HR: 14:30h
AN: U33A-03    [Abstracts]
TI: Chicxulub Impact and K-T Mass Extinction in Mexico and Texas
AU: * Keller, G
EM: gkeller@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08544, United States
AU: Adatte, T
EM: thierry.adatte@unine.ch
AF: University of Neuchatel, Geological Institute, Neuchatel, CH-2007, Switzerland
AU: Berner, Z
EM: zsolt.berner@img.uni-karlsruhe.de
AF: University of Karlsruhe, Institute for Mineralogy and Geochemistry, Karlsruhe, 76128, Germany
AU: Stueben, D
EM: doris.stueben@img.uni-karlsruhe.de
AF: University of Karlsruhe, Institute for Mineralogy and Geochemistry, Karlsruhe, 76128, Germany
AB: New cores and outcrops from El Penon, NE Mexico, and the Brazos River, Falls County, Texas, reveal the stratigraphic and temporal separation between the Chicxulub impact, the sandstone complex (commonly interpreted as "impact-tsunami") and the K-T mass extinction. In NE Mexico, where deposition occurred in about 500 m water depth, the original Chicxulub impact ejecta was discovered in a 1.8 m thick impact glass spherule layer within undisturbed pelagic marls more than 4 m below the base of the sandstone complex. At Brazos, Texas, where deposition occurred in shallow waters (20-80 m), the original spherule ejecta layer was found in a 3 cm thick clay-altered impact spherule layer within undisturbed late Maastrichtian claystones, about 60 cm below the sandstone complex. In both localities, the base of the sandstone complex contains spherules and clasts from shallow nearshore areas, which were eroded from the original impact spherule layer and transported into deeper waters during the latest Maastrichtian sea level lowstand. The K-T mass extinction and Ir anomaly occurred at a much later time. The Chicxulub impact is dated at 300 ky before the K-T boundary and the sea level lowstand about 100 ky before. These data reveal that the K-T mass extinction was not directly related to either the Chicxulub impact, or the sea level lowstand. The discovery of the original Chicxulub impact ejecta spherule layer in Mexico and Texas permits evaluation of the biotic effects of this large impact upon marine faunas and floras in both deep and shallow water environments at 1000 km and 1700 km from the impact crater, respectively. Quantitative analysis of planktic foraminifera reveals a major surprise: No species extinctions or significant species population changes occurred at the time of the Chicxulub impact. The impact coincides with greenhouse warming associated with Deccan volcanism, but appears to have caused no significant environmental stress even within 1000 km, let alone globally. Current estimates thus vastly overestimate the biotic effects of large impacts. The mass extinction at the K-T boundary coincides with a renewed pulse of major Deccan volcanism and a global Ir anomaly that suggests another large impact for which no crater is known to date. Thus, the K-T mass extinction appears to have been caused by the combined effects of volcanism and a second large impact superimposed on long-term biotic stresses.
DE: 0444 Evolutionary geobiology
DE: 0456 Life in extreme environments
DE: 0459 Macro- and micropaleontology (3030, 4944)
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
SC: Union [U]
MN: 2007 Joint Assembly