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Clearing up the Chicxulub impact cloud: Geochemistry and petrography of uniquely preserved Cretaceous-Paleogene impact ejecta


Pim Kaskes
AMGC, Vrije Universiteit Brussel

ABSTRACT: In this talk, I will present the first results of my FWO project (FWO11E6619N) focused on newly discovered ejecta derived from the Chicxulub meteorite impact. The 66-million-year-old Chicxulub impact structure in Mexico is unique since it is the only crater on Earth known to date with impact ejecta preserved worldwide, which allows for direct geochemical and petrographic comparison between material from the source area and its global deposits. This can provide crucial insights into the role of target lithologies on the type of products that entered the atmosphere after the impact and their climatic and biotic effects linked to the Cretaceous-Paleogene (K-Pg) mass extinction.

Ongoing research at AMGC on suevite and impact melt rock from the recently obtained IODP-ICDP Exp. 364 drill core inside the crater1 has revealed that hydrothermal alteration is pervasive throughout the core and could have altered the original chemical signal of melt particles and target lithologies. Furthermore, proximal K-Pg boundary sites often show signs of slumping and reworking, whereas distal K-Pg localities are characterized by a very thin, condensed boundary interval lacking the time resolution needed to constrain the different depositional patterns of ejecta. The newly discovered Tanis K-Pg site in North Dakota2 may solve these problems, since it displays a seismically induced onshore surge deposit preserving a unique mixed marine/terrestrial mass-death assemblage together with Chicxulub impact ejecta. These include seemingly unaltered glassy microtektites embedded in fossilized amber, shocked minerals, microkrystites (crystalline spherules linked to condensation from the impact vapor cloud) and an iridium anomaly.

In this project, we apply a multi-proxy approach on the sediments and impact ejecta from the Tanis KPg deposit, starting with the characterization of the microtektites. SEM-EDS, Micro-XRF, LA-ICP-MS and Synchrotron-based XRF and Fe-XANES have revealed that the Tanis amber-microtektites are wellpreserved, may even show a meteoritic component and display chemical variations similar to yellow and black glass from the proximal K-Pg site of Beloc in Haiti. Further geochemical characterization of
the other impact ejecta from Tanis is needed to document the full variability and temporal relationships inside the deposit. In addition, an in-depth comparison with ejecta and boundary slabs from other K-Pg sites (e.g. Gorgonilla Island in Colombia and US Western Interior sites), combined with ejecta modelling, could further constrain the timing and dynamics of the Chicxulub impact plume.

1 Kaskes, P., De Graaff, S. J., De Winter, N., Déhais, T., Smit, J., Goderis, S. & Claeys, P. (2018). Geochemical fingerprinting of the target lithologies within the Chicxulub impact breccia. Abstract #453195 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.
2 DePalma, R.A., Smit, J., Burnham, D.A., Kuiper, K., Manning, P.L., Oleinik, A., Larson, P., Maurrasse, F.J., Vellekoop, J., Richards, M.A. and Gurche, L., 2019. A seismically induced onshore surge deposit at the KPg boundary, North Dakota. Proceedings of the National Academy of Sciences, 116(17), pp.8190-8199.