A high-resolution stalagmite <italic>δ</italic>¹⁸O record from Sigangli Cave (Southwest China) reveals Early-Middle Holocene variability of the Indian Summer Monsoon

The Indian Summer Monsoon (ISM) is a major component of the Asian monsoon system and exerts a strong influence on hydroclimatic conditions in Southwestern China. However, its evolution during the Early to Middle Holocene, especially at the northeastern margin of the ISM domain, remains insufficiently understood. Southwestern Yunnan is located near the transition zone between the Indian Summer Monsoon and the East Asian Summer Monsoon (EASM), and is therefore highly sensitive to variations in monsoonal moisture transport and large-scale atmospheric circulation. In this study, we present a high-resolution speleothem oxygen-isotope record from Sigangli Cave to characterize regional ISM evolution history during the Early-Mid Holocene. The Sigangli Cave (23° 32′ N, 99° 33′ E; 1200 m a.s.l.) is located in Cangyuan County, southwestern Yunnan, China. The stalagmite SGL1220, collected from a branch of the cave, is approximately 1908 mm in height and 60 mm in diameter. Based on 15 230Th dates and 1038 oxygen-isotope analyses, we establish a δ18O sequence covering 8.9~5.4 ka B.P. with an average temporal resolution of about 3.7 years. Its variabilities are interpreted as the variation of regional ISM precipitation, with a more positive δ18O value indicates less ISM precipitation and vice versa.The SGL1220 record shows that ISM precipitation in the study area was relatively strong and stable during the Early Holocene (8.9~7.7 ka B.P.), followed by a significantly long-term decreasing trend, indicating a progressive weakening of monsoonal precipitation. This long-term trend is broadly consistent with the decline in Northern Hemisphere summer insolation and the southward migration of the Intertropical Convergence Zone (ITCZ). Comparison with multiple stalagmite records from the EASM region shows that the onset of this pronounced weakening in SGL1220, at ca. 7.7 ka B.P., occurred several centuries earlier than that recorded in the EASM region (7.4~6.9 ka B.P.). This offset suggests asynchronously evolution at the sub-orbital timescale among the Asian monsoon subsystems during the Early-Middle Holocene.Superimposed on this long-term trend, the SGL1220 δ18O record reveals three prominent centennial-scale weak monsoon events at 8.3~8.0 ka B.P., 6.7~6.5 ka B.P., and 6.25~5.8 ka B. P, respectively. The 8.2 ka B.P. event is characterized by a broadly synchronous weakening across the Asian monsoon region and is most likely related to North Atlantic freshwater forcing, which weakened the Atlantic Meridional Overturning Circulation (AMOC) and induced a southward displacement of the ITCZ. In contrast, during the Middle Holocene "6.3 ka B.P." event, the SGL1220 δ18O record displays an anti-phased relationship with stalagmite δ18O records from the core ISM region, such as Oman and India. This difference suggests that the mechanisms driving abrupt centennial-scale climate events during the Early and Middle Holocene maybe differed substantially. Indeed, the SGL1220 variations during the Middle Holocene are negatively correlated with total solar irradiance (TSI) and sunspot number (SN), indicating that centennial-scale abrupt climate events in the study area were closely linked to solar activity. Given that higher δ18O values in SGL1220 reflect reduced monsoonal precipitation, the results further imply a negative coupling between regional monsoonal precipitation and solar activity, namely that enhanced (reduced) solar activity corresponded to decreased (increased) monsoonal precipitation in the study area. Overall, the SGL1220 record provides new evidence for asynchronous evolution between the ISM and EASM during the Holocene, and highlights the spatial complexity of centennial-scale climate events superimposed on long-term monsoon evolution.