HR: 08:15h
AN: B31E-02    [Abstracts]
TI: Does He-3 Provide Evidence for Extinction-Causing Impacts?
AU: * Farley, K A
EM: farley@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology MS 170-25, Pasadena, CA 91125 United States
AU: Ward, P
EM: argo@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195 United States
AU: Garrison, G
EM: garrison@ess.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195 United States
AB: Elevated He-3 concentrations can provide strong evidence for an extraterrestrial component in many sedimentary rocks, but in themselves are only suggestive of a major impact. Because most extraterrestrial He-3 is attributable to interplanetary dust rather than large impactors, few-Myr episodes of elevated He-3 flux likely signal dust-producing solar system events (comet showers, asteroidal breakups) that may or may not be associated with enhancement of Earth-crossing bodies. Importantly, no post-Permian extinction boundary appears to correspond to such an event. For example, studies of the P/Tr boundary at Meishan, in Austria, and our new data from Opal Creek, Canada reveal no elevated He-3 levels. If impacts played a role in the P/Tr extinction, they were not associated with such solar system events. In contrast neither a few-Myr episode of enhanced He-3 accumulation at ~35 Ma (and two coeval large impact craters) nor a new event we find at ~8 Ma are associated with major extinctions. Thin He-3 rich layers or spikes have been reported in several boundary sediments including the P/Tr. These spikes and their reported association with He-3-bearing fullerenes have been attributed to a major impact event. We failed to detect such spikes at Meishan and in our new study at Opal Creek. Given the large magnitude of the reported spikes, it seems unlikely that our failure arises from differences in analytical sensitivity or procedures. If the previously reported high He-3 concentrations arise from a P/Tr impact, the deposition or preservation of the He-3 must be extremely heterogeneous.
DE: 5420 Impact phenomena (includes cratering)
DE: 3662 Meteorites
DE: 1040 Isotopic composition/chemistry
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting