Organitzat conjuntament CSIC-UB
Dimecres, 17 de gener de 2007, Sala d'Actes de l'Institut Jaume Almera, a les 12h.
Per Dra. Marie-Agnes Courty. CNRS-UMR 5198, TAutavel, France

The co-occurrence of a sharp dust peak, low lake levels, forest reduction and ice retreat at c.a. 4-kyr BP throughout tropical Africa and West Asia has been widely explained as the effects of an abrupt climate change. Severity and persistence of the drought is assumed to have precipitated the collapse of civilizations in the Aegean world, Mesopotamia, Palestine, Egypt, Indus valley, Central Asia, & China. In contrast, extensive study of various terrestrial and marine records provided us with evidence to alternatively explain the 4-kyr BP dust event as the global dispersal of a hot debris-jet formed in the Austral ocean by a cosmic impact.
The unusual 4 kyr BP dust assemblage has been comprehensively investigated by performing microfacies study, SEM and WDS microprobe analysis, TEM characterization, Raman microspectrometry, isotope geochemistry, and GC-IR-MS analysis.
The diagnostic tracers of the 4 kyr BP dust assemblage comprise: (i) vesicular silicate glass; (ii) carbonaceous materials associated to metallic particles; (iii) sedimentary clasts; & (iv) igneous-like rock clasts. They occur as nano-sized particles to fist-sized blocks, showing a great morphological diversity of angular fragments, glazed vesicular beads, spherules and filaments. The 4 kyr BP dust assemblage often occurs within a fired soil surface, sporadically associated to vesicular mass showing the fine imbrication of exogenous flow-textured glass and partially melted host materials. These distinctive facies are explained to result from pulverisation at the soil surface of a hot impact-ejecta consisting of solid debris and volatile-rich fluid condensates.
The impact indicators comprise: (1) lechatelierite schlieren within Ca-rich flow glass formed from incomplete melting under high pressure of silicate and carbonate sediments; (2) blue glass with a microspheroidal pattern similar to the Zhamanshite impactite; (3) diaplectic quartz, ballen-textured quartz and crystal anomalies of quartz typical of high pressure transformation; & (4) silt-sized baddeleyite clusters produced from impact-linked thermal decomposition of zircons. The presence of nano-sized clusters consisting of detonation-diamond, graphite, amorphous carbon, and polycyclic aromatic hydrocarbons (PAHs) indicates shock-induced formation of carbonaceous compounds. Identification of a sulphur phase with an isotopic anomaly indicating mass independent fractionation due to photolytic transformation suggests launching at great heights, beyond the O2-O3 UV shield.
Marine microfossils, organo-mineral markers, and the debris-fallout spatial pattern have allowed to identify an Antarctic provenance and a potential impact area in the vicinity of the Kerguelen plateau. 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 impact-ejecta.
Damage to habitations, occupants and lands by the surface heating and selective melting at temperature around 700-1000°C vary strongly at local scales, and seem to have rarely lead to irreversible destruction. High energy dust mobilisation originated from deflation of the fired soils subsequently to the hot-ejecta pulverization seem t