Elucidating the Sources Supplying Aerosol Iron, Zinc, and Cadmium to the Surface of the North Pacific Ocean with Stable Isotopes

Transition metals, such as iron (Fe), zinc (Zn) and cadmium (Cd), are found at just trace amounts within the global oceans yet are vital for many biological functions of phytoplankton. As a result, these metals influence the ocean’s efficacy for carbon sequestration. Atmospheric dust is the primary input of Fe to various surface waters and may also supply Zn and Cd to oligotrophic surface waters. Here, I present Fe, Zn, and Cd isotope data from the US North Pacific GEOTRACES Section GP15 (Alaska-Tahiti) from the low-dust season (Sept. – Nov. 2018). Using this data and aerosol enrichment factors, I show that aerosol Fe, Zn, and Cd are sourced primarily from Asian dust for northern deployments (52 to 32°N), while for southern deployments (20°N to 20°S) they are sourced from a mixture of Asian dust and natural, wildfire, and anthropogenic aerosols from North America. An important finding of this study is that both GP15 data and Fe deposition modelling from the CAM6-QFED model confirms an isotopically heavy aerosol Fe source to the North Pacific (which spans the full range of particle sizes), which I attribute to soil being entrained during wildfires and possibly Fe sourced from shipping. Soluble aerosol Fe is light (-1.28 to +0.02‰) in both Northern and Southern deployments, strongly indicative of anthropogenic combustion Fe. A two-component isotope mixing model calculates that 0.5-7% of bulk aerosol and 12-81% of soluble aerosol Fe in the North are anthropogenic, but the model breaks down in the South because at least three sources contribute to Southern deployments. For Zn and Cd, the whole GP15 section is dominated by non-crustal sources, reflected by light isotopic compositions (bulk Zn: -0.29 to +0.01‰; soluble Zn: -0.35 to +0.09‰; soluble Cd: -1.91 to -0.07‰) and enrichment factors (Zn: 32-693; Cd: 263-10775). Northern deployments indicate the presence of a large proportion of highly soluble, anthropogenic Zn, while southern deployments are lighter and more enriched in Zn, attributed to wildfires. In contrast, Cd exhibited no latitudinal trends, indicating a consistent source of anthropogenic Cd throughout the year. Although there is potential evidence of Fe and Cd aerosol influence on surface waters, the influence of Zn aerosols is not seen. Overall, this work highlights the utility of transition metal isotope tracers as powerful emerging tools for teasing apart distinct sources contributing to atmospheric aerosols, as well as the need to include additional sources such as wildfire and shipping with distinct isotopic signatures into models