Rare earth elements of shallow-water articulated brachiopods: A bathymetric sensor

Abstract

Variations in environmental physicochemical parameters result in corresponding changes in seawater rare earth element (REE) distributions. For an archive to be utilized in paleoceanographic and paleoenvironmental reconstructions, it should record these variations. Therefore, an archive's proxy potentiality should be assessed analogous to modern oceanic settings.Articulated brachiopods of several shallow water localities (<500m) of different water masses were evaluated for their REE contents. The affects of oceanographic factors such as depth, salinity, and temperature were investigated for REE uptake by the shells. Uptake of REE by dorsal and ventral valves of articulated shells are unequivocal, whereas shells of Thecideidines, the umbo region and primary layer of the Terebratulids are anomalous, and should be avoided. Water temperature and salinity appear to have minor to moderate impacts on the partition coefficients of REEs between seawater and brachiopod calcite.The mean REESN pattern of shallow-water brachiopods is significantly different from that of their deep-water counterparts, with lower enrichments in LREEs and MREEs, and depletion in HREEs. Depth exercises a major control on the LREE and MREE fractionation with a milder one on HREE fractionation. The Ce/Ce* anomaly is not affected by either of these parameters in shallow-water settings, but in deep settings the depth effect dominates. Due to the importance of depth on REE fractionation in the shells from shallow settings, their LREESN to HREESN ratios (L:H) were divided into six discrete intervals for the bathymetric sensor. The bathymetric sensor was applied to some fossils and whole rock in order to estimate their paleodepths. The evaluation was successful in placing the Permian Gyanyima, the Pennsylvanian Naco, Boggy and the Silurian Chicotte, Becscie and Jupiter Formations in shallow depths (<15–50m), which is consistent with their paleoenvironmental settings.