Quality Control Method and Application of High-Precision Dual-Inlet Analysis for Trace Speleothem Carbonate Carbon and Oxygen Isotopes

Speleothems have become the fourth pillar of paleoclimate research due to their advantages of high resolution, precise chronology, and widespread terrestrial distribution. The 690,000-year oxygen isotope composite sequence established by Chinese scientists has further served as a global benchmark on the orbital and millennial time-scale studies. However, the climatic interpretation of speleothem oxygen isotopes remains controversial, especially in the East Asian monsoon region, where accurate measurement of speleothem carbon and oxygen isotopes forms a crucial basis for paleoclimate reconstruction. Currently, high-precision dual-inlet isotope ratio mass spectrometry faces several major challenges, including insufficient analytical stability for trace-mass samples and the lack of unified inter-laboratory standards for data comparability. In addition, systematic quality control protocols for dual-inlet techniques remain underdeveloped, constraining the reliability and international consistency of high-resolution speleothem isotope records. To address these issues, this study establishes a comprehensive quality-control framework for dual-inlet analysis of trace carbonate δ¹³C and δ¹⁸O. The performance and data quality of a new-generation dual-inlet mass spectrometer (Isoprime PrecisION) were evaluated from multiple dimensions, including reference gas stability and repeatability of standard samples. Independent point-by-point reproducibility tests conducted on stalagmite DDH-B15 from Didonghe Cave, as well as cross-laboratory consistency tests using identical sample batches, confirm the robustness of the newly developed laboratory methodology. Furthermore, this paper explores the significant intensification event of the Asian monsoon during 20–17 ka BP (ka = thousand years, BP = before 1950) in the Last Glacial Period. We propose that the driving mechanism may involve glacial coastline retreat in Eastern Asia and El Niño-Southern Oscillation (ENSO) mode shifts triggered by western Pacific SST anomalies. Our results demonstrate that orbital timescale oxygen isotopes in central China represent an average state of moisture isotopes from eastern and western sources, highlighting the important influence of moisture provenance on speleothem δ¹⁸O. These findings provide valuable insights into the interpretation of speleothem δ¹⁸O records.