Oxygen Variations in the Northern South China Sea: Evidence From Environmental Magnetism of Sediments

The decline of dissolved oxygen (DO) in marginal sea ecosystems has been exacerbated by rising greenhouse gas concentrations and anthropogenic nutrient inputs. The northern South China Sea (SCS) has recorded multiple reductions in DO, making it an ideal region for studying the response mechanisms of magnetic minerals to DO under varying climatic conditions. Here, we reconstruct the variations in magnetic mineral characteristics, marine DO levels (V/Cr), productivity (Ba/Al and CaCO 3 ) and a nutrient proxy (P/Al) based on the northern SCS sediments over the past 16,000 years. The Younger Dryas (YD) interval was characterized by marked reductions in magnetic mineral content and DO concentrations, concurrent with a pronounced increase in productivity. Our results reveal that variations in magnetic properties were mainly driven by post‐depositional magnetite dissolution, rather than changes in sediment provenance. During the YD and the late Holocene, intensified East Asian Winter Monsoon‐driven oceanic mixing enhanced productivity in the northern SCS, reduced DO levels, and triggered near‐total magnetite dissolution. Under deoxygenated conditions, dissolved iron is converted into bioavailable forms, amplifying productivity through a positive nutrient feedback mechanism. Our results highlight the mechanisms of DO variations under varying climatic conditions in the northern SCS and the response of magnetic minerals to them.