Refining the Capitanian Sr/Sr Minimum: Conodont-Based Seawater Records and Implications for Mantle-Continental Coupling During Pangea Assembly

The pronounced minimum in Phanerozoic seawater ⁸⁷Sr/⁸⁶Sr during the Capitanian (Middle Permian) represents a fundamental perturbation to the global strontium cycle, yet its precise stratigraphic extent and underlying driving mechanisms remain debated. Here we present new 87Sr/86Sr data derived from conodont apatite and bulk carbonates at the Fengshan section, South China, the recently ratified Standard Auxiliary Boundary Stratotype for the base of the Lopingian Series. Conodonts, owing to their high resistance to diagenesis and superior stratigraphic resolution, provide minimally altered 87Sr/86Sr signatures across the Guadalupian-Lopingian boundary (GLB). The lowest 87Sr/86Sr value (0.70683) occurs just above the GLB, suggesting that the Capitanian minimum may extend into the earliest Wuchiapingian. This value is statistically indistinguishable from those reported in well-preserved brachiopod shells from South China and South America, yet appears slightly younger stratigraphically. Furthermore, our results show that 87Sr/86Sr values from bulk carbonates are consistently more radiogenic than those from co-occurring conodonts, despite the application of rigorous sequential leaching protocols. This discrepancy underscores the limitations of bulk carbonates in reconstructing seawater Sr isotope compositions due to potential post-depositional alteration. Comparison with published brachiopod and conodont datasets indicates that the Capitanian minimum is best defined by fossil-derived archives and spans the entire Capitanian Stage, characterized by persistently low seawater ⁸⁷Sr/⁸⁶Sr values around 0.70683. Steady-state Sr isotope mass balance model results demonstrate that maintaining a seawater ⁸⁷Sr/⁸⁶Sr ratio of ~0.70683 requires a mantle-derived Sr flux approximately 1.35 times greater than the riverine Sr input, indicating a mantle-dominated Sr budget during the Capitanian. A geologically plausible solution for this scenario involves a coupled reduction in riverine Sr delivery and enhancement of mantle-derived Sr input. The Capitanian minimum and its constraints on the Sr fluxes of continent and mantle could probably result from the mature assembly of Pangea, marked by reduced continental weathering under arid climates and shortened coastlines, together with intensified mantle heat flux and hydrothermal exchange.