Apatite for destruction: Isotopic and geochemical analyses of bioapatites and sediments from the Upper Devonian Escuminac Formation (Miguasha, Québec)

The paleoenvironmental context and diagenetic history of the middle Frasnian Escuminac Formation were investigated through geochemical (rare earth elements) and isotopic (Rb–Sr, Nd–Sm) analyses of fossil fish material and sediments. Samples of sediments and bioapatite were collected from the base to the top of the Escuminac Formation.Strontium isotope ratios (87Sr/86Sr) for bioapatites vary from 0.70804 to 0.70845 and overlap with Frasnian seawater Sr isotope compositions, though most of the biogenic apatites analyzed show a variable drift from marine values toward a more radiogenic continental signature. This trend is possibly the result of post-mortem Sr exchange between fossils and a fluid isotopically distinct from the waters in which they developed.All bioapatites are heavily enriched in rare earth elements (REE) and display a shale-normalized pattern that shows heavy-REE (HREE) depletion and a slight middle-REE (MREE) enrichment. The εNd (t) values of the sediments range from − 4.8 to − 6.4 and are consistent with a sedimentary provenance derived from an Appalachian source for the Escuminac Formation. In contrast, more radiogenic εNd (t) values (ranging from − 2.6 to − 4.6) were obtained for the bioapatites. The discrepancy between sediments and fossil material εNd (t) implies the presence of a second, diagenetic, Nd reservoir for the Escuminac bioapatites. We propose that this reservoir reflects, in part, seawater. The similarity between the Nd isotope compositions of the Escuminac fossils and Late Devonian conodonts from Poland argues for a close connection between waters of the Devonian Rheic Ocean and the Escuminac Formation collector basin.

► Paleoenvironment and diagenesis of the Upper Devonian Escuminac Formation
► Geochemical (REE) and isotopic (Rb–Sr, Nd–Sm) study of bioapatites and sediments
► 87Sr/86Sr ratios of fish material supports a brackish to marine environment.
► Bioapatite diagenetic REE enrichment partially derived from the enclosing sediments
► Bioapatite εNd (T) implies a Rheic Ocean component within the Escuminac Formation.