Chicxulub impact predates the K-T boundary mass extinction

Abstract

Since the early l990s the Chicxulub crater on Yucatan, Mexico, has been hailed as the smoking gun that proves the hypothesis that an asteroid killed the dinosaurs and caused the mass extinction of many other organisms at the Cretaceous-Tertiary (K-T) boundary 65 million years ago. Here, we report evidence from a previously uninvestigated core, Yaxcopoil-1, drilled within the Chicxulub crater, indicating that this impact predated the K-T boundary by Ϸ300,000 years and thus did not cause the end-Cretaceous mass extinction as commonly believed. The evidence supporting a pre-K-T age was obtained from Yaxcopoil-1 based on five independent proxies, each with characteristic signals across the K-T transition: sedimentology, biostratigraphy, magnetostratigraphy, stable isotopes, and iridium. These data are consistent with earlier evidence for a late Maastrichtian age of the microtektite deposits in northeastern Mexico.T he search for the K-T impact crater effectively ended in the early l990s with the discoveries of the Chicxulub crater and its estimated diameter between 180 and 280 km (1-4). The impact ejecta (microtektites) in Haiti and northeastern Mexico (5, 6) and melt rock in the Chicxulub cores have similar geochemistry, and the 39Ar͞40Ar ages are within Ϯ200,000 years (200 ky) of the K-T boundary (7,8). These observations made a convincing case for Chicxulub as the long-sought K-T boundary impact crater and cause for the end-Cretaceous mass extinction. But doubts persisted regarding the precise age and size of the impact crater (2-4, 9), the origin of the so-called megatsunami deposits (5,(10)(11)(12), and the nature of the mass extinction (13,14). To resolve these issues, the International Continental Scientific Drilling Program (ICDP) supported the drilling of a new core within the Chicxulub crater (Yaxcopoil-1 drilled between December 2001 and February 2002) with the stated objectives to determine the precise age of the crater and its link to the global K-T boundary layer, to unravel Chicxulub's role in the K-T mass extinction, and to study the cratering event and size of the impact crater (15).Chicxulub Core Yaxcopoil-1 (Yax-1) Yax-1 is located 40 km southwest of Merida, Mexico, and Ϸ60 km from the center of the Chicxulub structure (Fig. 1). A continuous core sequence was recovered from 400 to 1,511 m below the surface (15). † † ‡ ‡ Between 794.65 and 894 m, a 100-m-thick impact (suevite) breccia overlies a 617-m-thick sequence of horizontally layered shallow-water lagoonal to subtidal Cretaceous limestones, dolomites, and anhydrites. The first 85 m of the impact (suevite) breccia consist primarily of clasts from these underlying shallow-water lithologies, some crystalline rocks of continental basement origin, and devitrified glass (Cheto smectite) fragments and spherules. † † The upper 15 m of the breccia are stratified with alternating layers of upward fining clasts (3-5 cm at the base to 2-5 mm at the top), coarse cross-bedding structures, and gray friable sand layers in the top meter, all of which indicate...

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