Iron (Fe) is the fourth most abundant element in the Earth's crust (about 6.7%; Rudnick & Gao, 2003) but it is present at sub-nanomolar concentrations in seawater (<10 −9 mol L −1 ; Johnson et al., 1997). Yet, Fe is a key micronutrient for the growth and metabolism of all living organisms and especially phytoplankton for which it is essential for the proper functioning of the photosynthetic system (Behrenfeld & Milligan, 2013;Raven et al., 1999). Consequently, Fe has a direct influence on primary production (Martin et al., 1994;Sunda & Huntsman, 1995) and thus plays an important role on carbon export and sequestration in the ocean interior (Martin, 1990). Numerous natural fertilization studies have investigated the importance of iron, primarily in the Southern Ocean (Blain et al., 2007;Pollard et al., 2007), and have reported enhanced primary production rates and particulate organic carbon export efficiencies, which may influence the biological carbon pump (Morris & Charette, 2013). In the context of climate change (IPCC, 2021), characterizing the elements governing the efficiency of this pump is of great interest. Due to its importance, the number of dissolved Fe (DFe) concentration measurements has increased impressively in recent years thanks to the GEOTRACES program (https://www. geotraces.org/), particularly in the deep ocean. However, there is still a lack of data for some key ocean regions, such as the Western Tropical South Pacific (WTSP) Ocean.The WTSP Ocean (160°E-160°W) has recently been identified as a hotspot of dinitrogen (N 2 ) fixation with some of the highest rates recorded in the global ocean (Bonnet et al., 2017). Diazotrophy is a process favored in phosphorus-rich, nitrogen-poor waters, that fuels the ocean with new nitrogen, helping to maintain ocean productivity and carbon sequestration (Caffin et al., 2018). This region is characterized by two biogeochemical

Key Points Size‐fractionation reveals that colloidal iron is more important than soluble iron for distribution of dissolved iron in North Atlantic Colloidal iron is the predominantly active species at key supply and removal processes of the dissolved iron cycle Global comparison suggests that the colloidal iron fraction in mesopelagic zone is determined by the local lithogenic contribution

One of the recently recognized main sources of iron to the deep ocean inventory is the hydrothermal activity associated with mid-ocean ridges. Little is known about the oxidation 2 kinetics of iron(II) within these environments, especially the dependence on physicochemical parameters such as temperature (T), pH, particle size-fractionation and the effect of organic matter.Following sample collection during the GA13 section cruise, the iron(II) oxidation at six hydrothermal vent sites (Menez Gwen, Lucky Strike, Rainbow, Lost City, Broken Spur and TAG) along the Mid-Atlantic Ridge were investigated, revealing high variability. The Fe(II) oxidation rate constant analysis from multiple stations at two sites (Rainbow and TAG), revealed that factors, other than T and pH, controlled the oxidation process. Experiments on the effect of particle size-fractionation and organic matter at different pH showed that the presence of organic ligands and colloidal size particles delayed the oxidation process, while not affecting the overall pH dependency.Extending our analysis to the broader relationship between the Fe(II) oxidation rate constants across a range of temperatures (between 2 and 25 ºC) and pH (between 7 and 8) on a set of selected hydrothermal samples allowed us to derive a multiparametric equation to model the iron(II) oxidation rate constants in the ocean. This equation covers a larger range of temperatures than previous published equations, improving its applicability for global biogeochemical models.