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Communication dans un congrès Année : 2009

The characteristics of burnt facies and impact debris tracing cosmic airbursts and related hazards

Résumé

The better understanding of airburst processes from geological data and simulation suggests that the high-temperature jet of expanding gas formed from the projectile explosion in the Earth atmosphere would generate a descending fireball. The hypothesis of frequent low altitude airbursts in the recent past thus incites to question the possible occurrence in the Quaternary records of airblast-linked firing events. Here we compare the field/analytical characteristics of undisputable airblasts and impact craters to anomalous firing records. The reference impact situations comprise (1) the peat layer of the 1908 Tunguska explosion; (2) the Henbury crater field, Australia; (3) the early mid-Pleistocene tektite layers at Bose (south China); (3) the Darwin glass strewnfield, Tasmania. The debated impact-linked combustion records comprise in situ fired soil surfaces from the mid to late Holocene framed by AMS C14 dates, and distinctive dark-brown humic layers with charred fragments from continental deposits of West Europe dated at 0.8, 1.2-1.4, 2.4 Ma on the basis of magnetostratigraphy. All the situations display a similar assemblage of exogenous intact rock clasts, partly melted brecciated debris, and rapidly quenched heterogeneous glass, fluidal glass in the form of shards, spherules, beads, and vesicular blocks. The exotic mineral phases occur with native metals and carbonaceous components, all of terrestrial origin. The polymer films, vitreous carbon, carbonaceous fibres, graphite and the associated diamond polymorphs are shown to derive from high temperature pyrogenesis of biomass precursors and fossil combustible. The moderate heating under oxidised conditions of the host soil surface contrasts from the high temperature heating under reduced conditions of the melted debris. The coherent spatial pattern at small scales of the carbon-rich glassy debris within the burnt surface suggests local ignition by pulverisation of a hot debris-jet. The fine association of carbonaceous films with metal and glass droplets within the carbonized vegetation showing CO2 degassing demonstrates the great volatility of the pyrogenic components transported in the debris-jet. Local strewnfields of impact-ejecta debris lying on a heated soil surface are suggested to trace airblasts from isolated projectiles with local and short-term damage. The mosaic of impact-ejecta strewnfields at all spatial scales with local firing would express short periods of serial airblasts associated to subtle climate effects due to the production of carbonaceous aerosols. The early Pleistocene peaty dark layers would match exceptional long periods of high frequency airblasts with severe atmospheric disturbances triggered by the repeated biomass burning and carbon-delivery from the projectiles.
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Dates et versions

halshs-00457574, version 1 (17-02-2010)

Identifiants

  • HAL Id : halshs-00457574 , version 1

Citer

Courty M.A., N. Fedoroff, Ge T., Guo Z.T., Jean-Paul Raynal, et al.. The characteristics of burnt facies and impact debris tracing cosmic airbursts and related hazards. AGU 2009 Fall Meeting, Dec 2009, San Francisco, United States. pp.Abstract NH31D-10. ⟨halshs-00457574⟩
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