Evidence of an extraterrestrial origin for the demise of the dinosaurs 66 million years ago was first found in the form of a geochemical signature — an anomaly in the abundance of the element iridium — in the sedimentary rocks that separate the Mesozoic and Cenozoic eras. The iridium was derived from the impactor, which was largely vaporized during the impact and spewed upward in a vapor-rich cloud that eventually encircled the Earth. That globally distributed tell-tale signature of impact has been a stratigraphic benchmark for detailed studies of the environmental and biological upheaval that followed.

The point of impact is now known as the Chicxulub crater, an approximately 180-kilometer diameter structure on the Yucatan Peninsula of Mexico that has since been buried by Cenozoic sediments. In 2016, the International Ocean Discovery Program and International Continental Scientific Drilling Program drilled a new scientific borehole into the crater and recovered rock from more than 1.3 kilometers beneath the surface. The core provided evidence of how the crater’s peak ring formed and how the crater was modified by an extensive post-impact hydrothermal system.

In a new study led by Steven Goderis of Vrije Universitet Brussel (Belgium), the transition between crater lithologies and overlying sediments was scrutinized for evidence of the impactor similar to the tenuous millimeter-thick horizon of iridium detected elsewhere around the world. In fact, Goderis and an international team of colleagues found this iridium anomaly perfectly preserved in the recovered rock core. It appears that after the crater had been excavated and flooded by seawater, fine-grained debris from the projectile settled from the atmosphere through seawater to the crater floor, where it was then buried by marine sediments. Preservation of this unit is serendipitous because the high-energy environment of a fresh crater subject to erosion and hydrothermal alteration could have easily destroyed the iridium signature. The preserved iridium anomaly provides an important temporal marker in the reconstruction of biotic recovery within the crater. It also provides the strongest evidence to date directly linking the Chicxulub impact event to the mass extinction that defines the transition between the Mesozoic and Cenozoic eras. READ MORE