Dust deposition and iron cycling in the tropical western North Pacific based on thorium supply

Abstract

Dissolved ²³²Th and biologically-essential micronutrients, such as Fe, are simultaneously supplied to the open ocean mainly through the dissolution of dust aerosols. Both dust and associated Fe deposition fluxes are therefore able to be estimated by the long-lived Th isotopes (²³⁰Th and ²³²Th) in seawater in combination with the concentration and solubility of Th and Fe in aerosols. The vertical distribution of dissolved ²³⁰Th and ²³²Th in water columns and the solubilities of Fe and Th in aerosols were examined in the tropical western North Pacific during the GEOTRACES-China GP09 cruise. The solubilities of Fe ad Th in aerosols were 14.38 ± 1.02 % and 12.22 ± 2.46 %, respectively, giving a solubility ratio of S_Fe/Th of 1.24 ± 0.34. Estimated dust deposition fluxes integrated over the upper 500 m of the water column ranged from 0.58 to 2.35 g m⁻² yr⁻¹ based on the vertical distribution of long-lived Th isotopes in seawater and measured aerosol Th solubility. The dust-borne Fe deposition flux in the tropical western North Pacific was further estimated as 4.02–12.55 mg m⁻² yr⁻¹, using the measured S_Fe/Th. Both estimated dust and dust-borne Fe deposition fluxes agree well with the dust deposition model. The spatial variability of surface dissolved Fe and primary production were predominantly driven by dust-borne Fe deposition as supported by their significant correlation. Dust-borne Fe input regulated the spatial distribution of Fe:N supply ratios, which, in turn, affected the growth of marine phytoplankton, notably diazotrophs, by stimulating N₂-fixation. Quantification of dust-borne Fe fluxes may improve our understanding of the biogeochemical cycling of Fe and N₂-fixation process in the tropical western North Pacific.