Abstract: Y. Li, C. Huang, J. Lan, D. B. Kemp, J. G. Ogg, A. Sinha, L. Chen, X. Wang, H. Tang, X. Lin, K. H. M. Abdel-Raheem, R. Zhang, H. Cheng, R. L. Edwards, L. Tan (2025)

The Indian Summer Monsoon (ISM) contains both the Arabian Sea branch (ISMAS) and the Bay of Bengal branch (ISMBOB), 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 U-Th dating chronologies and oxygen isotope (delta 18O) records of three stalagmites from Shenqi Cave (SQ) from southwestern China, we reconstructed a high-resolution history of the ISMBOB over the past 9000 years. Our SQ delta 18O results demonstrate that the orbital-scale weakening of the Holocene ISMBOB was predominantly controlled by changes in Northern Hemisphere summer insolation (NHSI), consistent with the ISMAS and EASM record in the broad Asian summer monsoon region. However, in contrast to the strengthening of the ISMAS and EASM during the last 2000 years, our delta 18O record indicates a longtern weakening trend of the ISMBOB during this period. The ISMBOB 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 ISMBOB 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 delta 18O shows clear positive shifts, indicating transient ISMBOB weakening, which rapidly responded to the North Atlantic freshwater forcing. The ISMBOB was generally weak during the 4.2 ka event, with two decadal-scale monsoon strengthening intervals.

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