CHICXULUB PREDATES K-T MASS EXTINCTION

Yaxcopoil 1, drilled within the Chicxulub crater, was expected to yield the final proof that this impact occurred precisely at the K-T boundary 65 m.y. ago and caused the end-Cretaceous mass extinction. Instead, contrary evidence was discovered based on five independent proxies (sedimentologic, biostratigraphic, magnetostratigraphic, stable isotopic and iridium) that revealed the Chicxulub impact predates the K-T boundary by about 300,000 years and could not have caused the mass extinction. This is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest Maastrichtian zone CF1, corroborated by magnetostratigraphic chron 29R and characteristic late Maastrichtian stable isotope signals. These results were first presented by Keller et al. (PNAS, 2004). In this study we present more detailed evidence of the presence of late Maastrichtian planktic foraminifera, sedimentologic and mineralogic analyses that demonstrate that the Chicxulub impact breccia predates the K-T boundary and that the sediments between the breccia and the K-T boundary were deposited in a normal marine environment during the last 300,000 years of the Cretaceous. The pre-K-T age of Chicxulub and the presence of a global iridium anomaly at the K-T mass extinction indicates that two large impacts occurred at the end of the Cretaceous. We further demonstrate that based on the available impact crater record for the Phanerozoic and its correlation with extinction intensities, impacts leaving craters up to the size of 150 km wide do not appear to cause mass extinctions. The K-T mass extinction may have been caused by the combined effects of a large impact and major volcanism.