TWENTY-FIVE YEARS OF UNRAVELING THE CRETACEOUS-TERTIARY (KT) IMPACT MYSTERY

The buried Chicxulub impact basin records one of the most dramatic events recorded in Earth's history in the last 600 million years. When the 10-15 km comet or asteroid struck the shallow sea over what is now the northern Yucatan platform ~65 million years ago, it not only generated a 200-km diameter crater, it deposited impact debris worldwide. This discrete layer of ejected dust, ash, and spherules is temporally associated with the KT extinction event, one of the most profound excursions in our planet's biological evolution. Contained within the KT layer is indisputable evidence of its impact origin including distinct chemical and isotopic signatures of the extraterrestrial projectile as well as continental mineral fragments showing diagnostic indications of shock-wave damage. Other findings within the layer include isotopic and chemical characteristics that point to biomass burning on a global scale, acid rain, and global cooling. Unraveling the KT mystery to this point is a remarkable example of the synthesis that results when an intriguing hypothesis – presented in this case by Alvarez et al. in 1980 – draws together expertise and perspectives from diverse arenas. Yet important issues remain to be resolved. Given the accepted size of Chicxulub, production rate models based originally on lunar data suggest that impact events of this magnitude occur on average every 50 to 100 million years, implying that a half dozen or more such events were produced during the Phanerozoic. Yet, years of intensive stratigraphic studies have failed to identify any other global impact-derived clay layers within the Phanerozoic section, not to mention one that is associated with any of the other mass extinction events. So, why was the impact at the KT boundary stratigraphically and environmentally unique? Possible reasons for this will be discussed related issues will be explored during the presentation.