REE plus Y uptake in bioapatite revisited: Facies-controlled variability in coeval conodonts

REE + Y concentrations in bioapatite have been repeatedly shown to be ambiguous carriers of information for paleo-seawater reconstructions. The uptake of REE + Y begins rapidly after deposition, and bioapatite can, therefore, potentially record the pore water conditions during early diagenesis. In this study, we examined the REE + Y composition of conodonts belonging to a single species from three coeval facies across the Ordovician Baltic Paleobasin carbonate shelf. We show that the REE + Y concentrations, as well as Ce/Ce*, bell-shape index (BSI), Y/Ho and LaN/SmN ratios, vary systematically in conodonts from facies with different host rock compositions. Dissimilarly, REE + Y patterns and ratios of conodont dental elements from the same facies are uniform, indicating that the local depositional environment imposes principal control on the REE + Y uptake by bioapatite. Furthermore, our results show that even though REE + Y concentrations in different histological tissues of the same conodont vary significantly, no REE-specific fractionation occurs, resulting in similar REE + Y ratios in all tissue types. Our results illustrate that conodonts can record modified seawater-like characteristics of REE compositions if the input of Fe-Mn oxyhydroxides and particulate organic carbon (POC) is low. In contrast, under elevated POC and Fe-Mn oxyhydroxide loading, the conodonts show increasingly positive Ce anomalies and MREE enrichment. Collectively, these results suggest that, although primary seawater signatures are not captured in bioapatite, the conodont REE + Y patterns are controlled by local depositional environments and can record early diagenetic pore water conditions.