A 45‐Ma Environmental Magnetic Record of Paleoclimate and Tectonics From Eocene‐Present South China Sea Sediments

Abstract The magnetic fraction in marine sediments serves as an exceptional archive for reconstructing tectonic activities and paleoenvironmental changes in marginal seas, such as the South China Sea (SCS). However, understanding depositional processes over million‐year timescales has been hindered by the scarcity of long, continuous sedimentary sequences. Here, we present the long‐term environmental magnetic records from two long sedimentary sites at the International Ocean Discovery Program (IODP) Sites U1501 and U1505 in the North SCS. Our magnetic measurements, coupled with microscopic observations, reveal consistent substantial variations for both sites in magnetic concentration, grain size, and magnetic mineral composition over the past ∼45 million years. The magnetic carriers in the studied SCS sediments were primarily detrital (titano) magnetite, magnetofossils, and hematite, with a major greigite‐rich layer at ∼4.8 Ma. The enhanced erosion and stable sedimentary environments resulting from tectonic activities likely contributed to the elevated magnetic concentration since ∼16 Ma, while the intensified Northern Hemisphere glaciation drove the magnetic concentration peak at ∼3 Ma. Integrated with geochemical and mineralogical data, magnetic grain size primarily reflects sea‐level changes before ∼16 Ma and the weathering history since ∼16 Ma. The abundance of detrital (titano) magnetite, magnetofossils, and hematite is influenced by sediment budget and sea level, primary production, and weathering intensity with global temperature, respectively. Our study reveals that tectonic activities and paleoclimate conditions jointly exert a substantial influence on the formation of magnetic minerals within the Northern SCS sediments over million‐year timescales, offering new insights into long‐term sedimentary processes occurring in marginal seas.