HR: 08:00h
AN: U21E-01 INVITED    [Abstracts]
TI: Cause(s) of the elevated bombardment in the late Eocene
AU: * Claeys, P
EM: phclaeys@vub.ac.be
AF: Dept. of Geology, Vrije Universiteit Brussel, Brussels, 1050, Belgium
AB: The late Eocene (38 - 34 Ma) is marked by a high concentration of impact craters, ejecta layers and an elevated flux of interplanetary dust particles (IDP). The two largest craters in the Cenozoic formed in this period, the 100-km in size Popigai in Siberia, dated at 35.7 ± 0.2 Ma, and the 85-km in diameter Chesapeake Bay, offshore Virginia, dated at 35.5±0.6 Ma. Their almost coeval formation was triggered by the impact of projectiles, in the range of 4 to 5 km in diameter. Several other smaller impact structures, some of which are not as precisely dated, are also known in this late Eocene. The IDP, which arrival is recorded by an 3He anomaly in marine sediments, range in size from 1 to 1000 μm. The flux of both large bodies and small particles on Earth increased significantly over 2 to 3 Myr compared to the background. This anomalous bombardment is attributed to a comet shower in the inner Solar System, triggered by a perturbation of the Oort Cloud. The projectiles that formed the Popigai and Wanapitei (Canada, 8 km, 37± 2 Ma) craters were both ordinary chondrites; most likely L-chondrites based on platinum group elemental ratios, determined in their impact-melt lithologies. A composition not compatible with a cometary origin. Such objects were most likely derived from S-type asteroids located in the belt between Mars and Jupiter. Consequently, a major collision in the asteroid belt is another possible cause of the late Eocene elevated bombardment. The cosmic ray exposure ages of L-chondrites also support a collision on the L-chondrite parent body ~ 40 Ma ago. However, no asteroid family of that age range has so far been discovered in the belt and a particularly efficient delivery mechanism, so far unclear, is required to send large and small bodies at roughly the same time on Earth crossing orbits. In particular, considering the small fraction of large (5 km) projectiles likely to hit Earth, a huge number of fragments in this size range have to be injected into resonance positions capable of producing terrestrial impactors. Another possibility is to generate the fragments (large and small) by an asteroid disruption taking place in the region occupied by the Near Earth Objects. In this location, the break up produces a series of objects on orbits likely to impact Earth, reducing significantly the number of large fragments required. A cascade of collisions between the produced fragments could perhaps at the same time generate the continuous dust production over 2 Myr?
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 6022 Impact phenomena (5420, 8136)
DE: 6205 Asteroids
DE: 6210 Comets (6023)
DE: 8136 Impact phenomena (5420, 6022)
SC: Union [U]
MN: 2007 Fall Meeting