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Our study followed the seasonal cycling of soluble (SFe), colloidal (CFe), dissolved (DFe), total dissolvable (TDFe), labile particulate (LPFe), and total particulate (TPFe) iron in the Celtic Sea (NE Atlantic Ocean). Preferential uptake of SFe occurred during the spring bloom, preceding the removal of CFe. Uptake and export of Fe during the spring bloom, coupled with a reduction in vertical exchange, led to Fe deplete surface waters (<0.2 nM DFe; 0.11 nM LPFe, 0.45 nM TDFe, and 1.84 nM TPFe) during summer stratification. Below the seasonal thermocline, DFe concentrations increased from spring to autumn, mirroring NO3−and consistent with supply from remineralized sinking organic material, and cycled independently of particulate Fe over seasonal timescales. These results demonstrate that summer Fe availability is comparable to the seasonally Fe limited Ross Sea shelf and therefore is likely low enough to affect phytoplankton growth and species composition. Preferential uptake of soluble iron during the phytoplankton spring bloom implies that it is initially more bioavailable than colloidal ironPotentially growth limiting concentrations of bioavailable iron during summer in the Celtic Sea as a result of seasonal stratificationOrganic matter remineralization drives a seasonal increase in dissolved iron independent of particulate iron cycling

The supply and bioavailability of iron (Fe) controls primary productivity and N2fixation in large parts of the global ocean. An important, yet poorly quantified, source to the ocean is particulate Fe (pFe). Here we present the first combined dataset of particulate, labile‐particulate (L‐pFe), and dissolved Fe (dFe) from the (sub)tropical North Atlantic. We show a strong relationship between L‐pFe and dFe, indicating a dynamic equilibrium between these two phases whereby particles “buffer” dFe and maintain the elevated concentrations observed. Moreover, L‐pFe can increase the overall “available” (L‐pFe + dFe) Fe pool by up to 55%. The lateral shelf flux of this available Fe was similar in magnitude to observed soluble aerosol‐Fe deposition, a comparison that has not been previously considered. These findings demonstrate that L‐pFe is integral to Fe cycling and hence plays a role in regulating carbon cycling, warranting its inclusion in Fe budgets and biogeochemical models. Labile‐particulate iron has a key role in iron cycling and can increase the overall “available” iron pool by up to 55% in the OMZA strong relationship between particles and dissolved iron indicates that L‐pFe “buffers” the elevated dFe concentrations observedLateral shelf transport of available Fe (L‐pFe + dFe) supplied a similar magnitude of iron as atmospheric sources

Zinc is vital for the physiology of oceanic phytoplankton. The striking similarity of the depth profiles of zinc to those of silicate suggests that the uptake of both elements into the opaline frustules of diatoms, and their regeneration from these frustules, should be coupled. However, the zinc content of diatom opal is negligible, and zinc is taken up into and regenerated from the organic parts of diatom cells. Thus, since opaline frustules dissolve deep in the water column while organic material is regenerated in the shallow subsurface ocean, there is little reason to expect the observed close similarity between zinc and silicate, and the dissimilarity between zinc and phosphate. Here we combine observations with simulations using a three-dimensional model of ocean circulation and biogeochemistry to show that the coupled distribution of zinc and silicate, as well as the decoupling of zinc and phosphate, can arise in the absence of mechanistic links between the uptake of zinc and silicate, and despite contrasting regeneration length scales. Our simulations indicate that the oceanic zinc distribution is, in fact, a natural result of the interaction between ocean biogeochemistry and the physical circulation through the Southern Ocean hub. Our analysis demonstrates the importance of uptake stoichiometry in controlling ocean biogeochemistry, and the utility of global-scale elemental covariation in the ocean in understanding these controls.

The dispersal of dissolved iron (DFe) from hydrothermal vents is poorly constrained. Combining field observations and a modeling hierarchy, we find the dispersal of DFe from the Trans‐Atlantic‐Geotraverse vent site occurs predominantly in the colloidal phase and is controlled by multiple physical processes. Enhanced mixing near the seafloor and transport through fracture zones at fine‐scales interacts with the wider ocean circulation to drive predominant westward DFe dispersal away from the Mid‐Atlantic ridge at the 100 km scale. In contrast, diapycnal mixing predominantly drives northward DFe transport within the ridge axial valley. The observed DFe dispersal is not reproduced by the coarse resolution ocean models typically used to assess ocean iron cycling due to their omission of local topography and mixing. Unless biogeochemical models account for fine‐scale physics and colloidal Fe, they will inaccurately represent DFe dispersal from axial valley ridge systems, which make up half of the global ocean ridge crest. Hydrothermal venting along mid ocean ridges supplies large quantities of the trace metal iron to the ocean. Once it mixed with oxygenated seawater, precipitation leads to iron being lost from the dissolved phase to generate seafloor metal deposits. However, a small fraction of iron supplied escapes precipitation and remains in the dissolved phase. The processes that control the retention and ocean transport of hydrothermal dissolved iron is important as it has a disproportionate influence on the global carbon cycle. In this work, we examined the processes driving the dispersal of dissolved iron from a major site of hydrothermal venting on the northern mid Atlantic ridge. We found that the complex topography of the mid Atlantic ridge was crucial in steering the escape of dissolved iron in the colloidal size range out of the immediate mid ocean ridge system. This raises challenges for the large scale ocean models used to represent the global ocean iron cycle as they are typically not parameterized at sufficient spatial resolution. The use of multiple grids, with higher resolution nests, may offer a solution to the challenge of representing the interactions of tracer dispersal with complex topography. Iron is dispersed from trans Atlantic geotraverse predominantly northward within the axial valley and westward off axis, dominated by the colloidal size fractionA combination of fine‐scale processes are necessary to explain the dispersal both within and outside the axial valleyCoarse resolution models are impaired in their ability to constrain the broader influence of iron supplied from axial valley ridge systems Iron is dispersed from trans Atlantic geotraverse predominantly northward within the axial valley and westward off axis, dominated by the colloidal size fraction A combination of fine‐scale processes are necessary to explain the dispersal both within and outside the axial valley Coarse resolution models are impaired in their ability to constrain the broader influence of iron supplied from axial valley ridge systems

