Moisture Source Dynamics during the Penultimate Glacial Inception (MIS 7-6) in Northern Vietnam Stalagmite

Marine Isotope Stage 7 (~243-191 ka BP) represents a complex interglacial period characterised by multiple climate substages and gradual orbital forcing changes, culminating in one of the most rapid glacial inceptions of the Pleistocene at the MIS 7-6 transition (~201-187 ka BP). Despite its importance for understanding monsoon responses to glacial inceptions under differing orbital configurations, monsoon dynamics during this interval remain poorly understood due to a paucity of high-resolution paleoclimate records from Southeast Asia. We present a multiproxy speleothem record from Boi Cave in northern Vietnam spanning 201-187 ka BP. In contrast to characteristic glacial-interglacial δ¹⁸O shifts observed at other Asian monsoon sites, our record exhibits minimal amplitude change across the MIS 7-6 transition. The δ¹⁸O signal instead preserves sustained high-frequency variability throughout the interval. Trace element geochemistry, however, documents clear local hydroclimate changes, with peak wet conditions at 197 ka and rapid monsoon reorganisation at ~191.4 ka. These hydroclimate changes align with glacial inception timing in other Asian speleothem records and correlate with North Atlantic and Mediterranean records, suggesting hemispheric-scale reorganisation of atmospheric circulation. Modern climate analysis reveals this region receives rainfall from both the Southwest Summer Monsoon and the Northeast Winter Monsoon systems during the soil recharge period, with the balance between moisture sources varying interannually.We discuss how stable isotope and trace element proxies record different aspects of monsoon dynamics at this site. While trace elements document local infiltration in response to monsoonal rainfall, δ¹⁸O reflects the balance between Indian Ocean and Pacific moisture sources. This distinction arises from Boi Cave's unique geographical position, where both monsoon systems contribute to annual rainfall. This high-resolution reconstruction fills a critical spatiotemporal gap in understanding Southeast Asian monsoon dynamics during the penultimate glacial inception.