δ 18O and REE contents of phosphatic brachiopods: a comparison between modern and lower Paleozoic populations
δ 18O values of both carbonate (18.7-25.1‰) and phosphate (15.1-16.7‰) components of the francolite from the fossil brachiopod shells are interpreted as the result of a partial resetting of pristine isotopic compositions. The similarity of hat-shape REE patterns of some lower Ordovician linguloids with those of extant littoral specimens suggests the preservation of a 500 Myr-old record of REE marine chemistry. In agreement with the known paleogeography, the phosphatic brachiopods have been deposited in a coastal environment under shallow and oxygenated marine waters. However, the contents and distribution of REE in some strongly altered brachiopod shells reveal an alteration of the original negative Ce anomalies and both Nd and Sm enrichments that transformed the initial hat-shape patterns of linguloids into strongly convex or bell-shape patterns.