Chemostratigraphy Across the Cretaceous-Paleogene (K-Pg) Boundary: Testing the Impact and Volcanism Hypotheses

The mass extinction which marks the K-Pg has been linked to a catastrophic event. Cr and Os isotopes and 3 He/⁴He ratios of He encapsulated in fullerenes within Ir-rich K-Pg layer point to an extraterrestrial cause, while Hg/TOC spikes across the K-Pg boundary suggest Hg loading from the Deccan volcanism. Three Hg/TOC spikes are present in some classical K-Pg sections: (i) spike I within the CF2 planktic foraminiferal biozone, (ii) spike II at the K-Pg boundary layer, and (iii) spike III within the P1a planktic foraminiferal subzone. The spike II has, perhaps, resulted from Hg loading from the asteroid impact and volcanism. We suggest that higher ΣREE+Y values in the K-Pg layers are, perhaps, related to Deccan volcanism or to sea-level fluctuations, coeval to the K-Pg transition that enhanced continental influx. True negative Ce anomaly suggests predominance of oxidized surface waters during the K-Pg transition. In a δ²⁰²Hg versus Δ²⁰¹Hg plot, samples from the spike II and from Bidart-France lie within the Hg volcanic emission box. Samples from spikes I and III from Bidart lie within the volcanic emission/chondrite box. Small positive Δ²⁰¹Hg favors long-term atmospheric transport and supports Hg loading to the environment by Deccan phase 2 in three distinct episodes.