HR: 08:00h
AN: B31E-01    [Abstracts]
TI: The Chicxulub Impactites: What makes a crater lethal ?
AU: * Claeys, P
EM: phclaeys@vub.ac.be
AF: Dept. of Geology, Vrije Universiteit Brussel, Pleinlaan 2, Brussels, 1050 Belgium
AB: So far Chicxulub is the only impact crater clearly linked to a mass extinction event. Its impact-products differ from equivalent lithologies recovered at or around other impact structures, and may indicate that several factors together contributed to the devastation of the biosphere. There is certainly a size and depth effect. The impactites recovered in this 180-km in diameter peak-ring crater contain a high proportion of basement fragments and various types of melt, indicating intense excavation into the deepest part of the Yucatan crust. The carbonates and evaporites present in the upper 3-km of the target rock is another factor. The suevites are rich in carbonates, as clast and within the matrix. Calcite crystals, which morphology indicates a formation by quenching from a carbonate melt also occur. The presence of carbonate melts (~ 300 km3 or more) and the spontaneous recombination with CaO limits the amount of CO2 released by the impact in an already CO2-rich Upper Cretaceous atmosphere. The role of evaporites is critical, but its exact behavior during the cratering process remains poorly understood. Below the impactite at a depth of ~ 895 m, the Yacopoil drill hole encountered a sequence of Upper Cretaceous mega-blocks; based on this core, evaporites formed approximately 30% of the Yucatan target lithology. This more than the proportion (<10%) present in the suevites. Porosity in some suevite units may indicate the presence of now dissolved anhydrite phase, but in rather limited quantity. An explanation might be that that anhydrite was not present in the deepest part of the sedimentary target rock, where from the suevitic material most likely originated. The shallow cores drilled at the crater periphery contain anhydrite / carbonate breccias, deposited around the rim by the ejecta curtain, which probably sampled a more superficial part of the sedimentary target. Perhaps, the more superficial evaporite layers were completely vaporized upon impact. The climate perturbation models based on the injection in the atmosphere of a high abundances of S-bearing gases (~ 300 Gt) should then be favored. However, modeling also shows that beyond a given concentration in S aerosols, the climate forcing reaches a saturation level. Chicxulub appears thus as a rather unique event; it is unclear if similar size impacts on other target or smaller impacts on volatile-rich targets would have the same lethal consequence for the biosphere ?
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 1600 GLOBAL CHANGE (New category)
DE: 1630 Impact phenomena
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting