The identity of the Chicxulub impactor: KPg ejecta-encapsulated meteoric fragments

<p>The Cretaceous-Paleogene (KPg) boundary is demarked by two critical and intimately linked events: a global-scale impact caused by a massive extraterrestrial body, and a resultant catastrophic planet-wide mass-extinction, with pivotal long-term consequences for life on Earth. The site of impact has been identified as the ~180 km wide Chicxulub crater complex [Yucatan peninsula, Mexico] based on strong geochemical, sedimentological, and temporal evidence. However, the impactor, which was obliterated on contact during the cratering process, has eluded identification. Previous studies struggled to identify the projectile based on scant geochemical and isotopic traces admixed with the ejecta and melt-rock, plus a single heavily altered microscopic fragment possibly from the impactor. While those efforts helped to rule out some potential sources and narrowed down the best potential candidates to a CM, CR, or CO carbonaceous chondrite, the data was insufficient to provide an incontrovertible diagnosis, and the identity of the Chicxulub projectile remained a critical missing detail. Here we describe new, exceptionally-preserved fragments of cosmic origin that were directly associated with Chicxulub impact ejecta from a temporally constrained KPg boundary site in North Dakota (U.S.A.). The fragments occur as inclusions within unaltered glassy ejecta spherules, which likely protected them against chemical and physical degradation, leading to their marked preservation. The spherules themselves were also uniquely preserved in amber, which inhibited their usual breakdown to smectitic clay. Geochemical examination of the inclusions were undertaken using electron microprobe, laser-ablation inductively-coupled-plasma-mass-spectroscopy, and synchrotron X-ray techniques. Results provide multiple lines of evidence that support a cosmic origin and chemistry indicative of a CM subtype carbonaceous chondrite, offering new support of prior hypotheses. The data is also in agreement with projectile residues from the Chicxulub impact fallout. In addition to its utility in reconstructing the dynamics of the Chicxulub event, this data helps provide extra dimension to our understanding of major impact processes and their effects on Earth.</p>