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Marie-Agnes Courty, CNRS (France)
Michel Mermoux, CNRS (France)
David Smith, MNHN (France)
Mark Thiemens, Univ. of California (United States)
Xavier Crosta, CNRS (France)
Nicolas Fedoroff, CNRS (France)
Thierry Ge, INRAP (France)
François Guichard, CEA (France)
Kliti Grice, Univ. of Technology (Australia)
Paul Greenwood, Centre for Land Rehabilitation, School of Earth and Geographical Sciences (Australia)
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Land-sea correlation is of major importance to unravelling impact events that operated at multiple temporalities through a continuum of spatial scales. Our multidisciplinary study of marine and terrestrial records has provided clues to elucidate the conflicting pattern of the 4 kyr BP impact from the submicron to the global level. The markers of this impact comprise anomalous micro-facies and allochthonous debris with organic, mineral and metallic tracers characterized by petrography, XRD, Raman microspectrometry, SEM, GC-IR-MS, isotope and noble gas analyses. Dating on charred plant materials provides C14 radiometric ages at 4050-3950 + 50 yr BP. The allochthonous debris comprise intact rock-clasts, glassy components and crystallised breccia with a wide size range (a few mm down to a few m). Lechatelierite, baddeleyite, diaplectic quartz and graphite-derived diamond in the glassy phases confirm the impact origin. A micro-faunal-floral assemblage (foraminifera, diatoms and radiolaria) from subtropical, subpolar and austral seawaters in intact marine clasts gives a provenance from austral latitudes. Tasmanite-like bitumen and sandstone clasts with chromite, zircon, metal-rich quartz and alumina components supports an origin from the continental plateau somewhere around South of Australia.
The proximal emplacement of the impact-ejecta in the Austral Ocean (Adelie Land and Kerguelen Plateau) is traced by 7 to 12 m thick anomalous facies. Evidence of heating of local marine components and their mixing with the impact debris indicates seawater vaporisation by a hot debris jet. The identification of the 4 kyr BP tracers suggests that the Henbury crater field, the Boxhole crater, intermediate areas with glass debris, and the Edeowie glass field in South Australia, together with the Darwin glass field in Tasmania, could relate to projection of the hot debris jet to the nearby land areas. Association of the 4 kyr BP tracers to a wild-fire on the Reunion Island and to a giant tsunami along the north-west Sumatra coast would respectively express local wildfires by dispersion of the firery debris and long-distance effects of the impact shock-wave.
The distal emplacement of the impact-ejecta in the northern hemisphere is represented by a widely-distributed pattern of scattered fine debris and an erratic pattern showing great concentrations of the 4 kyr BP allochthonous debris along a narrow band locally traced across Syria and France. They formed splashed deposits of layered pseudo-tektites, pillow-like slabs, highly-vesiculated glassy materials and breccia blocks associated with firing evidence. The spatial pattern of the fire traces indicates micro-scale ignition produced by fragmentation at the soil surface of blocks yielding a hot metal-rich carbonaceous melt due to their re-heating while re-entering the Earth atmosphere. The fine debris pattern is suggested to trace dispersion of ejecta blocks fragmented before reaching the Earth's surface.
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