CLUMPED ISOTOPE ANALYSIS OF AMMONITE PALEOECOLOGY AND LATEST CRETACEOUS CLIMATE VARIABILITY IN THE US GULF COASTAL PLAIN

Geochemical records derived from well-preserved shell carbonate are increasingly used to reconstruct the paleoecology and habitat of ammonoid cephalopods (ammonites) and the evolution of ancient marine ecosystems and climate. Carbonate clumped isotope (Δ₄₇) analysis is a promising tool which does not rely on assumptions of ancient seawater composition that hamper ‘traditional’ stable isotope studies. However, utility of Δ₄₇ measurements on extinct taxa like ammonites is unresolved because of the lack of extant species needed to calibrate for any potential isotopic vital effects. Here we present a new multi-taxon dataset from the type locality of the Maastrichtian Owl Creek Formation, Tippah County, Mississippi. This site (est. 50-70 m paleo-water depth) contains an exceptionally preserved molluscan fauna with original aragonitic shell material constrained by macro- and microfossil biostratigraphy to the final ~500 kyr of the Cretaceous (Discoscaphites iris Ammonite Range Zone; calcareous nannofossil subzone CC26b). Fossils of three ammonite genera (Discoscaphites, Eubaculites, Sphenodiscus), four genera of infaunal bivalves, and rare nautilids (Eutrephoceras) were systematically collected and sampled at 0.5 m intervals throughout a 9 m-thick section. Preservation was assessed using the SEM Preservation Index (PI). Clumped isotope palaeotemperatures from well-preserved shells reveal overlap in values among all three ammonite genera and benthic taxa, and close agreement in δ¹⁸O_Seawater values from all taxa consistently clustered around 0.5‰. This suggests that ammonites and benthic bivalves secreted their shells in isotopic equilibrium with seawater of the same composition and probably lived in similar environments. Ammonite genera preserved in the Owl Creek Formation probably had a nektobenthic mode of life and lived close to the seafloor. In contrast to recent studies, agreement between co-occurring bivalve and ammonite taxa provides no evidence that well-preserved ammonites exhibit “vital effects” with respect to the clumped isotope composition of their shells. They are, therefore, reliable archives for paleotemperature and water column reconstructions. These data provide new constraints on both the paleoecology of extinct cephalopod taxa and marine climate evolution in the Gulf Coastal Plain immediately prior to the end-Cretaceous mass extinction.