An astronomically validated U–Pb reference material for dating Quaternary speleothems

Abstract

Well-characterized reference materials (RMs) are essential for advancing laser ablation (LA) U–Pb carbonate geochronology, yet Quaternary carbonate RMs with stringent accuracy constraints are still lacking. Here, we present a comprehensive inter-laboratory study of the speleothem sample SB19 (U: 3.5 ± 1.8 μg g⁻¹) employing both LA and isotope dilution (ID) methods. LA-MC-ICPMS analyses of the stratigraphically distinct subsamples demonstrate excellent inter- and intra-laboratory reproducibility. High-precision measured δ²³⁴U data were also obtained for SB19, allowing accurate corrections for the initial disequilibrium. The resultant chronology reveals a robust correlation between the SB19 δ¹⁸O record and theoretical solar insolation curves on the precession scale, providing a verification of its age accuracy (±6 kyr, 2σ). Notably, (²⁰⁷Pb/²⁰⁶Pb)₀ ratios are quite similar among subsamples (RSD < 0.5%), which can be used as a monitoring reference for studying the initial lead isotope composition and is applicable to related research such as paleoclimate reconstruction. Syntheses of ID (n = 15) and LA datasets of the SB19 yield recommended values: age = 1.091 ± 0.006 Ma and (²⁰⁷Pb/²⁰⁶Pb)₀ = 0.809 ± 0.004. SB19 meets the stringent requirements of LA carbonate U–Pb dating RMs (∼0.5% relative uncertainty in age and initial Pb isotopic ratios at 2σ), particularly for dating Quaternary speleothem samples. This integrated work establishes a new paradigm for carbonate RM validation by combining radiometric dating, geochemical proxies, and insolation tuning.