WHERE DID THE LATE EOCENE IMPACTORS COME FROM ? THE ASTEROID BELT OR THE OORT CLOUD ?

During the late Eocene (~ 35 Ma), several projectiles impacted the Earth. Two distinct impact-ejecta layers are found in the sedimentary record: the global microkrystite (CPX-) layer and just above the younger North American microtektites. Based on geochemical data, they are linked respectively to the Popigai (Siberia, 100 km in diameter) and the Chesapeake Bay (80 km in diameter) craters, dated at 35.7 and 35.5 Ma. Less precisely constrained by isotope-dating, the smaller Mistastin, Wanapitei and Logoisk craters may be part of the same suite of events. The deep-water carbonates in the Massignano section (Italy) are enriched in extraterrestrial 3He, indicating a ~ 2,5 million year period of higher delivery of interplanetary dust particles (IDP) on Earth (Farley et al., 1998). This increase in the flux of IDP and the impact of larger objects (at least 2) is attributed to the arrival in the inner solar system of a comet shower, triggered by a perturbation of the Oort cloud. However, the concentrations in platinum group elements (PGE) measured in the impact melt rock of the Popigai crater indicate that its projectile was most probably an L-chondrite (Tagle and Claeys, 2004). Yet comets are believe to be more similar in composition to carbonaceous chondrites. More over, chromium isotope ratios measured on the CPX-spherules in ODP hole 709C exclude a carbonaceous chondrite origin for the projectile (Kyte et al., 2004). The Popigai bolide could have originated in the asteroid belt rather than in the Oort cloud, and was perhaps part of an asteroid shower that hit Earth in the late Eocene. Some 35 Ma ago, a major break up may have occurred in the asteroid belt, sending numerous fragments on resonances capable of then taking these projectiles towards Earth crossing orbits. If verified, this hypothesis may eventually question the occurrence of comet showers, which existence is only a postulate, based in part on the late Eocene data. The asteroid shower hypothesis now requires a dynamic scenario, capable of explaining how collisions in the main belt could send together large objects and fine dust towards the Earth over a ~ 2 to 3 million year period. References: Farley et al., 1998, Science 280, 1250; Kyte et al. 2004, Lunar and Planetary Science XXXV, CD-ROM #1824; Tagle and Claeys, 2004, Science 305, 492.