Hot carbonates deep within the Chicxulub impact structure as revealed by clumped isotope thermometry

Carbonate clumped isotope thermometry has increasingly been
used over the last 15 years to reconstruct absolute temperatures
in the geological record. Besides its common use in
paleoclimatology, clumped-isotope analyses have recently shown
great potential in constraining temperatures of carbonate phases
involved in impact ejecta processes. The ~200 km wide
Chicxulub impact crater in Yucatán, México, formed by a
hypervelocity impact event ~66 Myr ago, constitutes an ideal
natural laboratory to also apply this method on lithologies deep
within an impact structure. 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 meltbearing
breccias (suevites) and impact melt rock. However, the
role of these carbonates in the cratering processes, such as shockmelting,
devolatilization, and post-impact carbon cycle
perturbations, remain poorly constrained.
Hence, we present 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. Together with conventional isotope analysis (δ18O and
δ13C) and high-resolution petrography, we introduce three
scenarios 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 the
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 backreaction,
in which highly reactive CaO recombines with impactreleased
CO2 to form secondary CaCO3 phases. This dataset
provides the first physical and chemical evidence for backreactions
deep within the Chicxulub impact structure. This has
important climatic implications for the Cretaceous-Paleogene
mass extinction event, as current numerical models likely
overestimate CO2-release from the Chicxulub impact. We
therefore propose that the recombination effect to form
secondary CaCO3 phases needs to be accounted for in these
climate models.