Iron (Fe), an essential micronutrient for all the living organisms, plays important roles in photosynthesis, respiration and nitrogen fixation of marine phytoplankton (Moore et al., 2013). Although it is one of the most abundant elements in the crust, concentration of dissolved Fe in oceanic water is very low, limiting primary productivity and affecting phytoplankton ecosystem structures in many open oceans (Boyd & Ellwood, 2010;Jickells et al., 2005;Tagliabue et al., 2017). In high nutrient low chlorophyll (HNLC) regions (e.g., Southern Ocean, subarctic North Pacific and east tropical Pacific) which cover around 30% of global oceans, Fe supply directly limits or colimits photosynthesis and thus primary production (Jickells et al., 2014). Moreover, Fe supply limits or colimits microbial nitrogen fixation and thus also affects primary production over vast tropical nutrient-poor regions (e.g., tropical Pacific, South Atlantic, and Indian Ocean) (Jickells et al., 2014). There are evidence that increased supply of soluble Fe to some oceanic regions would increase primary productivity (Boyd et al., 2007;Tang et al., 2021), having important implications for CO 2 uptake and climate (Jickells et al., 2005). It has been proposed that increase in dust and thus Fe deposition during the last glacial maximum caused increase in carbon export to deep ocean and decrease in atmospheric CO 2 (Martin, 1990).Major external sources of Fe in surface oceans include aerosol deposition, riverine input, continental margins, hydrothermal activities and glacial sediments, and aerosol deposition is a major Fe source for open oceans (Conway & John, 2014;Jickells et al., 2005;Tagliabue et al., 2017). In addition, aerosol Fe (as well as other transition metals such as Cu and Mn) also plays important roles in chemical reactivity of aerosol particles (Alexander et al., 2009;Martin & Good, 1991) and their health effects (Daellenbach et al., 2020;Fang et al., 2017). However, a large fraction of aerosol Fe may not be bioavailable. Fe availability is in fact poorly defined and understood, and soluble Fe has been widely used as a proxy for bioavailable Fe (Baker & Croot, 2010;Meskhidze et al., 2019).
