ELEMENTAL CHARACTERIZATION OF THE EL KEF K-PG BOUNDARY STRATOTYPE: NO EVIDENCE OF AN ASTEROIDAL IMPACT

The K-Pg boundary represents the most studied mass extinction in geologic history. For forty years, the theory that an asteroidal impact caused the mass extinction has been the most widely accepted explanation of this event. The key evidence leading to the impact theory was the discovery of high levels of Ir in the boundary clay, an element present at concentrations a thousand times lower in the continental crust but enriched in chondritic meteorites. However, there is a remarkable coincidence of large flood basalt eruptions that occurred contemporaneously with mass extinctions in the overall geologic record, while an impact is associated only with the K-Pg event. Recent high-precision chronology of the Deccan flood basalts and plankton biostratigraphy has shown that the mass extinction was simultaneous with the second major pulse of magmatism in the Deccan. To better understand the geochemical processes that occurred to form the K-Pg boundary clay, we analyzed the samples taken from the global K-Pg stratotype location at El Kef, Tunisia, by laser ablation ICP-MS. The boundary clay at El Kef contained 17 ppb Ir in the clay fraction where it is correlated with Fe-oxyhydroxides. The Ir is higher than in the classic Gubbio section that contained approximately 7 ppb Ir in the clay fraction. The siderophile element pattern of the bulk clay is distinctly non-chondritic – particularly evident in the Os/Ir ratio (1.6 x chondritic) and the presence of a negative Pt anomaly. The boundary clay is enriched in Fe, Co and Ni in non-chondritic ratios, more typical of seawater-precipitated Fe-oxyhydroxides. The siderophile pattern of impact material was either extensively modified by post-impact weathering or was mainly derived from marine sources, the latter not requiring an impact origin. This work requires a revision of the classic impact theory and merits extensive reanalysis of all K-Pg boundary sections.