Interhemispheric monsoon response to bipolar forcing during Heinrich Stadial 4

Heinrich Stadial 4 (HS4) the most prominent iceberg collapse event of the Last Glacial is marked by significant changes in both high- and low-latitude records, making it a key example for studying millennial-scale events. Based on an improved chronological, we present high-resolution, annually laminated speleothem record from southeastern China, spanning the HS4, which allows us to quantify the temporal relationship of Heinrich Event 4 (HE4) with unprecedented age precision across the Asian Monsoon region, South American Monsoon region, and polar regions. Our data suggests that during the onset of HS4, the East Asian Summer Monsoon (EASM) and northern high-latitude climate experienced two rapid stages of change, tightly coupled by the rapid southward migration of the Intertropical Convergence Zone over the Pacific. In contrast, controlled by low temperatures in Antarctica, the strengthened cross-equatorial flow led to a gradual weakening of the Indian Summer Monsoon (ISM). Our records precisely date the onset of HE4 at 39.70 ± 0.06 ka BP, significantly narrowing the uncertainty in Greenland ice cores (GISP2 and GRIP) for this period. Following HE4 onset, northern high-latitude cooling and EASM weakening occurred, followed by a marked strengthening of the South American Summer Monsoon and a rapid increase in atmospheric methane about 100 years later. Atmospheric CO₂ rises and Antarctic warming after HE4 caused a weakening of the cross-equatorial flow, eventually triggering a significant decline in ISM.