Traces of major collisional events in the asteroid belt in late Eocene marine sediments in Italy

Based on extraterrestrial chromite grains (63−355 μm) in late Eocene sediments, the Chesapeake Bay impact crater (northeastern United States; 40−85 km diameter) likely formed from an H-chondritic projectile. This impact occurred 10−20 k.y. after the formation of the Popigai crater (Siberia, Russia; ∼100 km). The craters formed at the peak of a 2-m.y. period with a factor of 3−5 enhanced flux of ³He-rich extraterrestrial fine dust to Earth. The Massignano and Monte Vaccaro sections in Italy provide detailed sedimentary records of these events. Previous data from 1168 kg of acid-dissolved limestone at Massignano combined in the present study with 1663 kg of limestone from Monte Vaccaro, spanning the ³He anomaly, reveal a persistent decimeter-thick layer enriched in H-chondritic chromite grains at the stratigraphic level of the Chesapeake Bay impact. These well-preserved grains, free of shock features, likely were released from the regolith of the impactor as it approached Earth. We also report that the small Ir anomaly at the 6.19 m level at Massignano is not reproducible at Monte Vaccaro and may reflect diagenesis or reworking. We revise earlier interpretations and suggest that the late Eocene ³He and chromite anomalies, along with the two large impacts, likely relate to a major breakup event within the Koronis asteroid family, the source of today’s common H chondrites.