Fingerprinting the Cretaceous-Paleogene boundary impact with Zn isotopes

Abstract

Numerous geochemical anomalies exist at the K-Pg boundary that indicate the addition of extraterrestrial materials; however, none fingerprint volatilization, a key process that occurs during large bolide impacts. Stable Zn isotopes are an exceptional indicator of volatility-related processes, where partial vaporization of Zn leaves the residuum enriched in its heavy isotopes. Here, we present Zn isotope data for sedimentary rock layers of the K-Pg boundary, which display heavier Zn isotope compositions and lower Zn concentrations relative to surrounding sedimentary rocks, the carbonate platform at the impact site, and most carbonaceous chondrites. Neither volcanic events nor secondary alteration during weathering and diagenesis can explain the Zn concentration and isotope signatures present. The systematically higher Zn isotope values within the boundary layer sediments provide an isotopic fingerprint of partially evaporated material within the K-Pg boundary layer, thus earmarking Zn volatilization during impact and subsequent ejecta transport associated with an impact at the K-Pg.

Resolving impact volatilization and condensation from target rock mixing and hydrothermal overprinting within the Chicxulub impact structure

“…The empirical factor a for tektite data (0.999 to 0.9997) based on the Zn isotope ratio data determined for distal K-Pg boundary sites (Mathur et al, 2021), and the data collected in this work imply Zn isotopic fractionation from volatilization below that expected by theoretical Rayleigh distillation to vacuum (a % 0.985 in a vacuum). In all cases, the lower a is likely attributable to effects linked to diffusion in the thin layer of the ejecta, slowing down the evaporation (Moynier et al, 2009; and/or suppression of Zn isotopic fractionation during large-scale evaporation under high pressure (i.e., non-vacuum) conditions (Davis and Richter, 2014;Day et al, 2017;Wimpenny et al, 2019;Chernonozhkin et al, 2021).…”

“…This trend may suggest that the Zn isotope ratios for the upper sections of the drill core may have retained a hint of volatility-driven isotopic fractionation. While one should keep in mind that seawater nodules, tracing seawater Zn isotope ratios, can also display higher d 66/64 Zn values (Maréchal et al, 2000), the trends for K-Pg boundary sediments highlighted by Mathur et al (2021) are observed for both marine and terrestrial sites, confirming the robustness of Zn isotope systematics. In addition, no Zn carrier phases were identified in the Paleogene sediments and transitional unit samples studied here.…”

“…Recently, Mathur et al (2021) reported that the Zn isotopic compositions of the K-Pg sedimentary layers closely adhere to a Rayleigh distillation curve, whereby d 66/64 Zn increases as Zn concentration decreases. This observation was interpreted to reflect that partial Zn evaporation from impact-generated melt spherules was the process controlling the Zn isotopic and concentration sig-natures of K-Pg boundary spherules and ejecta layers.…”

“…In all cases, the lower a is likely attributable to effects linked to diffusion in the thin layer of the ejecta, slowing down the evaporation (Moynier et al, 2009; and/or suppression of Zn isotopic fractionation during large-scale evaporation under high pressure (i.e., non-vacuum) conditions (Davis and Richter, 2014;Day et al, 2017;Wimpenny et al, 2019;Chernonozhkin et al, 2021). While the a envelope of the distal K-Pg ejecta (between roughly 0.999 and 1; Mathur et al, 2021) is well below that of tektites (down to <0.999; Moynier et al, 2009;Rodovská et al, 2017), the Zn data for the core samples plot within an even narrower a range between 0.9995 and 1 (Fig. 8b-c).…”

“…In addition, stable Zn isotope ratios were successfully used to characterize melt sheet material from the Sudbury impact structure in Canada (Kamber and Schoenberg, 2020). Recently, Mathur et al (2021) performed Zn isotopic analysis of K-Pg boundary sediments, revealing heavier Zn isotopic compositions and lower Zn concentrations within the K-Pg boundary claystone relative to the surrounding sedimentary rocks. These results provide clear evidence of volatilization taking place during the Chicxulub impact event, but additional data on the source region and materials are needed to further refine this interpretation.…”