Volcanic Triggers and Depositional Complexity of Submarine Megabeds in the Marsili Basin, Tyrrhenian Sea

Megabeds, also known as "megaturbidites," are exceptionally large submarine sediment deposits likely formed by catastrophic geohazard events. These deposits are increasingly being identified with modern high-resolution geophysical data, yet their origins and characteristics remain debated. Five megabeds have been identified in the Marsili Basin of the Tyrrhenian Sea within the upper 70 meters of sediment. These deposits are hypothesized to have been triggered by explosive volcanic eruptions of the Campanian Volcanic Province, including the ~39.8 ka Campanian Ignimbrite (CI) super-eruption, which is among the largest known eruptions, having a volcanic explosivity index (VEI) of 7. These megabeds were intersected by Ocean Drilling Program (ODP) Leg 107 Site 650, where sediment cores were collected in 1986. However, their presence was not recognized at the time due to lack of appropriate geophysical data. To better understand the properties and origins of the Marsili Megabeds, we identified the megabeds within the ODP cores and conducted detailed sedimentological and elemental analyses, along with age dating, to determine their possible sediment provenance, depositional mechanisms, and potential triggering events. Elemental analysis and age dating suggest a potential link between these megabeds and known eruptions from the Campanian Volcanic Province, including the Neapolitan Yellow Tuffs eruption (14.9 ka), the Masseria del Monte Tuff eruption (29.3 ka), and the Campanian Ignimbrite super-eruption (39.8 ka). A new megabed discovered below the Y-7 tephra is older than 60,300 years but its triggering event is unknown. The re-examination of ODP cores reveals that not all megabeds conform to a megaturbidite morphology. In the Marsili Basin, the variety of sedimentological structures differs within and between megabeds, suggesting varying and complex depositional mechanisms. The findings reveal that the megabeds are more internally complex than previously thought, with variations in their depositional processes even in one basin.