, Colli Albani, Roccamonfina, Ischia, and Campi Flegrei volcanic systems. For the most part, the Fucino tephra layers have been associated to their corresponding proximal equivalent eruptive units, but, in some cases, also documented previously unknown eruptive events at these volcanic systems. Thus, the Fucino Basin’s succession provided new integrative information that allowed a more rigorous and complete reconstruction of the past explosive activity at these volcanoes. In the third case study, mid-proximal deposits from the Campanian Plain area, southern Italy, have been analysed (i.e., major, minor, trace elements, Sr and Nd isotope composition, 40Ar/39Ar dating), and allowed to trace back for the first time the proximal eruptive units of the widespread C-22, X-5, and X-6 MIS 5 Mediterranean tephra marker horizons. Furthermore, other two less-dispersed tephra markers, i.e., TM-24a and TM-24b, have been associated to eruptive units from the Campanian Plain as well. Data from this study thus allowed to confidently associate these tephra marker horizons to the Campi Flegrei volcanic system, documenting an intense and frequent explosive activity at this volcano during the 110-90 ka time interval. In conclusion, results from this Thesis provided new integrative information that allowed refining the explosive history of the Quaternary Italian potassic volcanic systems during the Middle-Upper Pleistocene. Furthermore, the tephra layers investigated in the three case studies and presented in this Thesis have been correlated to tephra layers hosted in other Italian and central Mediterranean sedimentary archives, thus consolidating the Mediterranean tephra lattice. Finally, the approach employed in this Thesis, which combines the study of distal tephra sequences hosted in sedimentary successions with investigations in near-vent volcanic areas, showed to be an effective approach for confidently correlating tephra layers of unknown origin to their volcanic source.