A REGIONAL RECONSTRUCTION OF LATEST CRETACEOUS (MAASTRICHTIAN) OCEAN TEMPERATURES IN THE GULF COASTAL PLAIN USING CLUMPED ISOTOPE THERMOMETRY IN BIVALVE MACROFOSSILS

Roughly 66 million years ago, more than 75% of species on Earth went extinct at the Cretaceous-Paleogene (K-Pg) boundary. 150-300 kyr prior to the extinction and impact of the Chicxulub bolide, major inputs of greenhouse gases from Deccan Traps volcanism caused the Latest Maastrichtian Warming Event. The Gulf Coastal Plain, USA has a rich fossil record of the impacts of these events on the marine realm. We collected marine bivalve macrofossils (mostly oysters) from the D. iris ammonite zone and/or CC26b nannofossil subzone from 5 Gulf Coast K-Pg boundary sites: Owl Creek, Ellis Sand Pit, and Trim Cane Creek, MS, Braggs Road, AL, and Malvern, AK. We imaged fragments of these samples using scanning electron microscopy (SEM) to check for microstructural preservation, then measured carbon and oxygen isotopes (δ¹³C, δ¹⁸O) and carbonate clumped isotopes (Δ₄₇) to reconstruct past ocean conditions. Mean δ¹⁸O_sw values of the 5 sites ranged from 0 to 1‰, enriched relative to the often-assumed ‘ice-free Cretaceous’ δ¹⁸O_sw value of -1‰. Use of this assumed lower value in typical δ¹⁸O_carb-based paleotemperature calculations therefore resulted in temperatures that were too cold. The observed regional enrichment in δ¹⁸O_sw over mean ocean values is also seen today in the central Gulf of Mexico and around Florida and is due to surface evaporation in the warm tropics and extra-tropics. Our findings suggest that similar evaporative enrichment occurred in the Maastrichtian Gulf Coastal Plain. Mean benthic Δ₄₇-temperatures from each site ranged from 22°C to 26°C. Adjacent sites showed closest agreement, producing a coherent picture of the regional conditions. This reconstructed paleoenvironment is warmer than the modern Gulf of Mexico, despite being further from Earth’s equator. Warmer temperatures align with the greenhouse gas conditions during the Maastrichtian. At sites with higher temporal sampling resolution, individual samples showed minimal variability through the measured intervals. These results indicate that climate in the Gulf Coastal Plain was stable prior to the bolide impact.