Sea-level-driven strait closures enhance South China Sea Intermediate Water ventilation with impacts on North Pacific

Glacial-interglacial sea-level fluctuations affect meridional overturning dynamics in the marginal South China Sea, although their influence on intermediate water exchange with the North Pacific remains unclear. Here, we reconstruct the evolution of South China Sea Intermediate Water since about 31,000 years ago using radiocarbon dating, grain size, redox-sensitive trace elements, and ichnological data from core S17 at 751 m depth. The results reveal more intense flows, better ventilation, and higher oxygenation during the glacial interval. These conditions stemmed from the closure of shallow seaways (Taiwan and Karimata, <50 m sills; Balabac, ~100 m sill) during lowered sea level, which impeded outflow and enhanced downwelling of the South China Sea Surface Water. Comparisons with North Pacific Intermediate Water proxies indicate warmer, saltier South China Sea Intermediate Water outflows modified mixed-North Pacific Intermediate Water during Heinrich Event 1 and the Younger Dryas, potentially weakening deep stratification and facilitating carbon dioxide outgassing. Closures of shallow straits impede South China Sea surface water outflows and amplify intermediate water ventilation via downwelling and mixing with deep water, according to combined analysis of radiocarbon dating, grain size, trace elements, and ichnological data.