The sinking of particulate matter largely controls the resident timescales of carbon sequestered in the ocean interior and the level of atmospheric CO₂. Owing to a lack of approaches to constrain particle settling, the particle dynamics in the ocean interior remains poorly understood than those in the upper ocean. Here, ²¹⁰Po, ²¹⁰Pb, and ²²⁶Ra were analyzed to examine the cycling and settling of particulate matter across the water column of the South China Sea. In the euphotic zone, ²¹⁰Pb is in excess relative to ²²⁶Ra (A.R. = 1.46 ± 0.32), indicating the dominance of atmospheric deposition of ²¹⁰Pb. ²¹⁰Po is in deficit relative to ²¹⁰Pb (A.R. = 0.50 ± 0.08), revealing plankton-induced rapid settling of particulate matter. In the intermediate and deep water, the deficit in ²¹⁰Pb (A.R. > 1.0) indicates that particles continuously remove ²¹⁰Pb from seawater during settling to depth, supporting the use of ²¹⁰Pb–²²⁶Ra as a tracer for particle sinking in the ocean interior. In contrast, the ²¹⁰Po-²¹⁰Pb disequilibrium exhibits spatial heterogeneity. On the northern slope, a ²¹⁰Po deficit (A.R. < 1.0) is observed, attributed to the cross-shelf/slope transport of particles. In the southwestern basin, the riverine materials resulted in a depletion of ²¹⁰Po in the Mekong plume region. In the southeastern basin with little influence of shelf and river, a ²¹⁰Po deficit was only observed down to 1000 m; ²¹⁰Po reached equilibrium with ²¹⁰Pb (A.R. ≈ 1.0) in the deep water. In bottom water, the concentrations of particulate matter were 2–3 times those in the overlying water, pointing to the benthic nepheloid layer in the South China Sea and resulting in the enhanced ²¹⁰Po deficits. Using an inverse model, the settling velocity (SV) varied from 6 to 151 m d^-1, averaging 23 ± 14 m d^-1. The sinking flux of particles ranged from 0.1 to 11.6 g m^-2 d^-1, with an average of 2.0 ± 1.1 g m^-2 d^-1. These results preliminarily elucidated the differential characteristics of particle cycling and settling at various depths in the South China Sea.

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