Iridium at the K-Pg Boundary: Correlation with Deep Mantle Structure and a Testable Prediction

The Cretaceous-Paleogene (K-Pg) mass extinction at 66 Ma is conventionally attributed to the Chicxulub bolide impact, with iridium (Ir) enrichment in boundary clays cited as primary evidence for an extraterrestrial source. However, multiple anomalies in the global Ir distribution remain inadequately explained by the single-impact model: (1) proximal sites near Chicxulub show lower Ir fluence than the global mean, (2) no correlation exists between Ir concentration and distance from the crater beyond the Gulf of Mexico, and (3) osmium isotope ratios in boundary clays exceed chondritic values. Here we present a novel spatial analysis demonstrating that peak Ir concentrations at K-Pg boundary sites correlate more strongly with proximity to the margins of Large Low Shear Velocity Provinces (LLSVPs) at the core-mantle boundary (r = -0.744, log-Ir vs. LLSVP margin distance) than with distance from Chicxulub (r = +0.677, positive: farther = higher). These findings motivate an alternative hypothesis: K-Pg boundary Ir originated from internal Earth processes via mantle plume eruptions at LLSVP margins. This paper moves the Ir origin debate to an untested discriminatory axis: 186Os/188Os ratios in K-Pg boundary clays, which can distinguish outer core contributions from meteoritic input through coupled 186Os-187Os enrichment patterns. To our knowledge, this measurement has not been performed. A single isotopic analysis could resolve the origin question.