Fingerprinting the Cretaceous-Paleogene boundary impact with Zn isotopes

The Cretaceous-Paleogene (K-Pg) boundary event, which resulted in the extinction of non-avian dinosaurs and many other life forms, has been a subject of scientific debate for nearly 50 years. While evidence strongly supports an extraterrestrial bolide impact as the primary cause, geochemical evidence has largely been circumstantial, demonstrating changes in sediment and atmospheric composition but lacking a clear mechanistic link to the impact process. Previous research has identified anomalies in iridium (Ir) and other platinum group elements (PGEs), carbonaceous chondrite-like Cr isotope anomalies, and Os isotope data suggesting Deccan Traps volcanism preceded the K-Pg boundary. Geochronological and geophysical data pinpoints the Chicxulub crater in the Yucatan Peninsula as the impact site. However, a process tracer directly linking geochemical anomalies to the impact mechanism has been missing. This study focuses on the potential of stable Zn isotopes to provide such a tracer, leveraging Zn's volatility as a key process in large bolide impacts. Previous research on tektites, nuclear blast ejecta, and impact melt sheets, as well as experimental work, has shown that Zn volatilization during high-temperature events leads to lower Zn concentrations and heavy Zn isotope enrichment in the residuum. This study investigates whether this phenomenon can be observed in K-Pg boundary sediments, providing a direct mechanistic fingerprint of the impact.

The literature extensively documents the K-Pg boundary event, supporting the impact hypothesis. Geophysical, geochronological, and geochemical evidence, including Ir/PGE anomalies, impact ejecta (spherules, shocked minerals, Ni-rich spinels), and a CM2 carbonaceous chondrite-like Cr isotope anomaly, points toward a bolide impact. Osmium isotope data suggests that Deccan Traps volcanism predates the K-Pg boundary, diminishing the role of volcanism as the main driver of the mass extinction. However, the existing geochemical evidence primarily indicates changes in composition, not the mechanisms of the impact itself. This paper addresses this gap by focusing on the unique properties of Zn isotopes as a potential process tracer.

Zn isotope compositions and concentrations were measured in K-Pg boundary sedimentary rock layers from five different locations across North America, ranging from proximal (Mississippi, Missouri) to intermediate (Montana, New Jersey) distances from the Chicxulub crater. The selected stratigraphic sequences contain characteristic spherule layers and ash associated with the impact. Samples originated from well-studied outcrops and drill cores, with depositional environments ranging from terrestrial to deeper marine settings. Two sampling tactics were used to capture both micro- and macro-scale Zn isotope signatures. For high-resolution analysis, multiple samples were collected from within the boundary clay layer at several locations, alongside samples from layers above and below. Limestone from the base of the boundary sequence was also sampled to provide a pre-impact baseline. Visible type 1 spherules were also hand-picked from Mississippi marine samples. Samples were powdered, and Zn was purified using anion exchange resin before analysis via multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Zn concentrations were measured using quadrupole ICP-MS. Data were reported relative to the JCM Lyon standard, and corrected for mass bias using the exponential law and standard-sample-standard bracketing. Data were interpreted using binary mixing models and Rayleigh distillation modeling to understand the impact-related processes.

The key finding is that K-Pg boundary layer sediments exhibit higher δ⁶⁶Zn values correlated with lower Zn concentrations compared to surrounding rocks. This is inconsistent with explanations based on magmatic differentiation, inheritance from target rocks, meteoritic material, or secondary alteration processes. Magmatic processes associated with the impact site melting do not fractionate Zn isotopes to the extent observed. Comparison with the Permian-Triassic boundary, where isotopically light Zn is associated with volcanism, further refutes volcanic origins. The K-Pg boundary Zn isotopic signature is also incompatible with inheritance from target rocks or meteorites. Secondary alteration processes also do not produce the observed scale of fractionation. The study concludes that the high δ⁶⁶Zn values and low Zn concentrations are most likely due to partial Zn volatilization during the impact event, with drag heating during ballistic outfall as the primary mechanism. Binary mixing models, using tektites as a proxy for devolatilized impact material, suggest that approximately 10% of the Zn in terrestrial and transitional sediments comes from partially volatilized ejecta. Rayleigh distillation modeling, comparing the data to tektite datasets, confirms the dominance of Zn volatilization. While the volatilization signature is attenuated by mixing with endogenous materials, the correlation between δ⁶⁶Zn and 1/Zn concentration strongly supports partial Zn evaporation during the impact. A notable exception with unusually low Zn isotope values is observed in a sample from the top of the Caribbean boundary, potentially representing complementary isotopically light Zn fallout.

The observed Zn isotope and concentration signatures provide compelling evidence for Zn volatilization as a key mechanism during the K-Pg impact event. This offers a novel mechanistic fingerprint that directly connects geochemical anomalies to the impact process. The findings corroborate previous work on tektites and other impact-related materials and refine our understanding of the processes involved. The study demonstrates the utility of Zn isotopes as process tracers in impact events, offering a new tool for investigating past impacts and their consequences.

This research demonstrates that Zn isotope systematics provide strong evidence for impact-related Zn volatilization at the K-Pg boundary. The systematic higher δ⁶⁶Zn values coupled with lower Zn concentrations in boundary layer sediments, across diverse depositional environments, are best explained by partial evaporation during the impact. This work highlights the utility of Zn isotopes as mechanistic tracers of impact-related processes and calls for further investigation of Zn isotopes in other impact-related sedimentary layers. Future research should focus on identifying the isotopically light Zn reservoir required by mass balance, and expanding this methodology to other impact events.

The study acknowledges the complexities of material mixing and other factors affecting Zn isotope fractionation during the impact and subsequent sedimentation. The binary mixing models are simplified approximations, and the exact contributions of different Zn sources cannot be precisely quantified. The limited number of samples from some locations might also affect the statistical certainty of some interpretations, especially regarding marine sites. Future research could address these limitations by expanding the dataset and incorporating more sophisticated modeling techniques.