Courty, M., Abbott, D.H., Cortese, G., Crisci, A., Crosta, X., de Wever, P., Fedoroff, M., Greenwood, P., Grice, K., Mermoux, M. and Scharer, U., 2006, December. Scenario Of The 4 kyr Extraterrestrial Impact: Crater Location, Ejecta-Dispersion and Consequences. In AGU Fall Meeting Abstracts (Vol. 1, p. 1055).


The 4 kyr BP impact event has been identified from deep-sea, soil and archaeological records as the worldwide pulverisation of a volatile-rich debris jet(1). High resolution sequences show two stages of ejecta fallout linked to the impact-triggered doublet gaseous regime(2): scattered solid debris at the ground surface and spray of the vaporized hot fireball with thermal blast and local ignition. Ejecta debris consist of flow- textured impact glass, impact breccia and incompletely melted marine clasts: volcano-clastic sandy mudstone, calcareous mud, granite-gneiss, schists, volcanic breccia, kerogen and algal mud. Marine microfossils, organo-mineral markers, and the debris-fallout spatial pattern indicate two potential impact craters: an Antarctic source with an admixture of volcanic glass and ice rafted debris, from the vicinity of Heard Island and the Kerguelen plateau; a low latitude, shallow water one with hydrocarbons possibly from the Gulf of Mexico. Fine mixing of target materials from far distant source craters emphasizes a unique impact-ejecta. This matches the theoretical view of a debris jet channelled along the corridor cut through the atmosphere by the incoming projectile, raised upward, and dispersed widely(3). The isotopic anomaly of the sulphur phase in the kerogen volatile-component, indicating mass independent fractionation due to photolytic transformation, suggests launching at great heights, beyond the O2-O3 UV shield, responsible for climate disturbances. The incomplete melting of target rocks and global dispersion of impact breccia out of the craters would result from splash of small-sized projectile at rather great water depth and a low angle impact (10-15 degrees) into porous, highly compressible marine sediments. The spatially variable distribution of the organo-mineral and melt components, and the wide range of phase transformation reflect nonequilibrium shock-melting and micro-scale thermal processes in the heterogeneous vapor plume ejected from the impact sites. The 4 kyr BP event provides the first opportunity to compare on a variety of scales the environmental effects of globally dispersed ejecta from a multiple-site oceanic impact and the complex responses of human societies to frightening manifestations. [1] Courty, M.-A. et al., 2006. Geoph. Res. Abs., vol. 8, A-01812 [2] Alvarez, W., Claeys, P. and Kieffer, S., 1995. Science, 69, 930-936 [3] Kring, D.A., and Durda, D.D., 2002. J. Geoph. Res., 107E8: 1029