A MECHANISM FOR THE GLOBAL ACCUMULATION OF TERRESTRIAL SIDEROPHILE ELEMENTS ON THE K-PG BOUNDARY (Invited Presentation)

The global presence of an Ir-rich clay layer at the KPg boundary has long been interpreted as evidence for a catastrophic asteroid impact. The absence of a suitable terrestrial mechanism by which to concentrate siderophile elements in the boundary clay drove the argument for an extraterrestrial origin of the iridium. Fallout from a global fireball would be expected to create uniform siderophile element (e.g., Ir, Os, Pt, Pd, etc.) concentrations in the clay, but large-scale lateral variation in these elements and the lack of chondritic inter-element patterns has long been puzzling. At a minimum, siderophiles of an impact origin must have experienced terrestrial diagenetic modification to explain the inter-element patterns, but a more radical interpretation is that the siderophile excesses observed are of terrestrial origin. This explanation should not be surprising in view of the known enrichment of numerous trace metals, e.g. Ni and Co, in crustal ratios in the boundary clay. To unravel the complexity of elemental enrichment at the boundary, we applied laser ablation ICP-MS to investigate the distribution in a vertical transect at the K-Pg boundary at El Kef, Tunisia. Here, the boundary 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 crustal pattern, albeit super-enriched, with prominent negative Pt anomalies indicative of deposition from an oxidized solution. The Fe/Se ratio indicated an origin of the goethite by oxidation of sedimentary pyrite. The charged surfaces of freshly formed iron oxyhydroxides bind a wide assortment of metal oxyanions (including siderophile elements) from solution. Thus, the extreme enrichment of siderophile elements in goethite at the KPg boundary reflects long-term concentration of siderophile elements from percolating groundwaters at El Kef. Pyrite and its alteration products are ubiquitous phases in KPg sediments, so variable pyrite alteration is a plausible global mechanism for concentrating siderophile elements at global KPg boundary sites. 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.