Global climate–linked reorganization of upper-ocean structure in the northern South China Sea during 24–20 Ma

The Oligocene–Miocene transition was characterized by major reorganizations in global climate and ocean circulation, yet their expression in low-latitude upper-ocean structure remains incompletely resolved. Here we present paired stable isotope and Mg/Ca-derived temperature records from planktic foraminiferal surface-dwelling Globoturborotalita brazieri and thermocline-dwelling Paragloborotalia siakensis at IODP Site U1501 in the northern South China Sea, spanning ~24–20 Ma. The records document pronounced variability in upper-ocean hydrography across the Mi-1 glaciation, including surface cooling, a transient reduction followed by an increase in surface–subsurface δ18O gradients, reflecting a reduction in vertical separation between surface and subsurface waters, and a subsequent recovery reflecting a temporary weakening and subsequent strengthening of upper-ocean thermal stratification. Additionally, a sustained increase in surface–subsurface δ13C gradients indicates progressively enhanced vertical nutrient differentiation (reduced nutrient resupply to the surface). These patterns indicate intervals during which thermal and nutrient stratification evolved either coherently or independently. Paired δ18O–Mg/Ca reconstructions (δ18Osw-ice) further indicate spatially heterogeneous surface hydrological responses during post–Mi-1 warming, with enhanced freshwater input in the northern South China Sea that contrasts with contemporaneous records from interior continental regions. Comparison with coeval records from the Arabian Sea and the broader Indo-Pacific suggests that the observed upper-ocean changes were closely associated with large-scale climatic transitions across the Oligocene–Miocene boundary, while regional tectonic and paleogeographic influences likely played a more limited role. In combination, these observations provide new constraints on low-latitude thermocline–nutricline dynamics and their response to early Miocene climate variability.