Indian summer monsoon variability from 184 to 25 ka inferred from sedimentary magnetic properties in Heqing Basin, Southwest China

The Indian summer monsoon (ISM) is a major component of the Asian monsoon system, and exerts a critical influence on regional climate variability. However, continuous, high-resolution paleoclimate records spanning a full glacial-interglacial cycle remain limited, constraining our understanding of the ISM dynamics across different timescales. In this study, we reconstruct ISM variability from 184 - 25 ka using geochemical and environmental magnetic proxies from a lacustrine sediment core in the Heqing Basin, Southwest China. Rock magnetic results indicate that higher concentration-dependent magnetic parameters (χ, ARM, SIRM) occur during weaker monsoon (drier) phases, whereas lower values reflect stronger monsoon (wetter) phases. The proxy records reveal a pronounced dry-wet cycle throughout 184 - 25 ka: Phase I (184 - 131 ka) featured relatively weak ISM intensity and drier conditions; Phase II (131 - 69 ka) saw an overall increase in ISM intensity accompanied by higher rainfall; Phase III (69 - 57 ka) reverted to arid conditions; Phase IV (57 - 25 ka) exhibited a generally wetter climate, though punctuated by frequent major drought events. On glacial-interglacial timescales, ISM variability was primarily paced by Northern Hemisphere summer insolation (NHSI). On millennial timescales, the ISM weakened during ~ 57 - 25 ka (Phase IV, MIS 3). Weak ISM, as indicated by proxies in the PT2 core, showed consistent variations with Atlantic meridional overturning circulation (AMOC) weakening, as indicated by ²³¹Pa/²³⁰Th, and with low-latitude monsoon circulation. This pattern suggests that the region was jointly controlled by high-latitude boundary conditions and tropical ocean-atmosphere coupling processes. Therefore, variations in the ISM across different timescales are driven by multiple factors rather than solely by solar radiation.