CONSTRAINING THE OXYGEN ISOTOPIC COMPOSITION VALUES OF THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY USING BENTHIC BIVALVES

We present data of the oxygen isotopic composition (δ¹⁸O) of benthic bivalves from the epicontinental Western Interior Seaway (WIS) that connected the northern Boreal Sea and the southern Tethys Sea across central North America during the Late Cretaceous. In the late Campanian (75-72 Ma), the WIS began to recede and was completely dried out by the early Paleogene (~65 Ma). Models suggest that the WIS should have δ¹⁸O values that are lower than the open ocean normal marine settings due to significant continental freshwater input and the limited restricted connection to the open ocean. We measured δ¹⁸O of a variety of well-preserved epifaunal (Anisomyon, Endocostea, Inoceramus, Ostrea, and Pteria) and infaunal (Cucullaea, Cymbophora, Geltena, Lucina, Nucula, and Tenuiptera) bivalves from the upper Campanian and lower Maastrichtian (72-69 Ma) WIS. We compare our WIS measured bivalves’ δ¹⁸O values to previous other WIS studies’ δ¹⁸O values of bivalves and ammonites in the WIS, and the δ¹⁸O values of those of foraminifera, bivalves, and ammonites obtained from a literature survey of the open ocean environments. Overall, the δ¹⁸O values of WIS organisms ranged from -14‰ to +1‰. The lowest WIS δ¹⁸O values reported here most likely result from diagenetic alteration. However, most WIS δ¹⁸O values range from -6‰ to 0‰, which largely overlaps with those of open ocean studies (which ranged from -5‰ to +2‰). A relationship between δ¹⁸O values and proximity to the WIS’s shoreline was observed, with lower values closer to the shoreline and higher values towards the WIS’s interior. Notably, even when only considering the best-preserved samples, the WIS demonstrates a larger spread of δ¹⁸O values than the open ocean dataset. This spread may be due to (1) freshwater input into the WIS and/or (2) enhanced evaporation in a shallow epicontinental sea. These data support the hypothesis that WIS waters were not significantly different isotopically from the open ocean at this time. Further, even as the WIS began to recede in the late Campanian, WIS δ¹⁸O values remain similar in the early Maastrichtian. This suggests that the diminished connection to the open ocean did not substantially modify δ¹⁸O values in the WIS.