Phosphate nodules from marine black shales and nonmarine gray shales from the Mississippian Fayetteville Shale of north-central Arkansas and northeastern Oklahoma are strongly enriched in middle rare-earth elements (MREEs) as is common in Paleozoic phosphate deposits. The nodules commonly preserve conspicuous europium anomalies that are not typical of phosphate deposits elsewhere in the geologic record, suggesting extremely reducing diagenetic conditions. Contrastingly, minor cerium anomalies imply oxidizing conditions that are not unexpected in the light to medium gray nonmarine host shales. The most likely explanation for the Ce anomalies in the marine phosphate nodules is that bottom waters in the marine environment reverted from the anoxic waters that probably characterized the basal Fayetteville Formation to weakly oxidizing waters that permitted survival of the fauna that occurs in a well-defined zone. All of the Ce anomalies probably formed during an earlier diagenetic stage than the Eu anomalies that developed later, as decaying organic matter consumed what little oxygen was available.