Thorium distributions in high- and low-dust regions and the significance for iron supply

Abstract

Thorium and uranium isotopes (²³²Th, ²³⁰Th, ²³⁸U, and ²³⁴U) were investigated to refine their use for estimating mineral dust deposition and Fe delivery to the ocean. U concentrations and isotope ratios were consistent with conservative behavior and can safely be described using published U‐salinity relationships and global average seawater isotopic composition. Near Barbados, waters affected by the Amazon outflow contained elevated ²³²Th. This signals one region where the thorium‐dust method is inaccurate because of a confounding continental input. Dissolved ²³²Th fluxes in this region suggest that Amazonian Fe supply to the adjacent open ocean is much larger than local atmospheric deposition. The colloidal content of dissolved Th south of Bermuda was found to be quite small (2–6%), similar to that found north of Hawaii, despite the order of magnitude higher dust deposition in the Atlantic. This finding supports the assumption that dissolved ²³²Th and ²³⁰Th are scavenged at the same rate despite their different sources and also sheds light on the increase of dissolved ²³²Th fluxes with integrated depth. Outside the region influenced by Amazon River waters, dissolved ²³²Th fluxes are compared with Bermudan aerosol Fe deposition to estimate that fractional Th solubility is around 20% in this region. Finally, new dissolved and soluble Fe, Mn, and Cr data from the subtropical North Pacific support the idea that Fe concentrations in the remote ocean are highly buffered, whereas ²³²Th has a larger dynamic range between high‐ and low‐dust regions. Keywords: aerosol dust; Amazon River; Bermuda; Hawaii; Barbados