The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

The atmospheric dust deposition is the dominant source of micronutrients, such as Fe, to the sea surface in open oceans, which regulates primary production and nitrogen fixation and their spatial variability. Directly estimating dust flux in remote oceans is difficult due to limited observation opportunity and the poorly constrained solubility of specific elements. In this study, the vertical distribution of dissolved ²³⁰Th and ²³²Th in water columns at 16 stations in the western subtropical North Pacific as well as the solubilities of Fe and Th in aerosols were examined during the GEOTRACES-China GPpr15 summer and winter cruises to estimate dust fluxes. The solubilities of Fe ad Th in aerosols were 16.01 ± 8.57% and 11.73 ± 6.59%, respectively, giving a ratio of these two solubilities (S_Fe/Th) of 1.49 ± 0.58. The dust deposition fluxes were estimated in the range of 0.51-10.08 mg m^-2 y^-1 based on the long-lived Th isotopes in seawater and measured aerosol Th solubility with an integration depth of 400 m in the upper water column. The dissolved Fe deposition flux in the western subtropical North Pacific Ocean was further estimated as 0.07-1.20 mmol m^-2 y^-1, in combination of the measured S_Fe/Th. The inventory of dissolved Fe in the upper water column showed a positive correlation with the dissolved Fe deposition flux, suggesting a determine role of dust deposition on the Fe distribution. Furthermore, the spatial variabilities of primary production and nitrogen fixation were predominantly driven by dissolved Fe deposition under the nitrogen-limited condition as supported by their significant intercorrelation. Dissolved Fe deposition did not significantly stimulate primary production and nitrogen fixation in stations with potential phosphorus limitation. Such assessment of dust deposition are important for predicting biogeochemical effects of Fe input in the western subtropical North Pacific Ocean with low nutrient and low chlorophyll concentration.
 

subtropical North Pacific, 230Th-232Th, aerosol solubility of Th and Fe, dust deposition flux, dust-associated Fe flux