Reduced CO2 release from the Chicxulub impact event as inferred from carbonate clumped isotope thermometry on crater impactites

The ~200 km wide Chicxulub impact structure in Yucatán, México, formed by a hypervelocity impact event ~66 Myr ago, constitutes an ideal natural laboratory to apply carbonate clumped isotope thermometry on lithologies deep within an impact crater. The target stratigraphy at Chicxulub consists of a ~3 km thick Mesozoic carbonate-evaporite platform, and carbonates are present as clasts, fine-grained matrix particles, and secondary precipitates in impact melt-bearing breccias (suevites) and impact melt rock. However, the role of these carbonates in the cratering processes, such as shock-melting, devolatilization, and post-impact carbon cycle perturbations, remain poorly constrained. Hence, this study presents the first clumped-isotope (Δ47) analysis on drill cores from a transect throughout the Chicxulub crater that preserve hot signatures of impact-related thermodynamic processes. The clumped isotope dataset is supplemented by conventional isotope analysis (δ18O and δ13C) and high-resolution petrography.
Three scenarios are introduced to explain the high temperatures. 1) Outside the Chicxulub crater, the proximal ejecta blanket shows traces of thermal processing of carbonate material during ejection (>100°C). 2) Within the crater the influence of a widespread hydrothermal system is determined in all lithologies (>35.5°C) except post-impact sediments. 3) Superimposed on a burial diagenetic and a hydrothermal overprint, highly elevated temperatures (up to 327 ± 33°C) in lower suevites and impact melt rocks are measured in microcrystalline calcite phases. This calcite resembles microcrystalline petrographic features produced by laser-melting experiments on limestones. We interpret that these features likely formed by impact-induced decarbonation and rapid back-reaction, in which highly reactive CaO recombines with impact-released CO2 to form secondary CaCO3 phases.

Our dataset provides the first physical and chemical evidence for back-reactions deep within the Chicxulub impact structure. This has important climatic implications for the Cretaceous-Paleogene (K-Pg) mass extinction event, as current numerical models likely overestimate CO2-release from the Chicxulub impact, potentially up to a factor of ~60%. Therefore, the recombination effect to form secondary CaCO3 phases needs to be accounted for in these paleoclimate models.