RECONSTRUCTING OCEAN CIRCULATION ALONG THE EASTERN MARGIN OF THE WESTERN INTERIOR SEAWAY DURING THE LATE CRETACEOUS COOLDOWN

The Niobrara Formation and overlying Pierre Shale are prominent stratigraphic units that were deposited in the Western Interior Seaway (WIS) of North America during the Late Cretaceous as part of the Niobrara and Claggett transgressive sequences, respectively. Both stratigraphic units are exposed along the Manitoba Escarpment in southwestern Manitoba and northeastern North Dakota. Despite being deposited in similar nearshore (neritic) facies along the passive eastern margin of the WIS, the lithological character and fossil faunas of these two stratigraphic units differ markedly. Specifically, microfossil assemblages within the chalky strata of the upper Niobrara Formation (early Campanian) are dominated by calcareous foraminifers whereas those associated with siliciclastic strata of the Pierre Shale (late Campanian) are composed predominantly of radiolarians. It has been posited that this shift in microfossil assemblages signals a major change in ocean circulation where the calcareous assemblages of the Niobrara Formation reflect relatively warm Tethyan waters, while the siliceous assemblages of the Pierre Shale (Millwood member) are diagnostic of cooler, boreal waters. However, geochemical records (benthic foraminifer δ18O, TEX86) indicate that global climate cooled over the course of the Campanian; hence, the shift from calcareous to siliceous microfossil assemblages may simply reflect a global climatic signal with no change in local surface ocean currents. Here we use neodymium (Nd) isotope values measured from biogenic apatite (fossil fish teeth and bone) as a water mass tracer to test for an inferred change in ocean circulation. Preliminary analyses of Niobrara materials yield relatively radiogenic εNd(t) values (-7.5 to -9.3) consistent with a Tethyan affinity, and work is underway to determine the εNd(t) values of Pierre Shale materials. The perceived change in ocean circulation invoked to explain the shift from calcareous to siliceous microfossil assemblages would be corroborated if future analyses produce a resolvable difference between the εNd(t) values of materials collected from the Niobrara Formation and Pierre Shale.