FORAMINIFERAL TURNOVER AND PALEOENVIRONMENTAL CHANGES ACROSS AN EXPANDED CRETACEOUS/PALEOGENE BOUNDARY SECTION, NORTHERN ALBORZ, IRAN

An expanded Cretaceous/Paleogene (K/Pg) boundary section from northern Iran provides a unique snapshot of environmental changes associated with the end-Cretaceous mass extinctions on the northern margin of Tethys. A high-resolution study of planktic and benthic foraminiferal assemblages coupled with bulk carbonate stable isotope analysis, magnetic susceptibility, clay mineralogy, and major and trace element geochemistry record conditions prior to and immediately following the K/Pg event, and the subsequent recovery of the marine ecosystem at the Galanderud section, northern Alborz. Benthic foraminifera indicate outer neritic-uppermost bathyal depths with some perceivable bathymetric changes in the studied interval. Planktic foraminifera show a catastrophic extinction pattern at the base of a clay layer where 72% of the Cretaceous species became extinct. The K/Pg boundary clay is marked by a -2.2‰ δ¹³C excursion, a sharp drop in magnetic susceptibility, and elevated siderophile element contents (e.g., Ni, Co) including an Ir concentration of 3.6 ppb. Increased abundances of smectite (30-50%) in the uppermost Maastrichtian may indicate a volcanic source, but an abrupt increase in illite (>60%) at the K/Pg boundary indicates a change in climate or provenance after the event. Benthic foraminifera experienced a major reorganization and species turnover across the boundary reflecting significant environmental changes on the seafloor. The Galanderud section differs from other K/Pg sections by the presence of three chalk beds, interbedded with claystone in the basal Danian, composed almost entirely of calcareous dinoflagellates. The chalks each display positive δ¹³C and δ¹⁸O excursions suggesting upwelling of cooler waters and increased productivity. Increased abundances of illite and reduced kaolinite/smectite ratios in Zone P0, especially during chalk deposition, are compatible with either a cooler, drier climate than during the latest Maastrichtian, or a change in provenance of the clays. This section highlights the dynamic nature of ecosystem perturbation and recovery in the immediate aftermath of the mass extinction event. A stepped recovery of foraminiferal assemblages and bulk δ¹³C values records the gradual restabilization of a ‘new normal’ in the sea surface and at seafloor after Zone P0.