ABSTRACT The present-day ocean-climate system configuration took shape during the Miocene Epoch. Toward the end of the epoch, in the late Tortonian at ca. 8.5 Ma, there was an exceptional event: collisional disruption of an >150-km-diameter asteroid, which created the Veritas family of asteroids in the asteroid belt. This event increased the flux of interplanetary dust particles rich in 3 He to Earth and probably caused a period of increased dust in the atmosphere, with consequent alteration of global and local environmental conditions. A late Miocene 3 He anomaly likely related to the Veritas event has been registered in deep-sea sediments from Ocean Drilling Program (ODP) Site 926 (Atlantic Ocean), ODP Site 757 (Indian Ocean), and in the late Tortonian–early Messinian Monte dei Corvi section near Ancona, Italy. Here, we report the results of a study in the Monte dei Corvi section aimed to recover extraterrestrial chrome-spinel grains across the 3 He anomaly interval, as has been done for the similar late Eocene 3 He anomaly in the nearby Massignano section. In this study, three ~100 kg samples were collected from the Monte dei Corvi section: two within the 3 He peak interval and one outside the anomaly interval as a background reference sample. In total, 1151 chrome-spinel grains (>63 µm) were recovered, but based on chemical composition, none of the grains has a clear extraterrestrial origin. This supports the inference that the 3 He anomaly is indeed related to the Veritas event and not to an approximately coeval breakup of a smaller H-chondritic body in the asteroid belt, an event registered in meteoritic cosmic-ray exposure ages. Spectral studies of the Veritas asteroids indicate that they are made up of carbonaceous chondritic material. Such meteorites generally have very low chrome-spinel concentrations in the grain-size range considered here, contrary to the very chromite-rich ordinary chondrites. The terrestrial grains recovered were classified, and their composition showed that all the grains have an ophiolitic origin with no substantial compositional and distributional change through the section. The source area of the terrestrial grains was probably the Dinarides orogen.

Two long-lived peaks in extraterrestrial 3 He flux have been identified in the sedimentary record of the Cenozoic Era: at 8.2 Ma (late Miocene) and 35.8 Ma (late Eocene). These peaks document the occurrence of important events in the recent history of the solar system. The timing and temporal evolution of the dust event in the late Miocene as well as the absence of major terrestrial impact craters are consistent with an origin in the catastrophic collision that produced the Veritas asteroid family at this time. In contrast, there is no known asteroid collision corresponding to the late Eocene peak. Instead, the late Eocene event has elements consistent with a comet shower produced by a close stellar encounter. Both the rise time and the fall time of the enhanced dust flux and the occurrence of two major terrestrial impacts at the peak of the event are predicted by considerations of comet shower dynamics. However, debris from one of these impacts has a Cr isotope ratio that appears to exclude a carbonaceous chondrite impactor. If the presumption that comets have a carbonaceous chondrite–like composition is accurate, then an alternative mechanism for the late Eocene event may be required. The recent suggestion of an asteroid shower, where the 3 He-bearing dust resulted from lunar impacts, is one such possibility, but it too fails to account fully for existing observations. The cause of the late Eocene event thus remains uncertain.