SEARCHING FOR THE K-PG ANOMALY IN CENTRAL ARKANSAS

Iridium is one of the rarest occurring elements in the Earth’s crust, with an average concentration of 10-12 ppb. Global documentation of thin, stratigraphic upticks in the concentration of iridium are associated with the impacts of large asteroids. The most well-known of these iridium anomalies is the Cretaceous-Paleogene (K-Pg) Boundary, the result of the Chicxulub asteroid impact. In central Arkansas, exposed Cretaceous-aged nepheline syenite is overlain by Paleocene and Eocene sedimentary layers that represent a potential iridium anomaly across the K-Pg boundary. This outcrop is located near Bauxite, Arkansas, in formerly mined lands where stockpiles of aluminum ore are stored.

In March 2024, four stratigraphic sections were measured in the Sardis Pit across the K-Pg boundary. Sixty samples were collected, labeled, and then individually ground using a porcelain mortar and pestle. Each sample was analyzed using a Thermo Scientific hXRF instrument. These data were used to create a series of litho- and chemo-stratigraphic correlations of elements of interest including lanthanum, niobium, barium, silver, uranium, and others. Laser Ablation-Inductively Coupled Plasma-Mass Spectroscopy (LA-ICP-MS) analyses were conducted on twelve of the sixty powdered samples, which yielded quantifiable concentrations of various elements including iridium. An iridium spike characteristic of the K-Pg boundary iridium anomaly could not be identified with this sample size, but the analyses did report nine samples with a greater mean concentration of iridium than the average concentration within the Earth’s crust. These data could represent sediment transported into the study site from areas with a greater concentration of iridium.