QUANTIFYING EXTINCTION, ENVIRONMENT, AND ECOLOGICAL CHANGE ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY ON SEYMOUR ISLAND, ANTARCTICA

The highest southern latitude onshore record of the Cretaceous-Paleogene (K-Pg) mass extinction is found on Seymour Island, Antarctica. The boundary occurs within the upper levels of the ~1100 meter-thick López de Bertodano Formation, a highly expanded (sedimentation rates of 0.2 cm/yr) and abundantly fossiliferous sequence of silty sandstones with occasional glauconite-rich horizons. The boundary is marked by a small iridium anomaly linked to the Chicxulub bolide impact and the last occurrence of Cretaceous ammonites and microfossil taxa. These occur ~30 cm below a 1 to 5 m-thick glauconitic sandstone interval which outcrops continuously across southern Seymour Island. Previous work has recognized the profound change in the richness and composition of the marine micro- and macrofauna at this horizon. However, there has been little or no quantitative work examining ecological change across the boundary at high latitudes, and persistent debate about the timing of extinction and environmental changes seen in this succession prior to the K-Pg boundary itself.

During the 2023-24 field season we conducted a detailed study of the K-Pg boundary interval on Seymour Island, measuring multiple sections and making quantitative fossil collections in the uppermost Maastrichtian and lowermost Danian of the López de Bertodano Formation along strike for ~2.5 km. Our new collections extend the stratigraphic ranges of several marine invertebrate taxa closer to, and in one case across, the K-Pg boundary. They also indicate that several undescribed species of benthic mollusc are present in the upper López de Bertodano Formation below the K-Pg. Data from our quantitative field sampling indicate that while existing macrofossil collections from Seymour Island may provide an accurate picture of species richness, they do not adequately capture the ecological composition of the marine ecosystem in either the pre- or post-boundary intervals. Shifts in the dominance and abundance of particular taxa do occur within the final ~1 million years of the Maastrichtian, before a profound ecological change at the K-Pg to an early Danian community numerically dominated by the large infaunal bivalve Lahillia larseni. These ecological changes through time, as well as their environmental context and drivers, are the focus of this presentation.