Biogenic Magnetite Indicating the Evolution of Bottom Water Oxygenation in the Northern South China Sea Since the Last Glacial

This study investigates variations in bottom-water oxygen concentration (BWO) in the northern South China Sea (NSCS) since the Last Glacial Maximum (LGM) using biogenic magnetite as a novel proxy. Through analysis of sediment cores via first-order reversal curve (FORC) diagrams and low-/high-temperature magnetic measurements, we identify four distinct BWO phases: (1) a prolonged low-oxygen period during the LGM (~28-18 ka BP), (2) rapid oxygenation during deglaciation (18-11 ka BP), (3) peak levels in the early-mid Holocene (11-5.8 ka BP), and (4) gradual late Holocene decline. The BWO record shows strong coherence with equatorial Pacific and Indian Ocean patterns but differs from Southern Ocean millennial-scale variability. Our findings demonstrate that LGM hypoxia resulted from enhanced ventilation of western Pacific intermediate/deep waters (IDW) coupled with increased productivity, while deglacial reoxygenation reflects intrusion of oxygenated IDW through the Luzon Strait. Crucially, the reconstructed BWO changes precede atmospheric CO₂ fluctuations, supporting the hypothesis that deep-ocean carbon release contributed to deglacial CO₂ rise. This work establishes biogenic magnetite as a robust BWO proxy in marginal seas and provides new insights into ocean-atmosphere carbon exchange during glacial-interglacial transitions.