Volcanic origin of the mercury anomalies at the Cretaceous-Paleogene transition of Bidart, France

Abstract

The timing and mechanisms of the climatic and environmental perturbations induced by the emplacement of the Deccan Traps large igneous province (India) and their contribution to the Cretaceous-Paleogene (K-Pg) mass extinction are still debated. In many marine sediment archives, mercury (Hg) enrichments straddling the K-Pg boundary have been interpreted as the signature of Deccan Traps volcanism, but Hg may also have been derived from the Chicxulub (Mexico) impact. We investigated the Hg isotope composition, as well as the behavior of iridium (Ir) and other trace elements, in K-Pg sediments from the Bidart section in southwest France. Above the K-Pg boundary, Ir content gradually decreases to background values in the Danian carbonates, which is interpreted to indicate the erosion and redistribution of Ir-rich fallouts. No significant enrichment in Ir and W, or Zn and Cu, is observed just below the K-Pg boundary, excluding the hypothesis of downward remobilization of Hg from the boundary clay layer. Positive Δ199Hg and slightly negative values in the upper Maastrichtian and lower part of the early Danian are consistent with the signature of sediments supplied by atmospheric Hg2+ deposition and volcanic emissions. Up section, large shifts to strongly negative mass-dependent fractionation values (δ202Hg) result from the remobilization of Hg formerly sourced by the impactor or by a mixture of different sources including biomass burning, volcanic eruption, and asteroid impact, requiring further investigation. Our results provide additional support for the interpretation that the largest eruptions of the Deccan Traps began just before, and encompassed, the K-Pg boundary and therefore may have contributed to the K-Pg mass extinction. © 2021 The Authors