Magnetic Fingerprints for the Paleoenvironmental Evolutions Since the Last Deglaciation: Evidence From the Northwestern South China Sea Sediments.

Rapid global changes since the last deglaciation can be well documented in marginal sea sediments, while isolating individual paleoenvironmental signals is still challenging. Here, we identified magnetic minerals and unmixed their variations in sediments from the northwestern South China Sea to decipher environmental variations since the last deglaciation. The variation in the hematite to goethite ratio indicates a relatively drier mid‐Holocene and a wetter early and late Holocene in the Red River catchment. The position shift and intensity variation of Western Pacific Subtropical High may account for the Holocene precipitation changes. A higher chemical weathering intensity accompanied by a relatively more fine‐grained magnetite input during the last deglaciation suggests decoupling of the Asian Summer Monsoon and chemical weathering in the catchment, which might have been caused by the shelf exposure during the deglaciation. In addition, the relative abundance of biogenic magnetite increased with the sea level and possible deep‐water temperature rise. Therefore, magnetic minerals such as hematite, goethite, detrital and biogenic magnetite are markedly potential fingerprints for continent‐ocean environmental changes. Plain Language Summary: The global climate, which has changed rapidly since the last deglaciation, has been recorded by marginal sea sediments. However, the mixing of environmental factors renders it difficult to isolate individual information. Here, we unmixed the magnetic mineralogical signals of sediments from the northwestern South China Sea since the last deglaciation. We found that a higher hematite to goethite ratio (Hm/Gt) in our core represents higher precipitation in Red River catchment. The Hm/Gt variation indicates a relatively drier mid‐Holocene and a wetter early and late‐Holocene in the Red River catchment, which is consistent with the precipitation records in Southern China but opposite to that of Northern China. The chemical weathering of the Red River catchment was decoupled from the Asian Summer Monsoon, exhibiting a higher weathering extent (more fine‐grained magnetite was generated) during the last deglaciation due to secondary weathering on the continental shelf. Moreover, the relative abundance of magnetofossils in our core was consistent with changes of the sea level and deep‐water environments. Therefore, the isolation of magnetic mineral information from marginal sea sediments can help to reconstruct the continent‐ocean environmental changes. Key Points: A drier mid‐Holocene and wetter late‐Holocene occurred in Red River catchment controlled by the shift of Western Pacific Subtropical HighA decoupling between chemical weathering in Red River catchment and summer monsoon was caused by shelf exposure during the deglaciationThe relative content of magnetofossils in northwestern South China Sea is controlled by the sea level and deep‐water environment changes [ABSTRACT FROM AUTHOR]