REY Enrichment Mechanism in Bioapatite and Its Implications for Sr Isotope Chronometry During the Paleocene‐Eocene Thermal Maximum

Fish teeth are widely used as paleoceanographic archives for seawater rare earth elements and yttrium (REY) signatures and Sr isotope chronostratigraphy. However, the reliability of this proxy during extreme climatic perturbations, when both seawater chemistry and diagenetic conditions shift, remains poorly constrained. To elucidate these mechanisms, we conducted multiscale analyses on Paleocene‐Eocene Thermal Maximum (PETM) fossil fish teeth from the eastern equatorial Pacific, a key stratigraphic interval characterized by sharp environmental perturbations. Our integrated petrographic‐geochemical approach, incorporating in situ micro‐analyses (Fourier‐transform infrared spectroscopy, Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry) shows that primary productivity promoted fish teeth abundance and increased REY enrichment in bulk seafloor sediments by providing phosphorus and REY ions. However, enhanced productivity and diagenetic conditions that elevated bottom nutrient fluxes did not result in REY enrichment in the bioapatite. We find that reaction kinetics‐based lattice substitution dominated REY incorporation into the bioapatite, which was significantly enhanced by elevated paleotemperatures (ΔT ≈ 3–5°C) during the PETM. On the other hand, the Sr isotopic signature of the fossil teeth was decoupled from global seawater Sr trends during the PETM because of diagenesis, which limits the direct application of Sr chronometry for fish teeth deposited during extreme climatic perturbations. Our results highlight that the Sr isotope chronometry of bioapatite can be compromised during hyperthermal events, and that temperature‐driven substitution kinetics must be considered when interpreting REY enrichment in fossil fish teeth as providing a robust paleoceanographic signals.