Gradual changes in upwelled seawater conditions (redox, pH) from the late Cretaceous through early Paleogene at the northwest coast of Africa: Negative Ce anomaly trend recorded in fossil bio-apatite

Abstract

Marine vertebrate fossils from late Cretaceous–early Paleogene shallow marine phosphorites of Morocco were investigated for their trace element compositions and neodymium isotopic ratios in order to constrain marine and depositional conditions. The various analyzed fossils are separated into two groups with different geochemical compositions: (1) shark tooth enameloid with generally lower Cu, Ba, rare earth elements (REEs) and U, and higher Na, Zn and Sr concentrations than (2) dentine, bone, and coprolites. These differences are related to the originally different structures of these fossils.All the fossils revealed very similar shale-normalized REE patterns, with negative Ce anomaly and heavy REE enrichment mimicking the REE pattern of modern oxic-seawater. The results therefore suggest REE uptake by the fossils from an early diagenetic pore fluid that was dominated by seawater. Importantly, the Ce anomaly does not vary among the different types of fossil remains and the values are very similar in a given layer. Moreover, a step-wise shift towards lower Ce/Ce* values from older to younger beds is apparent. Nd isotope analyses across the phosphorite succession yielded minor variation with an average εNd(t) value of −6.2±0.4 (n=12) indicating no major changes in the REE source during the studied interval. The relatively radiogenic values reflect Tethyan connection and/or important contribution of a mafic Nd source from weathering in the region.The age-related trend in the Ce anomaly is interpreted to be due to changes in redox and/or pH conditions of the upwelling seawater on the shallow shelf areas. The driving force of these changes was probably an extended global oceanic circulation, especially enhanced connection and water exchange between the North and South Atlantic.