Recent GEOTRACES transects revealed basin‐scale patterns of dissolved iron in the global oceans, providing a unique opportunity to test numerical models and to improve our understanding of the iron cycling. Subsurface maxima of dissolved iron in the upper ocean thermocline are observed in various transects, which can play an important role in regulating marine productivity due to their proximity to the surface euphotic layer. An ocean biogeochemistry model with refined parameterizations of iron cycling is used to examine the mechanisms controlling the formation and maintenance of these subsurface maxima. The model includes the representation of three iron sources including dust deposition, continental shelves, and hydrothermal vents. Two classes of organic ligands are parameterized based on the dissolved organic matter and apparent oxygen utilization. Parameterizations of particle‐dependent scavenging and desorption are included. Although the model still struggles in fully capturing the observed dissolved iron distribution, it starts reproducing some major features, especially in the main thermocline. A suite of numerical sensitivity experiments suggests that the release of scavenged iron associated with sinking organic particles forms the subsurface‐dissolved iron maxima in high‐dust regions of the Indian and Atlantic Oceans. In low‐dust regions of the Pacific basin, the subsurface‐dissolved iron extrema are sustained by inputs from the continental shelves or hydrothermal vents. In all cases, subsurface ligands produced by the remineralization of organic particles retain the dissolved iron and play a central role in the maintenance of the subsurface maxima in our model. Thus, the parameterization of subsurface ligands has a far‐reaching impact on the representation of global iron cycling and biological productivity in ocean biogeochemistry models.
Observations of dissolved iron (dFe) in the subtropical North Atlantic revealed remarkable features: While the near‐surface dFe concentration is low despite receiving high dust deposition, the subsurface dFe concentration is high. We test several hypotheses that might explain this feature in an ocean biogeochemistry model with a refined Fe cycling scheme. These hypotheses invoke a stronger lithogenic scavenging rate, rapid biological uptake, and a weaker binding between Fe and a ubiquitous, refractory ligand. While the standard model overestimates the surface dFe concentration, a 10‐time stronger biological uptake run causes a slight reduction in the model surface dFe. A tenfold decrease in the binding strength of the refractory ligand, suggested by recent observations, starts reproducing the observed dFe pattern, with a potential impact for the global nutrient distribution. An extreme value for the lithogenic scavenging rate can also match the model dFe with observations, but this process is still poorly constrained.
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The Indian Ocean accounts for around one-fifth of the ocean net primary production (Behrenfeld & Falkowski, 1997a) and contains two of the largest oxygen (O 2) minimum zones (OMZs) of the world oceans in the northern part (the Arabian Sea and the Bay of Bengal) (Stramma et al., 2010). In these two regions, phytoplankton growth is generally limited by macronutrients because of the relatively shallow mixed layer and the Ekman downwelling that transports nutrients away from the euphotic layer. Furthermore, the low O 2 water in the OMZs promote nitrogen (N) loss through denitrification (Moore et al., 2013; Wang et al., 2019). In the northern Indian Ocean, the concentration of dissolved iron (dFe) is relatively high (∼0.6 nM in the surface and ∼1.5 nM in the subsurface 200-1,000 m water) due to relatively high dFe inputs from atmospheric deposition and reduced sediments over the continental shelves (Chinni et al., 2019; Nishioka et al., 2013). However, Fe can still be a limiting factor for the nitrogen-fixer diazotrophs, which have a higher demand for Fe than other phytoplankton (
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