Pyrite Oxidation Controlled Siderophile Element Accumulation on the K-Pg Boundary at El Kef, Tunisia

In a transformative contribution, Alvarez et al. (1980) discovered the iridium anomaly at several K-Pg boundary locations that they attributed to an extraterrestrial impact that triggered the end-Cretaceous extinction. The absence of a suitable mechanism by which to concentrate siderophile elements in the boundary clay drove the argument for an extraterrestrial origin of the iridium. They made the observation that fallout from a fireball would be expected to create a uniform distribution of Ir in the clay layer and puzzled over the scale of lateral variation in Ir observed even then. A detailed global study of the siderophile element (Ru, Rh, Pd, Ir, Pt Au) distribution at the K-Pg boundary found non-chondritic patterns concluding that some post-depositional process(es) must have affected the elemental distribution (Goderis et al., 2015). Such processes would mobilize siderophile elements into the surrounding strata. Here, we applied laser ablation ICP-MS, a microanalytical technique, to investigate the distribution of 60 elements, with an emphasis on the siderophile elements, in a vertical transect at the K-Pg boundary at El Kef, Tunisia, to search for elemental transport in or out of the K-Pg clay layer. The K-Pg boundary at El Kef consists of irregular mixed layers of clay, goethite and gypsum with marls above and below. The siderophile elements are concentrated in the goethite-rich component with a distinctly terrestrial crust pattern, albeit super-enriched, with prominent negative Pt anomalies indicative of deposition from an oxidized solution. The Fe/Se ratio indicates an origin of the goethite by oxidation of sedimentary pyrite. Iron oxyhydroxides are effective substrates for the binding of trace metal oxyanions from solution. The extreme enrichment of siderophile elements reflects long-term concentration of siderophile elements from percolating oxidized groundwaters at El Kef. The sulfuric acid produced by pyrite oxidation was neutralized by calcium carbonate in the marls to form gypsum. Selenium (normally a sulfur analog) is undetectable in the El Kef gypsum endmember (Se/S~0), unlike marine gypsum, supporting a formation by pyrite oxidation. This observation potentially explains the ubiquitous non-chondritic siderophile patterns observed globally and the variable Ir enrichments that puzzled geochemists since Alvarez et al. (1980). In view of this observation, siderophile element enrichment in the K-Pg layer can no longer be taken as unambiguous evidence of an extraterrestrial impact.