The Cretaceous–Tertiary Mass Extinction: Theories and Controversies
Published:January 01, 2011
Gerta Keller, 2011. "The Cretaceous–Tertiary Mass Extinction: Theories and Controversies", The End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas, Gerta Keller, Thierry Adatte
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The Cretaceous–Tertiary boundary (KTB) mass extinction is primarily known for the demise of the dinosaurs, the Chicxulub impact, and the frequently rancorous thirty-years-old controversy over the cause of this mass extinction. Since 1980 the impact hypothesis has steadily gained support, which culminated in 1990 with the discovery of the Chicxulub crater on Yucatán claimed as the KTB impact site and “smoking gun” that virtually proved this hypothesis. In a perverse twist of fate, this discovery also began the decline of the impact hypothesis, because for the first time it could be tested directly based on the impact crater and impact ejecta in sediments throughout the Caribbean, Central America, and North America. Two decades of multidisciplinary studies amassed a database with a sum total that overwhelmingly reveals the Chicxulub impact as predating the KTB mass extinction in the impact-crater cores, in sections throughout northeastern Mexico and in Brazos River sections of Texas, U.S.A. This paper recounts the highlights of the KTB controversy, the discovery of facts inconsistent with the impact hypothesis, and the resurgence of the Deccan volcanism hypothesis as the most likely cause for the mass extinction.
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The End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
One of the liveliest, contentious, and long-running scientific debates began over three decades ago with the discovery of an iridium anomaly in a thin clay layer at Gubbio, Italy, that led to the hypothesis that a large impact caused the end-Cretaceous mass extinction. For many scientists the discovery of an impact crater near Chicxulub on Yucatan in 1991 all but sealed the impact-kill hypothesis as proven with the impact as sole cause for the mass extinction. Ever since that time evidence to the contrary has generally been interpreted as an impact-tsunami disbturbance. A multi-disciplinary team of reserachers has tested this assertion in new cores and a dozen outcrops along the Brazos River, Texas. In this area undisturbed sediments reveal a complete time stratigraphic sequence containing the primary impact spherule ejecta layer in late Maastrichtian claystones deposited about 200-300 thousand years before the mass extinction.