Millennial-scale hydroclimate variability of the Asian monsoon during MIS 9 from stalagmite records

The evolution and millennial-scale variability of the Asian Summer Monsoon (ASM) during Marine Isotope Stage 9 (MIS 9) remain insufficiently constrained due to the limited availability of high-resolution archives. Here, we present a reconstruction of ASM dynamics between 340 and 285 ka BP, based on high-resolution (∼68-year) stalagmite δ¹⁸O records from Jinfo Cave, southwestern China. Results demonstrate that orbital-scale ASM variability was primarily modulated by Northern Hemisphere summer insolation, allowing subdivision into five substages: MIS 9e, MIS 9d, MIS 9c, MIS 9b, and MIS 9a. Synchronous comparisons with other speleothem datasets corroborate a spatial pattern characterized by pronounced variability in southwestern China and relative stability in the southeast. Within MIS 9c, three distinct millennial-scale weak monsoon events were identified, indicating substantial hydroclimate instability during interglacial conditions. Spectral analyses reveal dominant ∼2–4 kyr periodicities, analogous to Dansgaard–Oeschger (D-O) events of the last glacial period, likely linked to Atlantic Meridional Overturning Circulation (AMOC) instability under enhanced insolation forcing. Comparisons with MIS 5–4 highlight convergent responses in insolation-driven forcing, critical ice-volume thresholds, and monsoon evolution, suggesting an intrinsic propensity of the climate system toward quasi-cyclic oscillations.