FY 2025

Dust deposition to the Sargasso Sea: A comparison of estimates using aluminum in the surface ocean versus aerosols and rainwater

Williams, T.E., P.N. Sedwick, B.M. Sohst, K.N. Buck, S. Caprara, R.J. Johnson, D.C. Ohnemus, J.A. Resing, L.E. Sofen, A. Tagliabue, and B.S. Twining

Geophys. Res. Lett., 52(8), e2024GL113217, doi: 10.1029/2024GL113217, View open access article at AGU/Wiley (external link) (2025)

As a major component of mineral aerosol that partially dissolves in seawater, aluminum (Al) has been used to estimate time-averaged dust fluxes to the ocean, based on dissolved aluminum (DAl) concentrations and an assumed replacement time of DAl (τ_DAl) in the surface mixed layer, along with the fractional solubility (Al_s) and mass abundance of Al in dust. We apply this method in the Bermuda region using seasonal water-column DAl concentrations, and compare our results with fluxes estimated from Al measured in Bermuda aerosols and rain. Inventories of DAl over the maximum mixed-layer depth of 200 m and empirically derived Als values yield mean dust fluxes of 5.3–11.2 g m⁻² yr⁻¹, around 4- to 9-fold higher than fluxes estimated from aerosols and rain, which average 1.2 g m⁻² yr⁻¹ over a 318-day sampling period. This discrepancy likely reflects an underestimate in τ_DAl and lateral transport of DAl in the surface ocean.

Plain Language Summary. The rate at which soil dust from the continents is deposited to surface ocean waters is important, because dust is a major source of some chemical elements to the ocean, including the essential micronutrient iron, which controls primary production in some ocean regions. In this study we compare two different methods of estimating dust deposition rate to the North Atlantic Ocean near Bermuda, using dust, rain and seawater samples collected during calendar year 2019. Both methods use the chemical element aluminum (Al), which is present in soil dust in a relatively constant proportion. The first method estimates dust deposition using measurements of Al in dust and rain collected on Bermuda, and the second uses measurements of dissolved Al in the upper ocean near Bermuda. We find that the second method yields deposition rates 4- to 9-times higher than the first method, probably because the amount of time dissolved Al remains in the upper ocean in the study region is longer than typically assumed. Increasing that time can bring the two estimates into agreement, although that may introduce problems due to lateral movement of ocean waters, such that the estimated dust deposition is not representative of the ocean region around Bermuda.