Indian summer monsoon variability across the last 9000 years: New evidence from stalagmites of southwestern China

The Indian Summer Monsoon (ISM) contains both the Arabian Sea branch (ISM_AS) and the Bay of Bengal branch (ISM_BOB), which may act differently during various timescales. Accurately dated, high-resolution climate records from different regions of the ISM domain are needed to fully understand the spatiotemporal variability and dynamic mechanisms of the ISM. Based on high-precision UTh dating chronologies and oxygen isotope (δ¹⁸O) records of three stalagmites from Shenqi Cave (SQ) from southwestern China, we reconstructed a high-resolution history of the ISM_BOB over the past 9000 years. Our SQ δ¹⁸O results demonstrate that the orbital-scale weakening of the Holocene ISM_BOB was predominantly controlled by changes in Northern Hemisphere summer insolation (NHSI), consistent with the ISM_AS and EASM record in the broad Asian summer monsoon region. However, in contrast to the strengthening of the ISM_AS and EASM during the last 2000 years, our δ¹⁸O record indicates a long-tern weakening trend of the ISM_BOB during this period. The ISM_BOB was at its strongest during 9.0–7.5 ka BP, and a rapid weakening occurred between 7.2 and 5.0 ka BP. We propose that the abrupt ISM_BOB weakening during 7.2–5.0 ka BP may have been linked to high-latitude Northern Hemisphere cooling, which enhanced the interhemispheric temperature gradient and drove a southward migration of the Intertropical Convergence Zone (ITCZ). During the 8.2 ka event, the SQ δ¹⁸O shows clear positive shifts, indicating transient ISM_BOB weakening, which rapidly responded to the North Atlantic freshwater forcing. The ISM_BOB was generally weak during the 4.2 ka event, with two decadal-scale monsoon strengthening intervals.