Impact fingerprints of the 4 kyr BP dust event based on archaeological, soil, lake and marine archives

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Courty, M. A. , Cortese, G. , Crisci, A. , Crosta, X. , De Wever, P. , Fedoroff, M. , Guichard, F. , Mermoux, M. , Smith, D. and Thiemens, M. H. (2007): Impact fingerprints of the 4 kyr BP dust event based on archaeological, soil, lake and marine archives , Geophysical Research Abstracts, European Geosciences Union (EGU) General Assembly 2007, April 15th-20th, Wien (Austria), ISSN: .
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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 non-equilibrium 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.

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