A flow injection with chemiluminescence detection (FI-CL) method was used to determine the concentration of dissolved iron in seawater samples collected in the South Atlantic during the GEOTRACES GA10 cruise that took place from 24th December 2011–27th January 2012 on board the R.R.S. James Cook (cruise JC068). Six different sample collection and filtration strategies were used. Open ocean (shallow and deep) and coastal (shallow and deep) samples were collected and five sub-samples from each collection were filtered through a cartridge filter. For the deep open ocean sample, separate sub-samples were also filtered through a membrane disc filter. In addition, deep open ocean sub-samples were also taken from five separate sampling bottles. Each sub-sample (29 in total) was analysed six times (giving 174 discrete measurements in total) and the within sub-sample precision was in the range 1.4–12.2%. There was no statistically significant difference for the deep, open ocean sample between the mean results obtained with the two different filter types or the single sample bottle versusseparate sample bottle sub-samples. Application of classical ANOVA showed that the relative combined standard uncertainty for each of the six sampling strategies ranged from 2.3–3.8%. This approach did not include an estimation of sampling bias. Application of robust ANOVA to the deep open ocean samples showed that contributions to the total variance were 0% from the different sample collection and filtration strategies, 42% from the sub-sample precision and 58% from between sub-sample measurements.

The transport of warm and salty waters from the Indian Ocean to the South Atlantic by the Agulhas Current constitutes a key return route of the meridional overturning circulation. Despite the importance of the Agulhas Leakage on interoceanic exchange, its role on biogeochemical cycles is poorly documented. Here we present the first lead (Pb) concentration and isotope data for surface seawater collected during the GEOTRACES cruise D357 in the Agulhas current system. Lead in surface waters of the Cape Basin is described by three distinct end‐members: the South African coast, open South Atlantic seawater, and Indian Ocean seawater. The latter stands out in its Pb isotopic composition and can be tracked within two distinct Agulhas rings. High Pb concentrations in the Agulhas rings further corroborate an Indian Ocean provenance of waters and suggest that the Agulhas Leakage represents a major conduit not only for heat but also for trace metals. Lead isotopes for Cape Basin surface waters record mixing between three distinct end‐membersPb isotopes are able to track leakage of Indian Ocean seawater into the Atlantic via Agulhas ringsThe Agulhas Leakage is not only a key pathway for heat but also impacts biogeochemical cycles

New dissolved aluminum (dAl) data from the 2017 GEOTRACES process study GApr08 along 22°N in the subtropical North Atlantic are presented. They show an east to west increase in dAl concentration in the surface waters. Simulation of these data with a 1D advection‐dust deposition revealed that, (a) advection and dust dissolution are equally important dAl sources, (b) scavenging plays a minor role compared to advection in dAl removal, and (c) in addition to dust dissolution, another dAl source is required at the westernmost stations to fully explain our observations. We attribute this additional source to the dissolution of erosion products delivered to the western subtropical North Atlantic by the Lesser Antilles. For waters deeper than ∼200–300 m, an optimum multi‐parameter analysis allowed to separate the component of the dAl signal derived from water mass transport from its biogeochemical component. This revealed, (a) a major role played by water mass transport, (b) a net dAl removal between 200 and 800 m, attributed to scavenging at the subtropical North Atlantic scale, and (c) internal dAl inputs between 800 m and the seafloor, attributed to reversible scavenging. While the dAl oceanic distribution is usually considered to be dominated by the atmospheric dust input and removal by particle scavenging, this study highlights the important role played by advection, and the need to explicitly take this into account in order to quantitatively reveal the impact of external sources and dissolved‐particulate interactions on the Al cycle in the North Atlantic Ocean. Advection and dust deposition are the main processes driving the surface gradient of dissolved aluminum in the subtropical North AtlanticLesser Antilles erosion products are a possible source of dissolved aluminum to the western subtropical North AtlanticDissolved‐particle interactions play a major role in the dissolved aluminum distribution in the subtropical North Atlantic Advection and dust deposition are the main processes driving the surface gradient of dissolved aluminum in the subtropical North Atlantic Lesser Antilles erosion products are a possible source of dissolved aluminum to the western subtropical North Atlantic Dissolved‐particle interactions play a major role in the dissolved aluminum distribution in the subtropical North Atlantic