Determining the sources of aerosols delivered to the subtropical North Atlantic using stable isotopes

Determining the sources of aerosols delivered to the subtropical North Atlantic using stable isotopes

Deposition of atmospheric aerosols is a major source of bioavailable trace metals such as iron (Fe), zinc (Zn) and cadmium (Cd) to surface waters. Such aerosol deposition occurs through two modes: (1) dry deposition, or the settling of dry aerosol matter onto the open ocean, and (2) wet deposition, the delivery of solubilized aerosols during rain events. Although the effect of dry deposition on trace metal delivery to the oceans is reasonably well-studied, the influence of wet deposition remains extremely understudied. Here, we present the first dissolved Fe (δ56Fe), Zn (δ66Zn, and Cd (δ114Cd) isotopic compositions ever measured in rainwater. These samples, collected on Bermuda, are representative of the wet deposition aerosol flux to the subtropical North Atlantic during the summer. Our data shows dissolved δ56Fe ranged from -0.43‰ to +0.38‰, dissolved δ66Zn ranged from +0.19‰ to +0.34‰, and dissolved δ114Cd ranged from -1.03‰ to -0.08‰, compared to crustal values of +0.1‰, +0.3‰ and 0.0‰, respectively. Our finding of lighter-than-crustal compositions for all three elements indicate a contribution of aerosols from anthropogenic sources. Furthermore, for Fe, a two-component mixing model allows us to show that wet deposition aerosols are a mixture of Saharan dust and up to ~30% from anthropogenic combustion. Overall, trace metal addition via wet deposition can be influenced by addition from anthropogenic sources, and we anticipate further studies are necessary to investigate the isotopic variability of various trace metals deposited by wet deposition to the global oceans.