Biogenic Apatite Sr/ca Ratios: A Novel Dual-Proxy for Reconstructing Conodont Growth Dynamics, Paleotemperature and Phanerozoic Seawater Chemistry

Biogenic apatite provides one of the most continuous geochemical archives for reconstructing Phanerozoic ocean conditions. However, the application of many elemental proxies in this material remains poorly constrained. Here, we systematically evaluate the fidelity of the Sr-Ca system in conodont and ichthyolith specimens from a broad stratigraphic range. We demonstrate that (Sr/Ca)PO₄ is largely resistant to diagenetic overprinting. The analysis reveals that Sr incorporation is governed by a suite of parameters including biological, crystallographic and kinetic factors rather than dietary variations. Reproducible quadratic relationship between (Sr/Ca)PO₄ and specimen mass indicates a strong ontogenetic control on Sr incorporation. This growth-related signature is conserved across conodont taxa separated by ~300 Myr, establishing a taxonomic and size-based framework essential for interpreting stratigraphic Sr/Ca trends. Crucially, a consistent negative correlation between (Sr/Ca)PO₄ and the established paleothermometer δ¹⁸OPO₄confirms the high potential of (Sr/Ca)PO₄ as an independent paleotemperature proxy. Systematic offsets between shallow- and deep-dwelling taxa further confirm the thermal sensitivity of (Sr/Ca)PO₄. By coupling (Sr/Ca)PO₄with co-measured δ¹⁸OPO₄, we develop a dual-proxy framework that provides a mathematical bridge for the quantitative reconstruction of past seawater (Sr/Ca)sw from size-standardised, taxonomically constrained bioapatite. These findings establish (Sr/Ca)PO₄ as a key component of a dual-proxy approach to deep-time climate–ocean evolution.