Monsoon differences in South Asia and East Asia during the penultimate deglaciation revealed by speleothem oxygen isotopes

Difference between the South Asian Summer Monsoon (SASM) and East Asian Summer Monsoon (EASM) is observed in modern climate variations, but remains unclear on the orbital timescales. In this study, we reconstruct the monsoon history using three high-resolution (~ 17 years) and precisely dated ( ~ ± 400 years) speleothem δ¹⁸O records from Zhangjia Cave in Sichuan Basin, spanning 126.2-138.8 ka BP. In comparison with most ASM speleothem δ¹⁸O records in penultimate deglaciation, we unravel three differences in structure, amplitude, and trends between the western and eastern ASM, applied to distinguish the SASM from the EASM. First, the SASM in last interglacial seems stronger than that in Holocene, in contrast to the relatively stable EASM. Second, the different amplitudes of δ¹⁸O variations across the Marine Isotope Stages 6-5 transition further distinguish the two monsoons system. Third, the SASM speleothems δ¹⁸O-gradually depleted trend is significantly different with the relatively stable speleothems δ¹⁸O in EASM during Heinrich Stadial 11, indicating the different responses to cross-equatorial insolation gradients between SASM and EASM. These findings suggest that the SASM is more sensitive to cross-equatorial insolation gradients and cross-equatorial flow dynamics, distinguishing it from the EASM.