Mechanisms controlling dissolved iron distribution in the North Pacific: A model study

Abstract

Mechanisms controlling the dissolved iron distribution in the North Pacific are investigated using the Biogeochemical Elemental Cycling (BEC) model with a resolution of approximately 1° in latitude and longitude and 60 vertical levels. The model is able to reproduce the general distribution of iron as revealed in available field data: surface concentrations are generally below 0.2 nM; concentrations increase with depth; and values in the lower pycnocline are especially high in the northwestern Pacific and off the coast of California. Sensitivity experiments changing scavenging regimes and external iron sources indicate that lateral transport of sedimentary iron from continental margins into the open ocean causes the high concentrations in these regions. This offshore penetration only appears under a scavenging regime where iron has a relatively long residence time at high concentrations, namely, the order of years. Sedimentary iron is intensively supplied around continental margins, resulting in locally high concentrations; the residence time with respect to scavenging determines the horizontal scale of elevated iron concentrations. Budget analysis for iron reveals the processes by which sedimentary iron is transported to the open ocean. Horizontal mixing transports sedimentary iron from the boundary into alongshore currents, which then carry high iron concentrations into the open ocean in regions where the alongshore currents separate from the coast, most prominently in the northwestern Pacific and off of California.

The iron budget in ocean surface waters in the 20th and 21st centuries: projections by the Community Earth System Model version 1

“…This module simulates four phytoplankton functional type classes (pico/nanoplankton, coccolithophores, diatoms, and diazotrophs), one zooplankton class, five nutrients (dissolved nitrate, ammonia, phosphorus, iron, and silicate), inorganic carbon system geochemistry, and oxygen cycling. The model's skill in simulating ecology and biogeochemistry, including iron cycling, has been documented previously (Moore et al, 2004;Moore and Braucher, 2008;Doney et al, 2009a, b;Misumi et al, 2011Misumi et al, , 2013Moore et al, 2013).…”

“…Dissolved iron is assumed to be the only iron pool utilized by phytoplankton, and it is subject to particle scavenging. The ironscavenging rate is calculated by considering ambient sinking particle fluxes (particulate organic matter, dust, biogenic silica, and calcium carbonate) and, implicitly, the effect of complexation with binding ligands (for details see Moore and Braucher, 2008;and Misumi et al, 2011). Although in preliminary tests, we introduced spatially variable ligand concentrations into the BEC model , a conventional approach with a homogeneous ligand concentration (0.6 nM) was used for this simulation.…”

“…Because of the relatively short residence time of iron in seawater, favorable ocean bathymetry, circulation, or mixing conditions are required for iron to be transported from sedimentary or hydrothermal sources to surface waters of the open ocean; such conditions may occur, for example, downstream of islands (Blain et al, 2001) or where ocean currents flow swiftly (de Baar et al, 1995;Mackey et al, 2002;Slemons et al, 2009;Misumi et al, 2011). Complexation with organic ligands greatly lengthens the residence time of iron in seawater (e.g., Misumi et al, 2011), so the concurrent release of iron and organic ligands from hydrothermal vents (Bennett et al, 2008;Toner et al, 2009) may facilitate long-distance iron transport from a hydrothermal source. Most iron in rivers is thought to flocculate and be precipitated in estuaries, because seawater cations neutralize the negatively charged iron-bearing colloids (Boyle et al, 1977).…”

“…Various processes are responsible for the exogenous inputs: deposition of iron-bearing dust (Duce and Tindale, 1991;Tegen and Fung, 1994;Mahowald et al, 1999;Jickells et al, 2005); reduction and resuspension of sedimentary iron (Johnson et al, 1999;Elrod et al, 2004;Moore et al, 2004;Lam et al, 2006;Nishioka et al, 2007;Lam and Bishop, 2008;Moore and Braucher, 2008;Tagliabue et al, 2009;Misumi et al, 2011); and fluvial and hydrothermal inputs (Wetz et al, 2006;Tagliabue et al, 2010;Klunder et al, 2011;Nishioka et al, 2013;Saito et al, 2013). Most sedimentary and hydrothermal iron is supplied from continental margins and the ocean abyss to open ocean surface waters by physical ocean processes.…”

Spatial Distribution of the Particulate Organic Ligand in the South Pacific Ocean Surface Waters

“…POL, therefore, is a candidate of source of the dissolved organic ligand regarding Fe complexation in the ocean because Fe-binding ligands dissolved in seawater are released from bacterial degradation of sinking particles [14]. Knowledge of the spatial distributions of POL would be useful for development of ocean Fe biogeochemistry including biogeochemical modeling [15][16][17][18][19][20][21]. Furthermore, POL is a marker of biogenic Particulate Organic Matter (POM), because the organic ligand specified in PM has been detected in microorganisms such as bacteria, phytoplankton and zooplankton [12,13]).…”