FINDING A SPECIFIC NEEDLE IN A HAYSTACK: TRACING EJECTED ZIRCON FROM THE CHICXULUB IMPACT IN K-PG BOUNDARY SECTIONS

Numerical modeling of the Chicxulub impact event on the Yucatán Peninsula in Mexico suggests that shock-induced ejection and vaporization of target lithologies into the stratosphere caused severe environmental stress, culminating in the K-Pg mass extinction 66 Myr ago. However, the exact timing and mechanisms of these impact cratering processes are not yet fully understood. To better constrain particle transport physics in impact ejecta models, it is crucial to incorporate more physical and chemical proxy data from within the Chicxulub impact structure and from the global K-Pg ejecta layer to bolster the current proxies (grain size, ejecta thickness, and the global iridium anomaly). We performed U-Pb detrital zircon (DZ) analyses on K-Pg boundary sites, to provide crucial “tracer” ages linked to distinct pre-impact and impact lithologies, which can be used as an input parameter in ejecta models. Chicxulub target rocks include a newly dated ~334 Ma magmatic arc uplifted within the peak ring as well as the well-known ~550 Ma Yucatán basement age, previously studied in ejecta and suevite clasts within the crater. From the impact lithologies, researchers also recovered fully-reset 66 Ma grains. We investigated the K-Pg stratigraphy in multiple depositional environments (varying from deep marine to shoreface) and distances from the crater ranging from very proximal (<500 km) to intermediate (1000-3000 km). DZ signatures are complicated by competing impact-induced processes: mass gravity flows induced by earthquakes (<6 km/s), crater-sourced ejecta ballistically emplaced in proximal sites (<3 km/s) or following atmospheric transport and mixing in more distal sites, and tsunami that reworked previously deposited material (<0.2 km/s). K-Pg boundary samples show DZ signatures of Upper Cretaceous sediments typical of each region suggesting that these sediments were mobilized by earthquakes and/or reworked by tsunami waves. More importantly, we discovered ages that “fingerprint” the crater: 66 Ma, the most abundant “tracer” age in all sites and sparse Yucatán basement ages in the intermediate sites. The careful differentiation of these ages is critical to understanding the mechanisms that govern the K-Pg stratigraphy at each site including ejecta emplacement and erosion/deposition of Upper-K sediments.