Abstract

Among 21 whole-rock samples of the Upper Cretaceous Niobrara Formation from Colorado, the abundance of cerium relative to other rare earth elements (Ce anomaly), the weight percent organic carbon (%C org ), and the intensity of bioturbation all covary. This covariation is provocative because %C org and intensity of bioturbation track changes in the concentration of oxygen in the local water column at the time of deposition (Savrda and Bottjer 1989). Ce anomalies in apatite-rich fractions of the Maastrichtian Zumaya-Algorta Formation from France and Spain and the Miocene Monterey Formation from California show changes that also may coincide with changes in ancient oxygen levels. Our results for the Niobrara samples are the closest correspondence demonstrated between paleo-redox conditions and Ce anomalies, but we cannot yet determine whether the correspondence reflects a cause-and-effect relationship. Variation in Ce anomalies is influenced by a number of factors, including terrigenous input, depositional environment, and diagenetic conditions. Potential interplay of these factors prevents a unique interpretation of our whole-rock data; dissecting whole-rock Ce anomalies through analysis of isolated sedimentary components, though, is a promising avenue of research.

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