Evidence for a large-magnitude eruption from Campi Flegrei caldera (Italy) at 29 ka

The 40 ka caldera-forming eruption of Campi Flegrei (Italy) is the largest known eruption in Europe during the last 200 k.y., but little is known about other large eruptions at the volcano prior to a more recent caldera-forming event at 15 ka. At 29 ka a widespread volcanic ash layer, termed the Y-3 tephra, covered >150,000 km(2) of the Mediterranean. The glass compositions of the layer are consistent with Campi Flegrei being the source, but no prominent proximal equivalent in the appropriate chrono-stratigraphic position had been previously identified. Here we report new glass chemistry data and Ar-40/Ar-39 ages (29.3 +/- 0.7 ka [2 sigma]) that reveal the near-source Y-3 eruption deposit in a sequence at Ponti Rossi and a nearby borehole (S-19) in Naples. The dispersal and thickness of the deposits associated with this eruption, herein named the Masseria del Monte Tuff, were simulated using a tephra sedimentation model. The model indicates that similar to 16 km(3) dense rock equivalent of the magma erupted was deposited as fall. This volume and the areal distribution suggest that the Masseria del Monte Tuff resulted from a magnitude (M) 6.6 eruption (corresponding to volcanic explosivity index [VEI] 6), similar to the 15 ka caldera-forming Neapolitan Yellow Tuff (M 6.8) eruption at Campi Flegrei. However, the lack of coarse, thick, traceable, nearvent deposit suggests peculiar eruption dynamics. Our reconstruction and modeling of the eruption show the fundamental role that distal tephrostratigraphy can play in constraining the scale and tempo of past activity, especially at highly productive volcanoes.