STABLE ISOTOPE STRATIGRAPHY OF UPPER MAASTRICHTIAN SHALLOW MARINE RHYTHMITE DEPOSITS IN THE GULF COASTAL PLAIN, USA: CLIMATE VARIABILITY LEADING UP TO THE K-PG MASS EXTINCTION EVENT

The upper Maastrichtian Prairie Bluff Chalk (PBC) exposed in Starkville, Oktibbeha County, MS, records rhythmic bedding that possibly resulted from climatic variations associated with Milankovitch cycles. At this locality, the PBC consists of decimeter scale, micaceous, slightly glauconitic, silty marl interbedded with indurated grey to white slightly micaceous chalk. The Danian Clayton Formation unconformably overlies the PBC. The contact between the PBC and the Clayton Fm (K-Pg boundary) is undulating and heavily bioturbated. A detailed ammonite biostratigraphy has recently been constructed for this succession; however, a high-resolution geochronology is still lacking. In this study, we attempt to refine the temporal resolution of the latest Maastrichtian to better interpret paleoenvironmental changes leading up to, and following, the K-Pg mass extinction event. A preliminary geochemical analysis showed cyclical changes in stable isotope (δ¹³C_carb, δ ¹⁸O_carb) and % carbonate values that strongly match the observed lithological variations. However, the sampling intervals were too coarse to apply analytical methods to accurately infer orbital cycles. We resampled the section in 10 cm intervals up to, including, and above the K-Pg boundary. Microsampling was also conducted every 2 cm within a single rhythmite (chalk/marl) to identify any small scale stable isotope variations. To accurately quantify Milankovitch orbital cyclicity, a time-series analysis (REDFIT spectral estimation, wavelet transform) was applied to stable isotope and % carbonate data using the program PAST (PAleontologcal STatistics). δ¹³C, δ ¹⁸O, and % carbonate values show obliquity-scale cyclicity (~40 ky) throughout the section. The orbitally-forced sedimentary sequence at the Starkville locality will aid in correlation with upper Maastrichtian sections both regionally and globally.