Contrasting Patterns in Abrupt Asian Summer Monsoon Changes in the Last Glacial Period and the Holocene

Detailed Asian summer monsoon (ASM) variability across Heinrich stadials (HSs) 5 to 2 was reconstructed from four stalagmite oxygen isotopic (δ¹⁸O) records in central and southern China. For the last glacial period, these speleothem records, combined with previous cave records, reveal a rapid ASM decline at the onset of each HS. During this time, ASM intensity decreases immediately to the weakest level within approximately 50 years, which is followed by a gradual intensification in the mid-HS. Typically, this process of ASM weakening is synchronous with peak ice-rafted debris deposition and large freshwater outbursts into the North Atlantic, implying a tight link between the two. During the Holocene, however, a relatively gradual ASM decrease occurred at the start of the Bond events. Comparatively, the ASM decrease during the Bond events is generally accomplished within 110 years, and the weakest ASM occurs near the end. This difference implicates a further southward displacement of the Intertropical Convergence Zone and a stronger impact from the Atlantic meridional overturning circulation on the ASM in the early HS. Moreover, contrasting expressions of the ASM during HSs and Bond events suggest that a fixed phase relationship during bipolar climate changes cannot be expected.