6533b82cfe1ef96bd128f3c8

RESEARCH PRODUCT

Rare earth element contents of Jurassic fish and reptile teeth and their potential relation to seawater composition (Anglo-Paris Basin, France and England)

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The rare earth element (REE) chemistry of Jurassic shelf seawater from western Europe (Anglo-Paris Basin) was investigated by analyzing the fish and reptile teeth deposited in shallow to deeper water (<200 m) environments. REE patterns in apatites are controlled by the host sediments. Vertebrate teeth sampled from the siliciclastic sediments (calcareous sandstones and shales) show flat shale-normalized REE patterns that reflect the dominant influence of the continental source from which the REE were derived. Carbonate deposits, protected from the clastic sources, contain fish and reptile teeth whose REE patterns reflect more accurately the REE composition of the overlying water column. The REE patterns are similar through the Bajocian to the Oxfordian and are characterized by a depletion in heavy rare earth element (Dy/YbN=1.8–5.0) compared to modern seawater compositions (Dy/YbN=0.8–0.9). The fractionations among HREE in the Jurassic seawater are more efficient than in modern oceans and increases with depth instead of decreasing. These HREE removal processes operated as early as the Paleozoic and disappeared during the Late Cretaceous. The fractionation of HREE relative to LREE is correlated with the increasing depth of the basin as indicated by two independent criteria that are the sedimentary structures and the oxygen-isotope composition of coexisting brachiopods. The Dy/YbN ratio of the marine biogenic phosphates can be used as a proxy of paleo-water depths at the scale of a water mass. Ce anomalies (ΩCe) in biogenic apatite-bearing limestones are variable and systematically negative. Relative to modern surface seawater and Bathonian values (mean ΩCe=−0.63±0.10), weak Ce anomalies during the Callovian (mean ΩCe=−0.21±0.09) suggest the onset of more reducing conditions. This redox change coincided with an increase of seawater temperatures as suggested by the oxygen-isotope compositions measured on the same teeth. We speculated that the Callovian low ΩCe could result from a decrease in the pO2 due to the warming of seawater.