Persistent increases in bioaccessible iron deposition from anthropogenic processing activities to the North Pacific and Indian Oceans despite rapid decreases from fuel combustion sources

Abstract. The trend of atmospheric deposition of bioaccessible iron (BFe) from anthropogenic sources is driven by changes in aerosol column loading of total Fe (TFe) and Fe bioaccessibility, primarily due to the evolution of human activities. Black carbon (BC) is often used as a temporal and spatial proxy for BFe emissions from anthropogenic aerosols. However, derived changes in BFe concentrations from anthropogenic processing activities (e.g., smelting and abrasion) and fuel combustion have not been explored separately because of a lack of evaluation of BFe aerosols for separate anthropogenic sources. The Fe stable isotope ratio (δ⁵⁶Fe) of size-segregated aerosols is a useful tracer to disentangle anthropogenic Fe sources. Here, we evaluate the source contributions predicted by an atmospheric chemical transport model using size-segregated aerosol observations over the North Pacific. Using the improved model, we investigate the relative changes in BFe aerosols from anthropogenic processing activities and fuel combustion sources in the major continental outflow regions during 2002–2022. The model reveals a persistent increase in BFe supply to the North Pacific and Indian Oceans from anthropogenic processing activity sources from 2002 (18 Gg Fe yr⁻¹) to 2022 (20 Gg Fe yr⁻¹) despite a decrease from fuel combustion from 2012 (20 Gg Fe yr⁻¹) to 2022 (15 Gg Fe yr⁻¹). These findings may have important implications for projections of future micronutrient supply to the oceans, which regulates the magnitude and dynamics of marine primary productivity.