Ligand Binding Strength Explains the Distribution of Iron in the North Atlantic Ocean

Abstract

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.

Physical and Biological Controls of the Drake Passage pCO2 Variability

“…There are three sources of iron in this model including atmospheric deposition, hydrothermal vents and sediments. The atmospheric deposition field is taken from Ito et al (2016) with spatially variable solubility of dust iron, and the hydrothermal Fe fluxes are parameterized as a linear function of helium fluxes (Pham & Ito, 2019; Tagliabue et al, 2010). Sedimentary Fe fluxes are estimated offline according to Moore and Braucher (2008) using satellite productivity (Behrenfeld & Falkowski, 1997; Laws et al, 2011) and the ETOPO2v2 topography.…”

“…Detailed ecosystem model parameters are provided in (Dutkiewicz et al, 2015). Dissolved iron is a key limiting nutrient in the Southern Ocean, and an improved parameterization of iron cycling is implemented following Pham and Ito (2019). There are three sources of iron in this model including atmospheric deposition, hydrothermal vents and sediments.…”

Anthropogenic Iron Deposition Alters the Ecosystem and Carbon Balance of the Indian Ocean Over a Centennial Timescale

“…Scavenged dFe through this mechanism can also return to the water column by desorption from sinking particles. This return dFe flux was calculated from the vertical profile of sinking inorganic scavenged‐Fe flux, which was represented by a power function with a coefficient of −0.4 (Pham & Ito, 2019). Atmospheric deposition fields of N and dFe were taken from the output of atmospheric chemical transport model GEOS‐Chem (Johnson & Meskhidze, 2013).…”

“…The refined Fe scheme, which were developed and documented in our recent publications (Pham & Ito, 2018, 2019), encompassed various important processes in the ocean Fe cycling. These processes included external dFe inputs from dust deposition, continental shelves, and hydrothermal vents.…”

“…We focus on the response of the ocean ecosystem over a timescale of 100 years, which is relevant to human activities and policy decisions. To this end, we use a 3‐D global ocean ecosystem model (MITgcm), which represents major phytoplankton types (Dutkiewicz et al., 2014), coupled with a recently improved Fe cycling scheme (Pham & Ito, 2018, 2019). The Fe scheme includes many crucial processes controlling ocean Fe cycling and demonstrated improvements in the representation of dFe distribution as observed by the GEOTRACES cruises.…”