Conodont Oxygen Isotope Paleothermometry: Diagenetic Effects and Implications for Cambrian–triassic Paleotemperature Reconstruction

Conodont δ18O is a widely used proxy for Cambrian–Triassic paleotemperatures, yet intra-element heterogeneity between albid crown and basal body (up to 1–2‰) raises questions about the fidelity of absolute temperature estimates. The scientific challenge is distinguishing primary ontogenetic signals from secondary diagenetic overprinting and identifying which tissue best tracing original seawater δ18O. We applied high-resolution SIMS δ18O mapping, micro-Laser Raman spectroscopy (crystallinity), and in situ LA-MC-ICP-MS 87Sr/86Sr analysis to Early Ordovician and Triassic conodonts from South China. Results contradict the traditional assumption that the dense albid crown is most robust. Early diagenesis preferentially alters crown δ18O while increasing crystallinity, causing the color to change from the original amber to albid, whereas the more porous, cryptocrystalline basal body preserves a primary marine δ18O signature. Conversely, the crown better retains original 87Sr/86Sr and REE compositions against late-stage fluids. We propose a revised analytical protocol favoring basal body δ18O for paleothermometry, or alternatively other low-crystallinity parts that more closely resemble the original amber color，and provide a correction factor of +0.8‰ to +1.3‰ (3–5°C) for integrating legacy albid crown-derived data. This framework resolves tissue-specific diagenetic pathways and strengthens conodont-based Cambrian-Triassic climate reconstructions.