Lithogenic particle inputs play an important role in regulating trace metal supply and biogeochemical processes in oligotrophic marginal seas such as the South China Sea (SCS), yet their magnitude and spatial variability remain poorly constrained. Here, we combine dissolved long-lived thorium isotopes (²³⁰Th and ²³²Th) with aerosol Fe solubility measurements to quantify lithogenic particle inputs and associated dissolved iron (dFe) supply in the SCS. Dissolved ²³⁰Th_xs generally increased linearly with depth above 1000 m, consistent with its reversible scavenging behavior. Deviations from this pattern were also observed at stations near the shelf break, river plume and Luzon Strait, suggesting the effects of enhanced scavenging, riverine inputs, and lateral transport. Th-derived lithogenic particle fluxes ranged from 3.07 to 15.4 g m^-2 yr^-1 in summer and 3.84 to 18.5 g m^-2 yr^-1 in winter. Combining Th-derived lithogenic fluxes with aerosol Fe solubility, lithogenic dFe fluxes were estimated as 27.1 ± 13.9 mg m^-2 yr^-1 in summer and 31.6 ± 12.5 mg m^-2 yr^-1 in winter. Higher dFe deposition fluxes were generally observed in the southern and western SCS, consistent with the lithogenic input from the surrounding southeast Asian peninsula and island masses as indicated by the air mass back trajectory analysis. Positive relationships between lithogenic dFe fluxes and chlorophyll a and net primary productivity further imply a potential contribution of lithogenic Fe supply to biological productivity. However, the weaker productivity response in the southern SCS, despite its comparable or higher Fe supply, suggests that Fe stimulation is modulated by nutrient stoichiometry, particularly phosphate availability. Our results demonstrate that long-lived Th isotopes provide a useful constraint on lithogenic inputs and their biogeochemical consequences in complex marginal seas.

 

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