The sedimentary record of the Late Cretaceous North American Western Interior Seaway (WIS) is characterized by periods of enhanced organic carbon burial, including an interval that has been identified as Oceanic Anoxic Event (OAE) 3. The timing and duration of this event has proved difficult to constrain, calling into question whether a widespread organic carbon burial event actually occurred. In order to evaluate the nature of OAE 3 better, different localities are studied within the WIS to investigate whether a single anoxic, carbon burial event exists within Upper Cretaceous strata above the global OAE 2. New trace metal and Fe speciation records from the Niobrara Formation within the Sweetgrass Arch region in southern Alberta, Canada, highlight that there were several intervals of deoxygenation. Comparison of these new results with similar records from the southern portion of the seaway (Colorado, USA), allows seaway wide versus local changes in biogeochemical cycling to be differentiated. These results suggest that while enhanced organic carbon burial is favored in the WIS during the Niobrara transgression, there is no evidence for a single, discrete, seaway-wide anoxic event during the Coniacian−Santonian. In the Sweetgrass Arch region, enhanced organic carbon accumulation and the development of anoxia is controlled by the incursion of Tethyan waters into the region and may have been influenced by significant nutrient addition from ash deposition. A sea level control on the incursion of southern sourced water masses and, subsequent deoxygenation and enhanced organic carbon burial in the WIS, may explain why “OAE 3” is only identified in shallow and restricted marine environments that would be sensitive to changes in sea level. A combination of local and eustatic controls on sea level may also explain the diachronous timing of enhanced organic carbon burial in different regions.

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